F numbers (USA Face Numbers FF, FL ASTM, E1155 (metric), standard)
First used on a large-scale commercial project in 1983, the F-number system for measuring floor flatness (FF,) and levelness (FL), then had been perhaps the most significant factor in enabling the construction of flatter and more level (horizontal) concrete floors. However, because the dimensionless F-numbers are derived by processing the data from a large number of measurements, it can be difficult to grasp exactly what it means.