Soil Plasticity Index Calculator

Calculate the plasticity index (PI) of your soil samples with precision using our free online tool. Simply enter the liquid limit (LL) and plastic limit (PL) values to determine the PI and classify your soil type according to standard geotechnical engineering practices.

Liquid Limit (LL) (%)

Plastic Limit (PL) (%)

Calculation Result

0.0

Enter values to calculate

About Plasticity Index

The Plasticity Index (PI) is calculated using the formula:

PI = LL – PL

Where:

LL = Liquid Limit (%)
PL = Plastic Limit (%)

What is Plasticity Index?

The Plasticity Index (PI) is a measure of the plasticity of soil. It represents the range of water content over which soil remains in a plastic state, calculated as the difference between the liquid limit and plastic limit.

Soil Classification

Based on PI values:

0-15: Low plasticity
15-30: Medium plasticity
30+: High plasticity

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Frequently Asked Questions

What is the Plasticity Index (PI) used for?

The Plasticity Index is a crucial parameter in geotechnical engineering for soil classification (USCS and AASHTO systems). It helps engineers understand soil behavior, predict how soil will react to moisture changes, and determine its suitability for construction projects.

How do I determine the liquid limit and plastic limit?

The liquid limit (LL) is determined using the Casagrande apparatus or cone penetrometer test, measuring the water content at which soil transitions from plastic to liquid state. The plastic limit (PL) is found by rolling soil into threads and determining the water content when it just begins to crumble.

What does a high PI value indicate?

A high Plasticity Index indicates soil with significant clay content that exhibits high volume change characteristics when moisture content changes. Such soils require special engineering considerations due to their expansive nature and potential to cause structural damage.