Calculate concrete floor flatness F-number (FF) from a series of elevation readings using the TR-34 FACE method. Enter 3 to 10 readings at 1-foot spacing to get FF number and pass/fail against FF25, FF35, and FF50 warehouse specifications.
Achieve FF50+ slabs with a Somero laser-guided screed or Topcon 3D-MC2 machine control paving system.
Shop Express Tools →The FF number is calculated from the second differences of consecutive 1-foot elevation readings. The second difference (q) at each point equals the change in slope between adjacent 1-foot intervals — essentially the local curvature of the floor. A perfectly flat floor has all second differences equal to zero and an infinite FF number. A wavy floor has large second differences and a low FF number.
High FF slabs require laser-guided screed or laser-level controlled strike-off to eliminate waviness during concrete placement. The concrete mix design, slump, and finisher technique all contribute to final FF. Key practices for high FF slabs include: placing concrete wet enough for easy screeding but not so wet that bleed water accumulates; using a laser-guided screed (Somero or similar) for FF50+ requirements; finishing with a riding trowel machine for large slab areas; and avoiding traffic on the fresh slab until it has achieved adequate strength.
FF testing should be performed within 72 hours of concrete placement when possible — before significant moisture loss can cause curling. Slab curling from differential shrinkage is one of the most common causes of FF failures on slabs that appeared flat at placement. Post-tensioned slabs and slabs on deep fill may require additional attention to subgrade preparation and uniform support.
ASTM E1155 is the standard test method for determining FF and FL floor flatness and levelness numbers. The standard specifies the measurement procedure using a certified electronic dipstick profileograph, the number of measurements required per floor area, and the statistical method for computing floor-wide F-numbers. A minimum of 200 measurements (first differences) is typically required for a statistically representative FF result, and specifications often require 500 to 1000 or more for large warehouse floors.
For formal construction acceptance and dispute resolution, ASTM E1155 certified testing by an accredited testing laboratory is required. This calculator uses the same TR-34 FACE formula as ASTM E1155 but is intended for field estimation and pre-pour setup checks rather than formal acceptance testing.
The FF (floor flatness) F-number is a statistical measure of concrete floor flatness developed by Allen Face and published as the FACE system (TR-34). FF measures local flatness — the local curvature of the floor surface — rather than overall levelness. Higher FF numbers indicate a flatter floor. The FF number is calculated from the second differences of elevation readings taken at 1-foot intervals. FF50 is required for very narrow aisle (VNA) forklift warehouses; FF25 is the minimum for most industrial applications.
FF (flatness) measures local curvature — bumps and dips that affect vehicle ride quality and wheel contact. FL (levelness) measures overall waviness — how much the floor tilts or rises across longer distances. FF is measured from second differences of 1-foot readings; FL is measured from first differences of 10-foot readings. VNA warehouse applications typically require both high FF and high FL. This calculator computes FF from short 1-foot reading sequences. For full FL measurement, use a certified dipstick profileograph per ASTM E1155.
FF requirements depend on the material handling equipment used. Very Narrow Aisle (VNA) forklifts require FF50 or higher and FL60 or higher — these floors require laser-guided screed or laser-level control during placement. Standard reach truck warehouses typically require FF35 to FF50. Light industrial and manufacturing floors typically require FF25 to FF35. Retail and commercial floors are often specified at FF20 to FF25. Check with the equipment manufacturer for the minimum FF/FL requirement of your specific forklift model.
Use a digital level or precision straightedge to take elevation readings at 1-foot intervals along a straight measurement line across the slab. Readings should be taken in inches or millimeters on a freshly finished slab (within 24 hours) or on a cured slab using a certified dipstick. For this calculator, enter readings in inches at 1-foot spacing. The calculator computes second differences (curvature values) and returns the FF number using the TR-34 FACE formula.
This calculator provides an estimate of FF using the TR-34 FACE formula from a small sample of readings. For formal construction acceptance, use a certified F-number profileograph (dipstick) per ASTM E1155 with a minimum number of readings specified by the contract — typically 500 to 1000 readings per 10,000 square feet. This tool is useful for pre-pour setup checks, screed calibration verification, and early quality control assessments, but is not a substitute for certified ASTM E1155 testing when contract compliance is required.
Enter elevation readings in inches at 1-foot intervals along a single straightedge pass (TR-34 FACE method). Minimum 3, maximum 10 readings.