Fineness Modulus (FM)

Fineness modulus (FM) is a single number, typically between about 2.3 and 3.1, that summarizes the average particle size of a fine aggregate (sand) based on a standard sieve analysis. A higher FM means coarser sand; a lower FM means finer sand.

ACI 211 mix-design tables use fineness modulus alongside the maximum aggregate size to select the correct coarse-aggregate volume factor for a given mix — getting this lookup wrong is a common source of mix-design calculator errors.