FF/FL Concrete Floor Flatness and Levelness: The F-Number System Explained (With Chart)

FF and FL are F-numbers that grade a concrete floor. FF (flatness) measures how bumpy or wavy the surface is over short distances. FL (levelness) measures how closely it follows the design slope over 10 ft. Higher numbers mean flatter, more level floors. Both are measured under ASTM E1155. Typical specified values run from about FF20/FL15 for ordinary floors to FF50/FL50 and higher for superflat floors.

The FF/FL chart by floor use is right below. After it you’ll find an approximate straightedge conversion and a worked calculation you can check by hand. It also covers how to write the spec, what to settle before the pour, and how to fix a floor that fails.

Key takeaways

• FF = flatness (how bumpy the surface is over 12 in. and 24 in.). FL = levelness (how far it drifts from design elevation over 10 ft). Both have no units, and higher is better.
• Specify both an overall value (SOFF/SOFL) for the whole floor and a minimum local value (MLFF/MLFL) for each section. A common rule of thumb puts the local value at about two-thirds of the overall value.
• Common targets: about FF20/FL15 for conventional floors, FF25/FL20 for carpeted offices, FF35/FL25 for typical warehouses and thin-set flooring, FF45/FL35 for very flat floors, and FF50+/FL50+ for superflat floors.
• Have the floor measured soon after finishing, typically within 24 hours, so problems reach the crew before the next pour. ACI 117 sets 72 hours as the outer limit.
• Don’t specify FL on unshored elevated slabs. Deflection is a structural matter that the finisher can’t control.
• Very-narrow-aisle (VNA) warehouses and other defined-traffic floors need wheel-path tolerances such as F_Min, not just FF/FL.

FF/FL chart: typical F-numbers by floor use

This chart shows commonly specified FF/FL values by floor class. Overall values (SOFF/SOFL) apply to the whole floor; minimum local values (MLFF/MLFL) apply to each test section, as explained further down. The classes follow the categories in ACI 302.1R (Guide to Concrete Floor and Slab Construction) and ACI 117 (Specification for Tolerances for Concrete Construction and Materials). Values differ slightly between editions of those documents, so treat this as a starting point. Your designer of record and floor covering manufacturers set the actual numbers for your project.

Floor class	Typical uses	Overall FF / FL (SOFF / SOFL)	Minimum local FF / FL (MLFF / MLFL)	Typical placing and finishing approach
Conventional	Mechanical rooms, parking structures, back-of-house, areas under raised access floor or thick-set tile, exterior slabs	FF20 / FL15	FF15 / FL10	Strike off, bull float, then float and trowel
Moderately flat	Carpeted offices, light commercial, light industrial, schools	FF25 / FL20	FF17 / FL15	As above, plus restraightening with a highway straightedge
Flat	Thin-set tile and resilient flooring, retail, warehouses with moderate to heavy random forklift traffic	FF35 / FL25	FF24 / FL17	Laser screed or accurate wet screeds, multiple straightedge passes
Very flat	High-bay warehouses with random traffic, air-pad transporters, ice rinks	FF45 / FL35	FF30 / FL24	Laser screed or narrow strips, disciplined restraightening throughout finishing
Superflat	TV and film studios, high-bay random-traffic floors, some automated storage	FF50 / FL50 and higher (often 60 to 100+)	Typically about 2/3 of overall	Narrow strip pours, specialist superflat contractor, profiling during finishing

If the contract documents say nothing about floor tolerances, ACI 117 includes default values that apply. They broadly match the conventional row. Check the edition your spec references for the exact numbers and for elevated slab requirements.

Why FF/FL charts online don’t match

You will find several different floor flatness and levelness charts online, and the categories don’t always line up. Some come from older ACI editions. Others come from equipment makers or from the early profiling industry, which used finer steps. The chart from the original version of this article, below, is an example. It puts “conventional” at FF19/FL13 or FF25/FL17 depending on strike-off method, and it adds “good” and “ultraflat” classes. It isn’t wrong. It just uses different breakpoints.

The lesson for your job: the category name doesn’t matter. The numbers in your specification do. Never write “flat floor per ACI” without numbers.

Why the 10-ft straightedge spec was replaced

For decades, floors were specified as “1/8 in. in 10 ft”: put a 10-ft straightedge on the slab and the gap underneath can’t be more than 1/8 in. It sounds clear. In practice it caused arguments, and starting in the 1970s ACI committees worked toward a statistical replacement.

The problems with the straightedge spec:

• It can’t tell a gentle wave from a washboard. The two profiles in Figure 1 both pass 1/8 in. in 10 ft, but a forklift or a sheet of vinyl plank would treat them very differently.
• For a long time it had no standard test method. Where do you put the straightedge? How many placements? Do you count gaps at the ends? Do deviations add up over 30 ft? Every inspector answered differently.
• It doesn’t measure levelness. A floor can pass the straightedge everywhere and still be 1 in. out of level across a bay.
• Results depend on the person doing the test. Two people can test the same slab and get different results.

The F-number system fixes this. You take elevation readings at fixed intervals along sample lines and run the statistics. Two crews testing the same floor should get very close results, and the numbers mean the same thing on every job.

FF vs FL: what the F-numbers mean

F-numbers come from the work of ACI Committee 117 and early floor-profiling equipment, and they are now standardized in ASTM E1155. The F stands for floor. The subscript tells you which property is measured.

	FF (Floor Flatness)	FL (Floor Levelness)
What it measures	Bumpiness or waviness: how much the surface curves over short distances	Tilt: how closely the surface follows the design elevation or slope
Measurement distance	Readings 12 in. apart, combined over 24 in.	Elevation difference between points 10 ft apart
Driven mostly by	Finishing: bull floating, restraightening, troweling	Forming and strike-off: rail and form elevations, screeding, laser screed accuracy
Value in the formula	q = change in slope (curvature) over each 24-in. span	z = change in elevation over each 10-ft span
Typical range	15 to 100+	10 to 100+
On unshored elevated slabs	Specify it	Don’t specify it (deflection controls)

Both numbers scale in a straight line. An FF50 floor is roughly twice as flat as an FF25 floor, and an FL40 floor is roughly twice as level as an FL20 floor. That makes it easy to compare a floor against the spec: FF30 against a specified FF35 is about 15% short.

Search results sometimes run these terms together as “FFL” or “Ff/Fl concrete”, and some people confuse them with f’c, the specified compressive strength. They are different. f’c tells you how strong the concrete is. F-numbers tell you how flat and level its surface is. For a short definition you can share with an owner, see our glossary entry on floor flatness.

The FF and FL formulas (and what units to use)

ASTM E1155 reduces the readings with the two formulas below. Readers often ask what units the q and z values are in. For these inch-pound formulas they are inches. If your level or profiler gives decimal feet, multiply by 12 before you start. The standard also gives SI constants for readings in millimetres.

You won’t do this by hand on a real floor. The testing agency’s software processes hundreds or thousands of readings. It is still worth understanding the formula, because it shows you two things:

• Consistency matters most. The standard deviation is multiplied by 3, so a floor with a few big humps scores much lower than one with many small ripples.
• A consistent tilt costs you too. The mean term catches a floor that leans one way on average. That matters most for FL, where it shows up as a slab that is consistently high or low compared with design.

Overall vs local F-numbers (SOFF/SOFL vs MLFF/MLFL)

A good FF/FL spec has two tiers.

• Specified overall values (SOFF and SOFL) apply to the whole floor, or to each pour if the spec says so. They are the composite of every test section combined.
• Minimum local values (MLFF and MLFL) apply to each individual section. They are the floor below which a section must be repaired or replaced.

Why two tiers? Without a local minimum, one bad bay can hide inside an otherwise good floor. Eight sections at FF45 and one at FF18 could still give a passing overall number, while the forklift driver bounces through that one bay every day.

Local values are commonly set at about 60% to 67% of the overall value. ACI’s own tables follow roughly that ratio. A spec of FF35/FL25 overall would typically carry FF24/FL17 as the minimum local.

Two details often get missed:

• Define the section. The spec needs to say what one “minimum local area” is. It is usually bounded by column lines, construction joints or a set maximum area. If you leave it vague, you will argue about it at testing.
• Overall values aren’t a simple average. ASTM E1155 combines sections using a weighted formula that gives low-scoring sections more weight. One poor section pulls the overall number down more than you might expect.

FF/FL vs the 10-ft straightedge: approximate conversion

Owners, flooring installers and older specs still talk in straightedge terms. Many flooring manufacturers state their substrate requirements as a maximum gap under a 10-ft or 6-ft straightedge. So you will often need to translate. There is no exact conversion: the two methods measure different things, and a straightedge says nothing about levelness. The table below is a widely quoted approximation for FF only.

Approximate FF	Gap under an unleveled 10-ft straightedge	Metric (approx.)
FF12	1/2 in.	13 mm
FF20	5/16 in.	8 mm
FF25	1/4 in.	6 mm
FF32	3/16 in.	5 mm
FF50	1/8 in.	3 mm

Some practical points when the two specs meet:

• The classic “1/8 in. in 10 ft” is a demanding floor. It lines up with about FF50, which is well past what a typical office slab achieves. If a flooring spec asks for it, raise it early. You may need a self-leveling underlayment rather than trying to finish the slab to that standard.
• Don’t write both into the same contract without saying which one governs. A floor can pass one and fail the other.
• ACI 117 still includes a 10-ft straightedge method as an alternative. If your spec uses it, it needs to state the sampling and the percentage of measurements that must comply, just as an F-number spec does.

Random vs defined traffic floors, and FMin for warehouses

FF and FL assume random traffic: people, carts and forklifts can go anywhere, so every direction matters equally. Most floors are random traffic.

Defined-traffic floors are different. In a very-narrow-aisle (VNA) warehouse, guided turret trucks run in the same wheel tracks every time, often lifting loads 30 ft or more. A small difference in elevation between the left and right wheels can become several inches of mast sway at the top. Random-traffic F-numbers don’t capture that well. A floor can average FF60 and still have one bad dip in a wheel track.

Defined-traffic floors are specified and tested along the actual wheel paths:

• F_Min is a minimum F-number applied to the wheel paths. It is checked continuously along each aisle rather than estimated from random sample lines. Higher values mean tighter tolerances. Values in the F_Min 50 to 100+ range are commonly seen on VNA projects, rising with lift height.
• Property-based limits set maximum elevation differences across the aisle (between the wheel tracks) and along it (front to rear axle, and over set distances). ASTM E1486 provides a test method that uses waviness, wheel-path and levelness criteria.
• Testing uses a continuous-recording profilograph set to the truck’s wheel spacing. It is run down each aisle and reports the exact location of any non-compliant spot.

Where to get the numbers: the lift truck manufacturer and the rack designer publish floor tolerance requirements for their equipment and lift heights. Get those during design, not after the slab is poured. Many VNA floors are placed to a high random-traffic spec and then have the wheel paths ground to meet the truck tolerances once racking layouts are fixed. Allow for that in the budget and schedule. (On international projects you may see other systems, such as the UK’s TR34 classes or DIN 15185 in Europe. The principle is the same.)

How FF/FL is measured (ASTM E1155)

ASTM E1155 describes how to lay out, collect and reduce the readings. Your testing agency runs it, but the superintendent should know enough to check it was done right.

• Timing. Measure as soon as practical after final finishing. Specs commonly require results within 24 hours so problems reach the finishing crew before the next pour. ACI 117 sets an outer limit of 72 hours. Elevated slabs must be measured before shores are removed. Later measurements pick up curling and other movement, which is a separate issue from finishing quality.
• Sample lines. The tester lays out straight measurement lines across each test section, following the standard’s rules for minimum number and spacing of readings. Lines are usually kept clear of joints, slab edges, blockouts and penetrations, typically by about 2 ft. They are often run in two directions or diagonally to the pour strips, so the test sees the floor both along and across the strips.
• 12-in. intervals. Elevation readings are taken every 12 in. (300 mm) along each line. Consecutive readings give the slopes, and pairs of slopes give the q values for FF. Readings 10 ft apart give the z values for FL.
• Equipment. The standard allows any apparatus that records elevation differences at the required spacing and precision. That includes walking inclinometer-type profilers, rolling profilers and precise optical or laser levels.
• Reporting. Each section gets its own FF and FL, and these are combined into the overall values. The report should show line locations, section boundaries and pass/fail against both the overall and local values.

Build testing into the concrete pouring permit or pre-pour sign-off. That way the agency, access and timing are booked before the trucks are.

Worked example: calculating FF and FL in plain numbers

Here is a short, made-up sample line so you can see how the math works. Real testing needs far more readings than this to be statistically valid. Treat it as a teaching example only.

Step 1: Take readings. A profiler records relative elevation every 12 in. along a line. Twelve readings, in inches:

Point	1	2	3	4	5	6	7	8	9	10	11	12
Elevation (in.)	0.00	0.06	0.10	0.08	0.04	0.06	0.12	0.14	0.10	0.08	0.12	0.16

Step 2: Work out the 12-in. slopes. Subtract each reading from the next: +0.06, +0.04, −0.02, −0.04, +0.02, +0.06, +0.02, −0.04, −0.02, +0.04, +0.04.

Step 3: Work out q. q is the change between consecutive slopes, which is the curvature over each 24-in. span: −0.02, −0.06, −0.02, +0.06, +0.04, −0.04, −0.06, +0.02, +0.06, 0.00.

Step 4: Run the statistics. The mean of q is −0.002 in., so |mean q| = 0.002. The standard deviation of q (Sq) is about 0.046 in.

Step 5: Calculate FF. FF = 4.57 / (3 × 0.046 + 0.002) = 4.57 / 0.139 ≈ FF33.

Step 6: FL from a longer line. FL needs readings 10 ft apart, so it takes a longer line. Say the same test produced eight z values (elevation change over 10 ft), in inches: +0.20, −0.10, +0.25, +0.05, −0.15, +0.20, +0.10, 0.00. The mean z is about 0.069 in. and Sz is about 0.146 in. FL = 12.5 / (3 × 0.146 + 0.069) = 12.5 / 0.507 ≈ FL25.

What it means: against a “flat” spec of FF35/FL25 overall, this sample meets levelness but falls short on flatness. That points to finishing (bull floating and restraightening) rather than forms or screeding. Against the minimum local values of FF24/FL17, the section passes and wouldn’t need repair. This kind of diagnosis is why fast results matter: the finisher can adjust the very next pour.

What affects F-numbers on site

In general, FL is set by forms and strike-off, and FF is set by finishing. Here is what moves the numbers in practice.

Placement method and strike-off

• Forms, rails and wet screeds. FL can’t be better than the elevations the screed rides on. Shoot every form and screed rail with a level before the pour, and check them again after rebar and other trades have walked on them. For more on getting forms right, see our guide to formwork.
• Laser screeds strike off to a laser reference instead of forms. They typically raise FL well above hand screeding and allow large block pours. They don’t replace good finishing for FF.
• Strip width. Narrow strips between accurately set forms give the best results and are standard for superflat work. Wide block pours are faster but harder to keep level at the far edges.
• Subgrade and base tolerance. An uneven base gives an uneven slab thickness. Thickness changes alter set time and shrinkage across the slab, which shows up in both numbers.

Finishing sequence

• Bull floating and restraightening with a 10-ft highway straightedge, cutting high spots and filling lows, is the biggest single influence on FF. Specs for flat floors and above usually expect several restraightening passes, including after the first power float pass.
• Timing. Starting too early tears the surface. Starting too late means you can no longer correct anything. Inconsistent set times between trucks (from slump changes, temperature or admixture dosing) leave some areas ready while others aren’t, and FF suffers.
• Pans before blades. Power trowels with float pans flatten the surface. Blades alone tend to follow existing waves.
• Mix consistency. Keep slump and set time consistent load to load. Batching and placement records, such as a concrete pour request form, help you trace a bad zone back to a particular truck or delay.

Shrinkage and curling

Concrete shrinks as it dries, and the top dries faster than the bottom. Slab edges and joint edges curl upward, and FF and FL measured months later can be noticeably worse than the day-one results. That is the reason specs require early testing: it measures workmanship, not long-term movement. Joint spacing, sawcut timing, mix water content, curing and the vapor retarder layout all affect curling. Our saw cutting and coring guide covers joint cutting, and in cold weather the extended set times in our cold-weather concrete guide also affect finishing windows.

Elevated slabs and shoring deflection

On elevated slabs, the deck, beams and slab deflect under the weight of wet concrete and again when shores are removed. The finisher can’t control that. For this reason ACI 117 doesn’t apply FL to unshored elevated slabs, and they are normally specified with FF only. On shored slabs, FL can be specified, but it must be measured before the shores come out. Talk to the structural engineer about whether the slab will be placed to a constant thickness (following the deflected deck) or to a level elevation (adding concrete at midspan). The choice affects FL, the concrete quantity and the dead load.

Concrete slab finish types

F-numbers describe the shape of the surface. The finish describes its texture and appearance. You need to specify both. Here are the common slab finishes and where F-numbers come in.

Finish	How it’s made	Typical uses	Notes for flatness and spec
Float	Bull float, then hand or power float (pans). Leaves a slightly gritty, open texture	Under thick-set tile or toppings, exterior slabs that will get another treatment, some industrial areas	Floating is where FF is mostly won. A float finish can still meet a high FF/FL spec
Steel trowel (smooth)	One or more power or hand trowel passes after floating. Dense, smooth surface	Most interior slabs, under resilient flooring, carpet, coatings	The standard interior finish. Specify the number of passes or the required sheen if it matters
Hard-trowel / burnished	Repeated high-speed troweling until the surface is very dense and glossy	Warehouses, exposed industrial floors, some showrooms	ACI 302.1R warns against hard-troweling air-entrained concrete (above about 3% air) because of blistering and delamination risk. Watch for trowel burn
Broom	Broom dragged across a floated or lightly troweled surface	Exterior slabs, sidewalks, ramps, loading aprons, wash-down areas	Slip resistance is the priority. F-numbers are rarely specified, but drainage slope and FL to drains matter
Exposed aggregate	Surface paste removed (retarder and washing, or abrasive blast) to show aggregate	Plazas, walkways, architectural flatwork	Needs a mockup for color and depth of exposure. Aggregate selection and placement uniformity matter more than F-numbers
Polished	Ground with progressively finer diamonds, densified and polished	Retail, offices, schools, warehouses with exposed floors	Grinding shows any unevenness. Low spots stay dull and high spots expose more aggregate. Specify FF/FL and the polish level together
Stamped / textured	Imprinting tools or texture mats pressed into plastic concrete	Patios, decorative flatwork	Decorative. Tolerances are usually set by the mockup rather than F-numbers

Finish problems such as scaling, dusting, blisters, crazing and delamination are a separate topic from F-numbers. The Concrete Slab Surface Defects white paper explains their causes, prevention and repair.

What to do when a floor fails: grinding, toppings and replacement

First, check the test itself. Were section boundaries, line locations and timing per the spec? Were joints and edges excluded correctly? Was anything tested after shoring removal? If the results are valid, agree the fix in writing with the designer of record before anyone starts work. The spec should already say which remedies are allowed.

Problem	Common remedy	Watch out for
High spots, poor FF in local areas	Diamond grinding of the high spots (laser-guided grinders on larger areas)	Grinding removes the troweled surface and can expose aggregate, which changes appearance. Check remaining cover to reinforcement and slab thickness. Re-test afterward
Low areas, poor FL, or flooring requirements tighter than the slab	Cementitious self-leveling underlayment or patching compound	Surface prep and primer per manufacturer. Minimum and maximum thickness. Compatibility with the floor covering and moisture limits
Widespread out-of-tolerance area	Bonded topping, after scarifying or shot blasting the base slab	Added height at doors, elevators and transitions. Added dead load on elevated slabs (get the engineer’s approval)
Curled joints in service	Grinding joint edges. In some cases, stabilization by injecting grout under the slab	Curling may continue as the slab dries. Address joint filler and load transfer at the same time
VNA wheel paths out of tolerance	Precision grinding of wheel tracks after racking layout is fixed	Must be done to the truck manufacturer’s tolerances and re-profiled with the defined-traffic profilograph
Severe, structural or repeated failure	Remove and replace the section	Cost, schedule and joint detailing at the new edges. Usually the last resort

If you are adding a topping, a written request for concrete topping documents the location, thickness, mix and approval. The cementitious self-leveling underlayment white paper covers substrate prep for resilient and wood flooring.

How to write an FF/FL specification

Floor tolerances usually sit in CSI MasterFormat Section 03 30 00 (Cast-in-Place Concrete) or 03 35 00 (Concrete Finishing). A good specification:

• States performance, not method. Give the required F-numbers and leave means and methods to the contractor. If you specify both, you have muddied who is responsible when the floor fails.
• Gives overall and minimum local values for FF and FL, separately for each floor area or class where they differ (for example, office areas FF25/FL20, warehouse FF35/FL25).
• Defines the test section (column bays, pour strips or a maximum area) and what is excluded (for example, areas near joints, blockouts, drains and sloped areas).
• Names the test method and timing: ASTM E1155, results within a set time after finishing, and before shores are removed on elevated slabs.
• Drops FL on unshored elevated slabs and addresses deflection with the structural engineer instead.
• Handles defined-traffic aisles separately with wheel-path criteria from the equipment supplier.
• Says who tests and who pays for re-tests, and lists acceptable remedies and the approval process.
• Doesn’t overspecify. Each step up in flatness costs money in labor, equipment and sometimes strip pours. A good way to set realistic numbers is to measure an existing floor that already works well for the same use.

An illustrative spec paragraph (adapt it with your designer; don’t copy it into a contract as-is):

Floor tolerances, slabs-on-ground, warehouse areas: Specified overall values FF35 / FL25. Minimum local values FF24 / FL17. A minimum local area is the area bounded by column lines and construction joints. Measure per ASTM E1155 by an independent testing agency and report results within 24 hours of final finishing. Exclude areas within 2 ft of construction joints, slab edges and penetrations. Remedy sections below minimum local values by grinding or by approved underlayment, subject to the Architect’s review. Defined-traffic aisles: see separate wheel-path tolerance requirements.

Pre-pour coordination for an F-number floor

ACI 302.1R recommends a preconstruction meeting for floor work, and on any flat or better floor it’s worth the hour. Bring the owner’s representative, designer, GC, concrete contractor, finisher, ready-mix supplier, pump operator and testing agency. Cover these items, then carry them into your concrete pre-pour checklist:

1. Confirm the F-number requirements for each area, the section definitions and the exclusions. Make sure everyone has the same spec revision.
2. Confirm floor covering requirements (tile, vinyl plank, wood, coatings, polished concrete) and resolve any straightedge-based manufacturer tolerances.
3. Agree the pour layout (strips or blocks), joint layout, sawcut timing and pour sequence.
4. Set the strike-off method (laser screed, wet screeds, rails) and the survey check of forms and rails before the pour.
5. Agree the mix design, target slump and set time, and the admixture plan, including what happens in hot or cold weather.
6. Book the testing agency: equipment, timing (within 24 hours), access and who receives the results.
7. Agree the curing method and protection from early traffic.
8. Agree acceptance, remedies and re-testing in writing, before anyone has a reason to argue about them.
9. For important floors, pour a test panel or first strip and test it before committing to the full sequence.

Track the floor in your general quality program too. Our guide to controlling quality on the jobsite and the Construction Project Quality Checklist (250+ items) cover the inspection and hold-point side.

Common mistakes with FF/FL floors

• Overspecifying. Asking for FF50/FL50 throughout a building that needs FF25/FL20 in most areas adds cost with little benefit.
• Specifying FL on unshored elevated slabs. The finisher can’t control deflection, and the requirement can’t be enforced fairly.
• Testing too late. Measuring after curling or shore removal turns a workmanship test into an argument about structure and shrinkage.
• No minimum local values, or local values without a defined section size.
• Mixing straightedge and F-number specs without saying which governs.
• Writing “flat floor per ACI” without numbers. Category names differ between documents and editions.
• Assuming high FF/FL covers VNA traffic. Defined-traffic aisles need wheel-path tolerances.
• Ignoring the flooring installer. Their substrate tolerance may be tighter than the slab spec. Find out at submittals, not at installation.
• Poor rail and form checks. FL problems almost always start with elevations that weren’t checked again after other trades walked the deck.
• No agreed remedy. Without one, a failing floor turns into a dispute.

FAQ: floor flatness and levelness

What do FF and FL stand for in construction?

FF is the Floor Flatness number and FL is the Floor Levelness number. Both come from ASTM E1155 and are used in ACI 117 and ACI 302.1R. FF measures waviness over short distances (12 to 24 in.). FL measures how well the floor follows its design elevation over 10 ft. Neither has units, and higher numbers mean better floors.

What is a good FF/FL for a warehouse?

Warehouses with random forklift traffic are commonly specified around FF35/FL25 for moderate traffic and FF45/FL35 or higher for high-bay racking. Very-narrow-aisle warehouses with guided trucks need wheel-path tolerances (such as F_Min) from the truck manufacturer in addition to FF/FL. Confirm the numbers with your designer and equipment supplier.

What do FF35 and FF50 mean?

FF35 is a “flat” floor in ACI 302.1R terms, usually paired with FL25. It suits thin-set tile, resilient flooring and many warehouses, and roughly equals a 3/16 in. gap under a 10-ft straightedge. FF50 is about where superflat floors begin, roughly a 1/8 in. gap, and usually needs laser screeds or strip pours with a specialist finisher. For comparison, FF20/FL15 is a conventional floor.

What units is z in the FL formula?

Inches, for the inch-pound formula FL = 12.5 / (3·Sz + |mean z|). The same goes for q in the FF formula. If your readings are in decimal feet, convert them to inches first. ASTM E1155 also gives SI versions of the constants for millimetre readings.

How do I convert FF to a straightedge tolerance?

There is no exact conversion, but a widely used approximation is FF20 ≈ 5/16 in., FF25 ≈ 1/4 in., FF32 ≈ 3/16 in. and FF50 ≈ 1/8 in. under a 10-ft straightedge. It applies to flatness only. A straightedge doesn’t measure levelness.

Can you specify FL on an elevated slab?

Not on unshored elevated slabs. Deflection of the structure under the concrete’s weight controls levelness, and ACI 117 doesn’t apply FL there. On shored slabs, FL can be specified but must be measured before shores are removed. FF can be specified on both.

When should FF/FL testing be done?

As soon as practical after final finishing. Specs commonly require results within 24 hours so the crew can adjust on the next pour. ACI 117 sets an outer limit of 72 hours, and elevated slabs must be tested before shore removal.

What is the difference between FF/FL and FMin?

FF/FL statistically sample a random-traffic floor in every direction. F_Min applies to defined-traffic floors, such as VNA aisles, and is checked continuously along the actual wheel paths with a profilograph set to the truck’s wheel spacing. It catches individual bumps and dips that an average would hide.

Is there an FF/FL calculator?

Testing equipment and agency software calculate F-numbers automatically from ASTM E1155 readings. You can also set up a spreadsheet using the formulas and steps in the worked example above. That’s useful for checking a report or for training, but use the agency’s certified results for acceptance.

What are F1, F2 or F3 concrete finishes? Are they F-numbers?

No. In some British and Commonwealth specifications, F-classes (F1, F2, F3 and so on) and U-classes are surface-finish classes for formed and unformed concrete, describing appearance. FM and DM classes in the UK’s TR34 are warehouse floor categories. None of these are ASTM E1155 F-numbers, so check which system your spec uses.

Summary

FF measures how bumpy a floor is, and FL measures how level it is. Both are calculated from 12-in. readings under ASTM E1155. Specify overall and minimum local values that suit how the floor will be used. Leave FL off unshored elevated slabs, and give VNA aisles their own wheel-path tolerances. Most floors are won or lost before the first truck arrives, in the forms, the screed elevations, the mix and the agreement on testing and remedies. Settle those at the pre-pour meeting, test within 24 hours, and the numbers will mostly take care of themselves.