# Plastic Index

Written by Jerry Ratzlaff on . Posted in Geotechnical Engineering

Plastic index, abbreviated as PI, also called plasticity index, a dimensionless number, is the water content at which the soil changes from semi-solid state to solid state.

## plastic Index formula

 $$\large{ PI = LL - PL }$$

### Where:

 Units English Metric $$\large{ PI }$$  =  plastic index $$\large{dimensionless}$$ $$\large{ LL }$$ = liquid limit $$\large{dimensionless}$$ $$\large{ PL }$$ = plastic limit $$\large{dimensionless}$$