Instantly calculate concrete floor flatness (FF) and floor levelness (FL) F-numbers, check compliance against ASTM E1155 and ACI 117 tolerances, and get remediation guidance — for warehouses, offices, retail, and superflat floors across the USA.
Enter your floor measurements and specified F-numbers to check ASTM E1155 compliance and get remediation guidance.
Sets default FF/FL specification — adjust below if needed
ASTM E1155 requires F-Meter dipstick for certified F-numbers
FF measures surface waviness over 24-in intervals — higher = flatter
FL measures overall slope over 10-ft spans — higher = more level
Measure largest gap under a 10-ft straightedge placed flat on floor
Difference in elevation between two points 24 inches apart
Total floor area being assessed — used for remediation cost estimate
ACI 117 requires F-number testing within 72 hrs of slab placement
The F-number system is the US industry standard for quantifying concrete floor flatness and levelness, defined by ASTM E1155 and referenced in ACI 117 (Standard Specification for Tolerances for Concrete Construction). FF (Floor Flatness) measures local surface waviness — how bumpy the floor is over short 24-inch intervals — while FL (Floor Levelness) measures overall tilt and slope across longer 10-foot spans. The higher the F-number, the better the floor quality. F-numbers replaced the old straightedge gap method because they provide a statistically meaningful, repeatable measurement of floor quality across the entire slab — not just the worst single spot.
Think of FF as "bumpiness" — a floor with a low FF has noticeable ripples and waves that you feel when driving a forklift or rolling a cart. Think of FL as "levelness" — a floor with a low FL has an overall tilt or slope that causes liquids to puddle in one direction and makes shelving racks appear uneven. A floor can have a high FF (smooth surface) but low FL (tilted) — or vice versa. Both numbers must meet specification for the floor to be accepted per ACI 302.1R and typical US construction contracts.
ASTM E1155 (Standard Test Method for Determining FF Floor Flatness and FL Floor Levelness Numbers) is the only ASTM-recognized method for generating certified F-numbers in the USA. It uses an electronic F-Meter (dipstick device) that measures differential elevations at 12-inch intervals along test lines across the slab. A minimum number of readings based on floor area is required — ACI 117 specifies at least 10 sample readings per 1,000 sq ft of slab.
ACI 117 requires that F-number testing be performed within 72 hours of slab placement — before the concrete has been loaded, marked, or subject to post-pour shrinkage. This is critical because shrinkage cracking and curling at joints can significantly change F-number readings after the first few days. Testing an existing floor after curling has occurred yields lower FL numbers than testing would have shown immediately after placement.
Modern distribution centers and high-rack warehouses using very narrow aisle (VNA) forklifts require exceptionally flat and level floors — typically FF 50–100 and FL 40–80 or higher. Even small floor irregularities cause forklift mast sway at height, rack damage, and safety hazards. Amazon, Walmart, and major US 3PL operators routinely specify superflat floors (FF 100+) for automated storage and retrieval systems (ASRS).
The ASTM E1155 F-number system is based on statistical analysis of elevation readings taken with a dipstick device at 12-inch intervals. The calculator on this page uses a simplified field estimation method based on maximum differential readings — suitable for preliminary assessments and field checks. For legally binding contract compliance, only certified ASTM E1155 testing with an approved F-Meter device is accepted.
The formulas above provide a field estimation only — they use maximum readings which are more conservative than the statistical method used in ASTM E1155 (which uses standard deviation of all readings). A certified ASTM E1155 test using an approved F-Meter dipstick device will typically yield higher F-numbers than this field estimate because it accounts for the statistical distribution of all readings, not just the single worst measurement. Always use certified ASTM E1155 testing for contract acceptance on US commercial and industrial floor projects.
The table below provides the standard ACI 117 and industry FF/FL F-number specifications for common concrete floor types in the United States. These are the minimum values required at the time of placement testing — before shrinkage, loading, or joint curling affects the floor.
| Floor Type / Use | Min. FF | Min. FL | 10-ft Straightedge Max Gap | Typical Application | Standard |
|---|---|---|---|---|---|
| Residential / Light Duty | 13 | 10 | 3/8 in | Garage slabs, basements, patios | ACI 302.1R |
| Office / Retail | 25 | 20 | 3/16 in | Office buildings, retail stores, schools | ACI 117 |
| General Warehouse | 35 | 25 | 1/8 in | General storage, manufacturing | ACI 117 |
| High-Rack / VNA Forklift | 50 | 40 | 3/32 in | Pallet rack aisles, reach trucks | ASTM E1155 |
| Superflat / Wire-Guided | 100+ | 60+ | 1/32 in | ASRS, very narrow aisle, robotics | ASTM E1155 |
| Defined Traffic Floor | FF N/A | FL N/A | Special test | Wire-guided VNA — special ACI test | ACI 117 §4.8 |
The table below converts FF and FL F-numbers to maximum allowable differential elevations — helping contractors and inspectors translate abstract F-numbers into physical measurements they can check with a straightedge or level on US job sites.
| F-Number | Max 24-in Differential (FF) | Max 10-ft Differential (FL) | 10-ft Straightedge Max Gap | Quality Level | Typical Floor |
|---|---|---|---|---|---|
| F 10 | 0.218 in (5.5 mm) | 0.360 in (9.1 mm) | 3/8 in | Low | Residential / rough |
| F 20 | 0.109 in (2.8 mm) | 0.180 in (4.6 mm) | 3/16 in | Below Avg | Light commercial |
| F 25 | 0.087 in (2.2 mm) | 0.144 in (3.7 mm) | 5/32 in | Average | Office / retail |
| F 35 | 0.062 in (1.6 mm) | 0.103 in (2.6 mm) | 1/8 in | Good | Warehouse |
| F 50 | 0.043 in (1.1 mm) | 0.072 in (1.8 mm) | 3/32 in | Very Good | High-rack / VNA |
| F 100 | 0.022 in (0.55 mm) | 0.036 in (0.9 mm) | 1/32 in | Superflat | ASRS / robotics |
Achieving specified FF and FL F-numbers on US concrete floor projects requires careful attention to six interconnected factors — from subgrade preparation to finishing technique. Understanding these helps contractors prevent costly remediation after the pour.
The single most important factor for FL (levelness). An uneven or poorly compacted subgrade directly transfers to the finished slab surface. ACI 302.1R requires subgrade compaction to 95% modified Proctor density (ASTM D1557) and a subgrade tolerance of ±3/4 inch for typical floors and ±3/8 inch for superflat floors before concrete placement on US projects.
Higher slump concrete (above 5 inches) is easier to strike off level but prone to more bleed water and settlement, which can cause low FF numbers. ACI 302.1R recommends a target slump of 4–5 inches maximum for flatwork (or equivalent slump flow with water reducers). Superflat floors typically use mid-range water reducers (ASTM C494 Type A) to achieve workability without excess water.
Laser screed equipment (Somero, Allen Engineering) consistently produces FF 35–50+ on US warehouse floors that manual screeding struggles to achieve above FF 25. The laser screed's automatic grade control and vibrating head produce a uniform strike-off elevation independent of operator skill. For superflat floors (FF 60+), laser screeds are essentially mandatory on US projects.
Ride-on power trowels (36–46 inch diameter) improve FF more than any other single finishing operation. The overlapping circular paths of a ride-on trowel average out surface irregularities created during screeding. Most US superflat floor contractors use multiple power trowel passes — typically 4–6 passes — with the final pass using float pans at low pitch angles to produce FF 50+ surfaces.
Differential evaporation across a large slab causes the concrete to set at different rates in different areas, making it nearly impossible to achieve uniform FF numbers. ACI 305R hot weather precautions (fogging, windbreaks, concrete temperature control below 95°F) are critical to achieving high F-numbers in summer pours across the US Sun Belt, Great Plains, and Southwest states.
Slab curling at construction joints and control joints causes FL numbers measured after 28+ days to be significantly lower than measurements taken within 72 hours of placement. ACI 117 requires testing within 72 hours specifically to capture FL before curling develops. On long-strip and large-panel pours, joint spacing and subgrade moisture uniformity are critical to minimizing post-cure FL degradation on US industrial floors.
Once a concrete floor has been subjected to construction traffic, rack loading, or post-cure shrinkage, F-numbers will be significantly lower than immediately after placement — especially FL. ACI 117 Section 4.8.3 is explicit: F-number compliance testing must be performed within 72 hours of slab placement on the completed fresh surface, before any traffic or loading. If you miss this window on a US construction project, the only recourse is grinding/diamond planing remediation and negotiating with the owner on acceptable post-cure F-numbers.
Authoritative references for concrete floor F-numbers, testing methods, and tolerance specifications across the United States.
The definitive US standard for determining FF Floor Flatness and FL Floor Levelness numbers using the F-Meter dipstick device. Covers test procedure, required sample sizes, statistical calculation methods, and reporting requirements for certified F-number testing on US commercial and industrial concrete floors.
View ASTM E1155ACI 117 Standard Specification for Tolerances for Concrete Construction covers the F-number system, testing timing requirements (72-hour window), minimum FF/FL specifications by floor type, and defined traffic floor testing. The primary reference for floor tolerance specifications on US construction contracts.
View ACI 117ACI 302.1R Guide to Concrete Floor and Slab Construction is the comprehensive US guide covering subgrade preparation, concrete mix design, placing, finishing, curing, and F-number achievement for all floor classes from residential to superflat. The essential reference for any US contractor specifying or building concrete floors to F-number requirements.
View ACI 302.1R