Calculate plasticity index, implied liquid or plastic limit, liquidity and consistency indices, or a conditional inorganic fine-grained USCS chart symbol. Limits and water content use a dry-soil mass basis.

Limits and water content are mass of water per dry-soil mass; values can exceed 100%. PI alone does not classify soil. Presets are illustrative examples, not mineral or site specifications.

A preset overrides the limit fields; its PI is LL − PL. Choose measured limits to use your entries.

Related Calculators

Plasticity Index Formula

The following two example problems outline the steps and information needed to calculate the Plasticity Index.

PI = LL - PL

Variables:

  • PI is the Plasticity Index (%)
  • PL is the plastic limit (%)
  • LL is the liquid limit (%)

To calculate the plasticity index, subtract the plastic limit from the liquid limit. Report nonplastic (NP) if either laboratory limit cannot be determined or PL is greater than or equal to LL. PI alone cannot determine a USCS classification.

How to Calculate Plasticity Index?

The following steps outline how to calculate the Plasticity Index.

  1. First, determine the plastic limit. 
  2. Next, determine the liquid limit. 
  3. Next, gather the formula from above = PI = LL - PL.
  4. Finally, calculate the Plasticity Index.
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

plastic limit = 4

liquid limit = 6

PI = LL - PL = 6 − 4 = 2 percentage points (reported as 2%).