Atterberg Limits Calculator

Turn liquid and plastic limit results into the indices engineers actually use: plasticity index, position relative to the Casagrande A-line, the USCS fines symbol (CL, CH, ML, MH), and — with a natural water content — the liquidity index and in-situ consistency.

Atterberg Limit Values

Liquid Limit LL

Plastic Limit PL

Natural Water Content w (optional)

Enter liquid and plastic limits to compute plasticity indices.

The indices

PI = LL − PL · LI = (w − PL) ÷ PI · A-line: PI = 0.73·(LL − 20)

PI measures the width of the plastic range — how much water the soil can take while still behaving plastically. On the plasticity chart, soils plotting above the A-line are clays (C), below are silts (M); a liquid limit of 50 separates low (L) from high (H) plasticity, giving the four fines symbols. PI values of 4–7 just above the A-line fall in the hatched CL-ML zone.

Reading the liquidity index

LI locates the natural water content within the plastic range: below 0 means the soil is drier than its plastic limit (semi-solid, brittle); 0–1 is the plastic range; above 1 means the soil is wetter than its liquid limit — a flag for very soft, potentially sensitive material that can lose strength dramatically when disturbed.

Frequently Asked Questions

What does a high plasticity index mean?

A wide plastic range — more clay minerals and more water-sensitivity. High-PI soils shrink and swell with moisture changes, which is why PI limits appear in pavement subgrade and fill specifications.

What is the A-line?

The empirical boundary PI = 0.73(LL − 20) on the Casagrande plasticity chart separating clays (above) from silts and organic soils (below).

What is a normal liquidity index?

Stable natural deposits usually sit between 0 and 1. LI near or above 1 indicates a very soft, possibly sensitive clay; below 0 indicates a stiff, desiccated soil.