What is the plasticity index?

The plasticity index (PI) is the range of water content over which a soil behaves plastically. It equals the liquid limit minus the plastic limit (PI = LL − PL). A higher PI means the soil can hold more water while remaining plastic, which typically indicates a more expansive, clay-dominated soil.

What is a non-plastic soil?

A soil is considered non-plastic when a plastic limit cannot be determined — either because no thread can be formed, or PI = 0. Coarse-grained soils such as clean sands and gravels are typically non-plastic.

What does the A-line mean in USCS?

The A-line (PI = 0.73 × (LL − 20)) on the Casagrande plasticity chart separates clays (above the line) from silts and organic soils (below the line). Points above the A-line with LL ≥ 50 are classified CH; below are MH or OH.

What is the liquidity index?

The liquidity index (LI) is LI = (w − PL) / PI, where w is the in-situ water content. LI = 0 means the soil is at its plastic limit (stiff); LI = 1 means it is at its liquid limit (very soft). Negative values indicate a semi-solid or solid state.

How does PI affect construction?

High-PI clays swell when wet and shrink when dry, causing differential settlement in foundations and pavement heave. AASHTO limits PI to ≤ 6 for granular base course materials. Subgrade soils with PI > 20 often require lime or cement stabilization before paving.

Can LL be less than PL?

No. By definition LL ≥ PL because liquid limit represents a wetter state than plastic limit. If your lab results show LL < PL, there is a measurement or recording error that needs to be corrected.

Plasticity Index Calculator

Name: Plasticity Index Calculator
Availability: InStock
Author: Nham Vu

Enter liquid limit (LL) and plastic limit (PL) to compute plasticity index (PI = LL − PL) and get USCS/AASHTO plasticity classification.

Atterberg Limits

Liquid Limit (LL) %

Moisture content at liquid–plastic transition

Plastic Limit (PL) %

Moisture content at plastic–semi-solid transition

Natural Water Content (w) % — optional

Required for liquidity index (LI)

Enter LL and PL values, then click Calculate.

Plasticity Index

PI = LL − PL

Liquid Limit (LL)

Plastic Limit (PL)

USCS Plasticity Chart (A-Line)

A-line PI at LL = %

PI_A =

Plasticity Classification Reference

PI Range	Classification	Typical Soil
0	Non-plastic	Clean sand, gravel
1–5	Slightly plastic	Sandy silt
6–10	Low plasticity	Silt, silty clay
11–20	Medium plasticity	Clay of medium plasticity
21–40	High plasticity	Fat clay (CH)
> 40	Very high plasticity	Highly expansive clay

Summary

Enter liquid limit (LL) and plastic limit (PL) to compute plasticity index (PI = LL − PL) and get USCS/AASHTO plasticity classification.

How it works

Enter the liquid limit (LL) — the moisture content at which soil transitions from plastic to liquid behavior.
Enter the plastic limit (PL) — the moisture content at which soil transitions from semi-solid to plastic behavior.
The plasticity index is computed as PI = LL − PL.
The result is classified by plasticity (non-plastic to very high) based on PI value.
The USCS A-line equation (PI = 0.73 × (LL − 20)) is evaluated to guide CL vs. ML / CH vs. MH placement.
Liquidity index is computed if natural water content (w) is provided.

Use cases

Geotechnical site investigation and soil classification reports.
Foundation design — identifying swelling or shrinkage risk in fine-grained soils.
Pavement subgrade assessment using AASHTO plasticity criteria.
Embankment and earthwork design with fine-grained fill materials.
Educational exercises on Atterberg limits and USCS classification.
Quick field check of lab results from Casagrande cup and thread-rolling tests.

Frequently Asked Questions

Last updated: 2026-07-22 · Reviewed by Nham Vu