Atterberg Limits Calculator: Plasticity Index and USCS Classification

The Atterberg limits, liquid limit and plastic limit, describe the water contents at which a fine-grained soil transitions between liquid, plastic, and semi-solid behavior. From these two simple lab tests come the plasticity index, the liquidity index, the consistency index, and the Unified Soil Classification System symbol that governs how the soil is expected to behave under load and with changing moisture.

The core indices

• Plasticity index, PI = LL minus PL, the range of water content over which the soil behaves plastically
• Liquidity index, LI = (w minus PL) / PI, comparing the natural water content w against the plastic and liquid limits
• Consistency index, CI = (LL minus w) / PI, the complement of LI
• Activity, A = PI / clay fraction, comparing plasticity against the percentage of clay-sized particles present

Classifying on the Casagrande chart

Plotting PI against LL on the Casagrande plasticity chart is the standard way to assign a Unified Soil Classification System group symbol to a fine-grained soil. The A-line, defined as PI = 0.73 times (LL minus 20), separates clays (above the line) from silts (below the line). The U-line, PI = 0.9 times (LL minus 8), marks the practical upper bound most natural soils fall below. A liquid limit of 50 splits each group further into low plasticity (L) and high plasticity (H) subgroups.

Worked example

Consider a clay sample with LL = 45%, PL = 22%, a natural water content of 30%, and a clay fraction of 35%.

1. PI = 45 minus 22 = 23
2. LI = (30 minus 22) / 23 = 0.35, a plastic state consistency
3. CI = (45 minus 30) / 23 = 0.65
4. Activity = 23 / 35 = 0.66, an inactive clay
5. A-line value at LL=45: 0.73 times (45 minus 20) = 18.25, and since PI (23) exceeds this, the point plots above the A-line
6. With LL below 50, the soil classifies as CL, a low plasticity clay

These are the exact numbers the calculator returns when you click Load Example, so you can confirm the tool matches a hand calculation before applying it to real lab data.

Why this matters

The USCS group symbol drives expected engineering behavior: CH clays are prone to shrink-swell and long-term consolidation settlement, CL clays are more workable but still compressible, and inactive versus active clay behavior affects how a soil responds to changes in moisture around foundations and slabs. Liquidity index also gives a quick sense of in-situ consistency, with values above 1 flagging soils that may behave almost like a liquid if disturbed.

One thing that catches people off guard the first time they run this test: two soils with nearly identical liquid limits can behave completely differently in the field if their plasticity index numbers are far apart. A high PI clay will hold moisture and swell in ways a low PI silt never will, even if their LL values look the same on paper. That is exactly why geotechnical reports lean so heavily on the plasticity chart instead of liquid limit alone when classifying a soil.

Related Calculator and Reference

For correcting field SPT blow counts before combining them with soil classification data, see the SPT N-Value Corrections Calculator. For background on the Unified Soil Classification System, see the USGS reference on soil classification.