Q.: What does a specification for a floor flatness of 1/8 inch in 10 feet equal in F-numbers?

A.: Although there are no direct equivalents between F-numbers and straightedge tolerances, ACI 302, "Construction of Concrete Floors and Slabs," gives the following table of approximate values:

F-number Gap under an unleveled 10-foot straightedge
(fraction of an inch)
FF12 1/2
FF20 5/16
FF25 1/4
FF32 3/16
FF50 1/8