ACI 117, Standard Tolerances for Concrete Construction and Materials, gives tolerances for slabs up to 12 inches thick of +3/8 inch and -1/4 inch. For the past few years, I've been gathering data on thicknesses of concrete slabs, and I've found that most slabs don't conform to ACI 117. The ACI 117 tolerances of +3/8 inch and -1/4 inch are unrealistic for slabs on grade and probably were meant to apply to structural slabs cast on forms. Revised and more realistic tolerances are needed. Until new thickness tolerances are developed, designers and specifiers should be aware of realistic deviations that exist in thickness of floors on grade. To comply with the ACI tolerance, contractors would have to closely control fine grading of the subgrade or base course. Such extreme control may not be worth the cost in terms of improved slab performance. A realistic specification could read, "The average thickness of a floor slab shall be no less than the thickness shown on the drawings, and not more than 16% of the floor area shall be thinner than 3/8 inch less than that shown on the drawings."