The Plasticity Index (often abbreviated as PI) is a numerical measure of the plasticity of a soil. It corresponds to the range of moisture contents, expressed as percent water by dry weight of soil, within which the soil has plastic properties.

The Plasticity Index is calculated by subtracting the Plastic Limit (PL) from the Liquid Limit (LL). i.e., PI = LL - PL. Soils with a high PI tend to be predominatly clay, those with a lower PI tend to be predominatly silt, and those with a PI of 0 tend to have little or no silt or clay.