What Are FF and FL Numbers?

FF and FL are two separate statistical measurements used to describe how flat and how level a concrete floor is. FF (Floor Flatness) measures short-range waviness across the surface. FL (Floor Levelness) measures longer-range elevation change and slope. Both come from ASTM E1155/E1155M and are referenced in ACI 117 for specification purposes.

Most people outside the concrete trade hear “F-numbers” and assume it’s a single flatness score, similar to a grade on a report card. It isn’t. A slab specification written as “FF 35 / FL 25” is really two independent targets that happen to travel together. A floor can post an excellent FF result and a mediocre FL result on the same test, or the reverse, and both outcomes are common enough that ASTM International built two distinct calculations rather than one composite number.

The distinction matters because FF and FL fail for different physical reasons and get fixed by different means. Waviness (low FF) usually traces back to finishing technique, screed control, or concrete workability during placement. Elevation drift (low FL) usually traces back to formwork setup, screed rail accuracy, or slab curl over a wider span. Specifying only one number, or treating both as interchangeable, is one of the more common and expensive mistakes in slab documents reviewed on real projects.

What FF (Floor Flatness) Measures

FF quantifies short-distance waviness on the floor surface. ASTM E1155/E1155M-23 calculates it from elevation readings taken at 12-inch intervals, evaluating curvature between points 24 inches apart. A higher FF number means less bumpiness at that scale, according to ASTM International and the American Concrete Institute’s Floor Flatness Report.

Think of FF as the measurement that catches trowel marks, screed lines, and the small undulations a forklift wheel or a robot’s caster feels every few feet. Because the calculation window is only 24 inches, FF is sensitive to local surface texture rather than the floor’s overall pitch. A floor could tilt gently from one end of a bay to the other and still register a strong FF number, because that kind of drift happens over a much longer distance than the 24-inch curvature window used in the calculation.

The Floor Covering Reference Manual notes that F-number scales are linear for comparison purposes: an FF 30 floor represents approximately twice the measured flatness of an FF 15 floor. That linearity is useful when comparing bids or reviewing test reports, since the numbers scale in a way a percentage or pass/fail grade does not.

What FL (Floor Levelness) Measures

FL quantifies elevation change over longer distances, capturing gradual slope or drift across a floor. ASTM E1155/E1155M-23 evaluates elevation differences over roughly 10-foot (3000 mm) intervals. Higher FL values indicate more consistent elevation across that span, per ASTM International and the Floor Covering Reference Manual’s “Floor Flatness and Levelness” reference.

Where FF is concerned with the bumps you feel every couple of feet, FL is concerned with whether the floor as a whole tilts, dips, or drifts over a longer run. This matters for anything that travels a fixed, extended path: conveyor lines, long racking rows, automated guided vehicles moving down an aisle. A floor with strong local smoothness (high FF) can still have a slow, cumulative slope that shows up only when you measure across 10 feet or more, which is exactly what FL is designed to catch.

FL results are also more forgiving of minor tooling texture and more sensitive to formwork and screed-rail setup during placement. A contractor chasing a high FL number is usually solving a setup and control problem at the time of placement, not a finishing problem afterward.

FF vs. FL: Side-by-Side Comparison

FF and FL differ in what they measure, the distance over which they’re calculated, and what a low score usually indicates about the slab. The table below summarizes the practical distinction specifiers and testers rely on most often.

Typical F-Number Classifications (Conventional to Superflat)

F-number classifications range from Conventional to Superflat, with each tier defined by a minimum FF and FL value. The American Society of Concrete Contractors publishes a widely referenced classification scale, reprinted below, that specifiers use as a starting point before adjusting for traffic type and use.

This table, sourced from the American Society of Concrete Contractors’ “F-numbers and Textured Concrete Surface Finishes” (ACI Concrete International, May 2022), is one of the most frequently referenced tables in flooring specifications because it gives architects and engineers a common vocabulary before the project-specific numbers get set. Somero’s F-Numbers Handbook for Floor Flatness explains how this scale ties back to ACI 117 and, for Canadian projects, CSA A23.1 requirements, which makes it a useful cross-check when a spec references multiple standards at once.

These classification tiers are a starting point, not a substitute for project-specific numbers. A distribution center running very-narrow-aisle racking or automated vehicles will typically need values calculated for its specific traffic pattern rather than a generic “Superflat” label, which brings up the next distinction.

Random-Traffic vs. Defined-Traffic Floors (FF/FL vs. F-Minimum)

Random-traffic floors, where vehicles and foot traffic move in unpredictable paths, are properly evaluated using overall FF and FL numbers. Defined-traffic floors, where vehicles repeatedly follow the same fixed wheel path (such as very-narrow-aisle warehouse racking), require F-minimum criteria instead, since overall FF/FL statistics can mask localized irregularities along that specific path, according to Dipstick’s Flat Floor Library.

This is where a lot of warehouse and distribution projects go wrong. A general warehouse floor, a manufacturing floor, or a parking or service area with mixed, unpredictable vehicle movement fits the random-traffic model, and standard overall FF/FL numbers describe it well. A very-narrow-aisle rack aisle, where a lift truck or an automated vehicle tracks the same few inches of concrete thousands of times a week, does not fit that model. The overall statistical FF/FL result for the whole slab can look perfectly acceptable while the specific wheel path inside one aisle has a localized dip or hump that never shows up in the aggregate number.

That gap is precisely why F-minimum values exist. F-minimum criteria evaluate the floor along the actual travel path rather than across the whole tested area, catching the kind of localized irregularity that affects mast stability, load handling, or automated guidance in high-bay and automation-dependent facilities. Conspectus’s “Floor Slab Flatness & Levelness” technical sheet frames this as a specification-stage decision architects and engineers need to make deliberately, not by default.

A short way to self-identify which path applies to a given project:

• If vehicles and foot traffic move unpredictably across the floor (general warehouse, manufacturing floor, open production area): random-traffic FF/FL specification typically applies.
• If vehicles repeatedly follow the same fixed path (very-narrow-aisle racking, AS/RS, guided robotics, conveyor-adjacent travel lanes): defined-traffic F-minimum criteria typically applies.
• If the project mixes both (open receiving areas feeding into narrow rack aisles): different zones of the same slab may need different specification criteria.

For high-bay warehouses, automated storage and retrieval systems, and robotic or AMR fleets, this distinction is not a technicality. Small elevation changes along a fixed path affect mast movement, vehicle stability, and automation performance in ways that overall FF/FL averages simply won’t reveal.

What a Complete Specification Should Include

A complete flatness and levelness specification defines more than two target numbers. It should also state whether values are overall, local, or both, the governing standard edition, the testing deadline after placement, sampling pattern, who is responsible for testing, and the acceptance or remediation procedure, consistent with ASTM E1155 and ACI 117 practice.

Two numbers alone leave too much room for dispute once a floor is poured. A specification that only says “FF 35 / FL 25” doesn’t say whether that’s an overall requirement, a local minimum, or both, and it doesn’t say what happens if the floor comes in below target. Overall values describe the tested area as a whole using statistical averaging; local minimum criteria prevent a single failing patch from hiding inside an otherwise passing average. Both matter, and a specification silent on the distinction invites disagreement between contractor and owner exactly when it’s most expensive to resolve.

At minimum, a specification worth building from should identify:

• Required FF and FL values (or F-minimum values, for defined-traffic zones)
• Whether those values are overall, local, or both
• The governing test standard and edition (ASTM E1155/E1155M-23, current at time of writing)
• The testing deadline relative to placement date
• Sampling pattern and test section layout across the slab
• Who is responsible for testing and reporting results
• The acceptance and remediation procedure if results fall short
• Random- vs. defined-traffic designation for each zone of the floor
• Any separate requirements sitting beneath floor finishes, coatings, or fixed equipment

Somero’s F-Numbers Handbook for Floor Flatness is a practical reference contractors, finishers, engineers, and inspectors can use to check that a specification actually connects back to ACI 117 and ASTM’s testing procedure rather than using F-numbers as a loose stand-in for “flat enough.”

Why Testing Timing Matters

Concrete floors continue to change after placement due to drying shrinkage, curling, joint behavior, temperature gradients, and construction loading. ASTM E1155/E1155M-23 and standard project-specification practice call for testing to occur within a defined window after placement and before later construction activity alters the surface, so results reflect the floor’s actual as-placed condition.

Test too early and the slab hasn’t finished settling into its final profile. Test too late, after racking installation, equipment staging, or heavy trade traffic has already crossed the surface, and the results may reflect damage or wear rather than the original placement quality, undermining any warranty or acceptance decision tied to that data. Contract documents should specify the testing window explicitly rather than leaving it to informal agreement between contractor and owner, and that window should be set before the pour, not negotiated afterward.

Quick-Reference Guide for Specifiers and Testers

The core reference points from this page: FF measures short-range waviness (24-inch curvature window), FL measures longer-range slope (10-foot intervals), classification tiers run from Conventional (FF 20/FL 15) to Superflat (FF 60/FL 40), and random-traffic floors use overall FF/FL while defined-traffic floors need F-minimum values along the travel path.

Keep the classification table and the random- vs. defined-traffic decision cue from earlier sections close at hand when drafting or reviewing a specification. Those two elements, paired with the specification checklist above, cover the majority of the questions that come up between design intent and slab acceptance on medium-to-large commercial projects.

Flatness and levelness aren’t the only slab characteristics that determine whether a floor performs as intended. Moisture condition, tested under ASTM F2170 (in-situ relative humidity) or ASTM F1869 (calcium chloride), affects flooring adhesives, coatings, and finish longevity in ways that are independent of FF/FL results. Projects that specify F-numbers carefully sometimes overlook moisture testing entirely, only to run into flooring failures unrelated to the slab’s physical flatness.