• F_{L} (levelness) defines the relative degree to which a surface is horizontal
Flatness relates to the bumpiness of the floor, while levelness describes the tilt or pitch of the slab. The higher the FNumber, the better that characteristic of the floor.
FNumbers are linear, so an FF 20 is twice as flat as an FF 10, but only half as flat as an FF 40. FF/FL tolerances only apply to floors that support random traffic, be it vehicular or pedestrian traffic.
In the tiny percentage of floors that have defined traffic, where vehicles are restricted in their movement by wire or rail guidance, a different FNumber  F_{min}  is used. Most Superflat floors should use the Fmin System, since most of these slabs support defined traffic.
Why use FNumbers?
FNumbers replace the familiar "3mm in 3 metre" type specs that had proven unreliable, immeasurable and unrealistic. Did "3mm in 3 metres" mean ± 3mm in 3 metres (a horizontal 6mm envelope which is 3 metres long)  or  did it mean ± 1.5mm in 3metres (a horizontal 3mm envelope which is 3 metres long)
Before FNumbers, floors were only "measured" long after the fact, when someone didn't like the floors' general appearance. That's when the straightedge was finally hauled out by the aggrieved party in an effort to prove his case. Of course, no two people got the same results, since there was no standard for either the test method or for interpreting the results.
Although "3mm in 3 metres" has been used to specify millions of square metres of concrete, it was seldom, if ever, achieved. The typical industrial floor, for example, is closer to a horizontal 15mm by 3 metre envelope.
FNumbers control both the floor's "envelope" and its bumpiness. Or, if you think of the floor profile as a wave, FNumbers control both the wave's amplitude and its frequency. FNumbers have shown the ability to identify and to control floor characteristics, which are critical to the floor's usefulness.
Therefore, regardless of your location around the world, the "F Number" system provides specifiers with the ability to decide on the correct tolerance, provide contractors with a meaningful tolerance to construct and owners with a facility that works.
How are FNumbers are measured?
FNumbers are derived from a statistical analysis of the floor's elevation measured at 300mm intervals. The elevation differences over 600mm are used to determine F_{F}, while the differences over 3 metres are used to determine F_{L}. Basically, measurement lines are laid out on the floor, and elevation measurements are taken every 300mm down the line. Each measurement line should be at least 3.3 metres, and at least 34 individual elevation measurements should be taken for each 95m^{2} of floor area. Detailed rules for performing FNumber tests are set forth in ASTM E 1155.
After collection, the elevation readings are put into standard mathematical formulae to calculate the floor's FNumbers. Several devices are approved by ASTM for FNumber measurement, including the Dipstick Floor Profiler, of which, Bess Concrete are certified operators through the Face Company of Norfolk, Virginia. The table below details the different specifications for FNumber floor profiles.
Floor Profile Category 
Random Traffic Floors 
Defined
Traffic Floors 
Specific Overall Value 
Minimum Local Value 
F_{F} 
F_{L} 
F_{F} 
F_{L} 
F_{min} 
Conventional
(using bullfloat) 
19 
13 
13 
10 
19 
Conventional
(Using highway straightedge) 
25 
17 
13 
10 
25 
Good 
38 
25 
19 
13 
38 
Flat 
50 
33 
25 
17 
50 
Very Flat 
75 
50 
38 
25 
75 
Superflat 
100 
66 
50 
33 
100 
Ultraflat 
150 
100 
75 
50 
150 
