Used to classify Aggregate particles as elongated when they have a length (greatest dimension) of more than 1.8 of their mean sieve size, this size being taken as the mean of the limiting sieve apertures used for determining the size fraction in which the particle occurs.

The Elongation Index is found by separating the elongated particles and expressing their mass as a percentage of the mass of sample tested.

The test cannot be used for material that passes through a 6.30 mm test sieve or is held back by a 63.0 mm test sieve.