How to Figure Construction Grades

Subtract the elevation in question from the benchmark's elevation. Ignoring the decimal value, the difference is the number of feet below the benchmark. Negative values indicate elevations above the benchmark. For example, given a benchmark at 100.00 and a grade elevation at 101.43, the difference is -1.43, which indicates that the elevation is 1 foot and some odd inches above the benchmark.

Multiply the decimal part of the difference times 12 to find the number of inches. For example, 0.43 x 12 = 5.16. Ignoring the decimal part for now, that indicates that the elevation is 1 foot 5 inches above the benchmark.

Multiply the decimal part of the inches value times 8 to find the number of 1/8 inches. You could multiply by 16 to find 1/16s, but that much accuracy is unnecessary for building elevations. For example, 0.16 x 8 = 1.28. Round that off to 1, giving 1 foot 5 1/8 inches for the final elevation.