The length gauge determines aggregate elongation index. Aggregate particles become elongated when their length exceeds 1.8 times their nominal size. It is made of a hardwood base, a brass plate, and steel pins.

The elongation index is calculated as the mass of all length gauge elongated aggregates divided by the entire sample size.

The aggregate to be identified is divided into six sieve fractions ranging from 6.3 to 50 mm, and each fraction is analyzed individually.