What FF and FL Numbers Actually Measure

FF measures local flatness: the bumpiness or waviness of a slab surface over short distances, roughly 2 to 12 feet. FL measures levelness: the slope or tilt of the floor over longer spans, capturing gradual dips or high spots that a short reading would miss. Both numbers come from the same test data set but answer different questions.

A high FF number tells a racking installer that beam-to-beam surface variation won’t rock a pallet or bind a shuttle wheel on a robotics floor. A high FL number tells a drainage engineer that a mechanical room floor won’t pond water at one end. You need both numbers, and they’re not interchangeable. A slab can post a strong FF and a mediocre FL if it’s locally smooth but sags across a 40-foot bay, and that combination is exactly the kind of thing a straightedge sweep in a punch-list walk misses entirely.

Every reference to “flatness” in this article means FF unless stated otherwise, and every reference to “levelness” means FL. Keep that distinction in mind as you move through the chart below, because specs frequently list both numbers together and contractors sometimes read only the first one.

The Standards Behind the Numbers: ASTM E1155, ACI 117, and ACI 302.1

Three documents govern how North American slabs get measured and judged. ASTM E1155 defines the measurement method, ACI 117 sets the tolerance backbone that predates the F-number system, and ACI 302.1 translates both into usage-based guidance for specific floor types. Understanding how they stack matters more than memorizing any single number.

ASTM E1155, “Standard Test Method for Determining FF Floor Flatness and FL Floor Levelness Numbers,” is the only internationally recognized protocol for quantifying floor surface characteristics through the Face Floor Profile Numbering System. It tells you how to collect elevation data and calculate F-numbers from it, but it doesn’t tell you what number to target for a given building.

ACI 117R, “Standard Tolerances for Concrete Construction and Materials,” is the older, dimension-based reference standard. It sets acceptable variation in elevation, alignment, thickness, and placement of embedded items using straightedge-style tolerances measured in fractions of an inch, not F-numbers. Many older specs, and some GC contracts today, still cite ACI 117 directly.

ACI 302.1R, “Guide for Concrete Floor and Slab Construction,” bridges the two. It recommends describing flatness and levelness using the F-number system per ASTM E1155 and provides usage-based FF/FL targets by floor type, which is the guidance most of the facility-specific numbers in this article trace back to.

Quick Reference Chart: Recommended FF/FL by Facility Type

This table is the fastest way to find a starting FF/FL target for a given facility type before consulting your project spec. It draws on ACI 302.1 usage guidance and IFTI field data, and applies to slabs measured within 72 hours of placement per ASTM E1155.

These numbers are starting points, not substitutes for your project’s actual spec. Facility type alone doesn’t determine the correct target; equipment manufacturer requirements, aisle width, and lift height all push the number up or down, which is why an independent measurement against the specified value, not a generic chart lookup, is what actually protects a warranty claim.

Performance Tiers: From Conventional to Super Flat

Floor flatness performance is commonly grouped into five tiers, from Conventional at FF 20 up to Super Flat at FF 60, each tied to a typical use case. This classification helps non-specialists locate where their project sits on a continuous scale rather than treating FF/FL as a pass/fail line.

The F-number scale is linear, so a floor at FF 45 is roughly twice as flat as one at FF 22.5 in relative terms. In practice, most measured slabs land somewhere between FF/FL 12 and 45; anything specified above 60 typically calls for special placement techniques (laser screeds, ride-on power trowels, tight pour sequencing) and should be flagged early in preconstruction, not discovered during acceptance testing.

Overall vs. Minimum Local Values, Why Both Matter

Every FF/FL spec actually contains two numbers: a Specified Overall Value (SOFF/SOFL), which is the average across the whole test section, and a Minimum Local Value (MLFF/MLFL), which is the floor for any single test section. A slab can average out fine and still fail the spec if one bad patch drags the local minimum below its floor.

A common rule of thumb, drawn from Allen Face standard specification methodology and echoed in the American Concrete Institute’s “Floor Flatness Report” (WTRF6), sets minimum local values at roughly 60% of the overall target. So a spec calling for Overall FF 35 might carry a Minimum Local FF around 24, not 35. A PDH Online course on the topic (S130) walks through exactly this pairing: Specified Overall Value FF 35 paired with Minimum Local Value FF 24.

The Allen Face formulas approximate this relationship using a target Grade (G): Overall FF (OAFF) is roughly equal to G, Overall FL (OAFL) is roughly 3G/5, Minimum Local FF (MLFF) is roughly 3G/5, and Minimum Local FL (MLFL) is roughly 9G/25. These aren’t exact substitutes for a project’s actual specified values, but they explain why a slab spec sheet should always list four numbers, not two.

Testing Timing and Methods: What Happens On-Site

ASTM E1155 testing happens within 72 hours of slab placement, with elevation readings taken at 10-foot intervals along test lines laid out for statistical significance. Equipment includes Dipstick floor profilers, F-meters, or 3D laser scanning systems, all calibrated before use and cross-checked against site conditions that could skew readings.

The 72-hour window exists because concrete continues to move slightly as it cures and dries, and testing outside that window introduces variables the standard doesn’t account for. That’s a hard scheduling constraint, not a suggestion: if the pour finishes on a Friday, testing needs to happen by Monday, which means the testing team has to be booked before the pour, not after someone notices the floor looks wavy.

Before the profiler ever touches the slab, a competent testing setup includes reviewing the spec to confirm target FF/FL, assessing site conditions (temperature, humidity, lighting, obstructions like column lines or embedded items), coordinating timing with the GC and concrete contractor, calibrating equipment, and laying out test lines per ASTM requirements. Skipping any one of these steps produces numbers that won’t hold up if a warranty dispute ever puts the test report in front of an engineer.

Cross-Checking with ACI 117 Straightedge Tolerances and Flooring-Specific Requirements

Many specs still reference ACI 117’s straightedge-based tolerances alongside or instead of F-numbers, and flooring installers frequently work to a tighter tolerance than the structural slab spec, particularly for resilient flooring under ASTM F710. Reconciling these systems prevents disputes between the concrete contractor and the flooring trade.

ACI 117 sets an elevation tolerance of ±3/8 in from specified elevation at any point, a local flatness limit of a maximum 3/8 in gap under a 10-ft straightedge anywhere on the surface, and a thickness tolerance of minus 3/8 in from design thickness. Industrial floors tested under ASTM E1155 are generally expected to hit a minimum FF 35 / FL 25 for conventional warehouse use, roughly the equivalent flatness level ACI 117’s straightedge method was designed to catch, just expressed as a statistical F-number instead of a single-point gap measurement.

ASTM F710-21 is the one most GCs underestimate. It requires concrete floors receiving resilient flooring to be flat “to within the equivalent of 3/16 in in 10 ft… and within the equivalent of 1/32 in in 12 in,” verifiable through ACI 117R methods, ASTM E1155, or other recognized methods. That’s substantially tighter than a typical structural slab tolerance, which is exactly why a slab that passes its structural FF/FL spec can still require a leveling pass before resilient flooring or coatings go down. If your project includes a finish floor system, check the flooring spec’s tolerance separately from the structural slab spec before you assume one covers the other.

Field Checklist for Superintendents and QC Teams

This checklist condenses the testing and coordination steps above into an on-site action list for superintendents managing pour-to-test timing on a slab job. Use it to confirm nothing gets skipped between placement and the 72-hour testing deadline.

• Confirm the spec lists both SOFF/SOFL and MLFF/MLFL, not just an overall number.
• Book third-party ASTM E1155 testing before the pour date, not after.
• Schedule testing to land inside the 72-hour post-placement window.
• Verify test line layout covers the full floor area with statistically significant sampling, not just a few convenience strips.
• Cross-check the structural slab spec against any resilient flooring or coating spec (ASTM F710) if a finish floor system is planned.
• Flag high-rack or robotics zones for tighter FF targets early, before formwork and placement sequencing lock in.
• Keep calibration records for Dipstick, F-meter, or laser scanning equipment with the test report.