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Atterberg Limits Calculator

The plasticity index is simply PI = LL − PL, the moisture-content range over which a fine-grained soil behaves plastically.

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Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Enter the liquid limit (LL) and plastic limit (PL) as percentages from your Atterberg (Casagrande cup or fall-cone, and thread-rolling) test results.
  2. Optionally add the natural water content (w) to get liquidity and consistency indices, and the clay fraction (< 2 µm) to get Skempton's activity.
  3. Read off the plasticity index, the USCS plasticity-chart group (CL/CH/ML/MH), and the in-situ consistency, then verify the classification against your governing standard.

How it works

The plasticity index is simply PI = LL − PL, the moisture-content range over which a fine-grained soil behaves plastically. The tool then places the point (LL, PI) on the Casagrande plasticity chart: the A-line is defined by PI = 0.73 × (LL − 20). Points above the A-line are clays (C), points below are silts/organics (M/O), and the vertical LL = 50 line splits low plasticity (L) from high plasticity (H) — giving the USCS groups CL, CH, ML and MH, with a borderline CL-ML hatched zone at low PI.

When natural water content w is supplied, liquidity index LI = (w − PL) ÷ PI and consistency index CI = (LL − w) ÷ PI describe where the in-situ moisture sits within the plastic range (LI ≤ 0 is stiff/semi-solid, LI ≈ 1 is near-liquid). If the clay fraction (percentage finer than 2 µm) is entered, Skempton's activity A = PI ÷ clay-fraction indicates the clay mineralogy — inactive (< 0.75, kaolinite), normal (0.75–1.25, illite) or active (> 1.25, montmorillonite).

Worked example

Fat clay: LL 52, PL 24, natural w 30. A clayey soil tests at liquid limit LL = 52% and plastic limit PL = 24%. Plasticity index PI = 52 − 24 = 28%. The A-line at LL 52 sits at PI = 0.73 × (52 − 20) = 23.36%. Because PI (28) plots above the A-line and LL ≥ 50, the soil classifies as CH — a clay of high plasticity (fat clay). With natural water content w = 30%, liquidity index LI = (30 − 24) ÷ 28 = 0.214, so the soil is in a firm, plastic in-situ state.

Common mistakes

  • Swapping LL and PL. The plastic limit is always the lower moisture content; if PL > LL the test or the entry is wrong, and the tool returns an error rather than a negative PI.
  • Treating a below-A-line, high-LL soil as a clay. If PI plots below the A-line it is a silt (ML/MH) or organic soil, not a clay, regardless of how high the liquid limit is.
  • Forgetting that activity needs the clay-size fraction (< 2 µm), not the total fines passing 0.075 mm — using the wrong percentage badly skews the activity value.

Frequently asked questions

What is the difference between liquid limit and plastic limit?

The liquid limit (LL) is the water content at which a remoulded soil changes from a plastic to a near-liquid state (measured with the Casagrande cup or fall-cone). The plastic limit (PL) is the water content at which it changes from plastic to semi-solid (the 3 mm thread-rolling test). The gap between them, PI = LL − PL, is the plasticity index.

How does the plasticity chart classify the soil?

The point (LL, PI) is plotted against the Casagrande A-line, PI = 0.73 × (LL − 20). Above the line the soil is a clay (C); below it, a silt or organic soil (M/O). The LL = 50 line separates low plasticity (L) from high plasticity (H), giving CL, CH, ML and MH, with a small CL-ML borderline zone where PI is 4–7.

What does liquidity index tell me?

Liquidity index LI = (w − PL) ÷ PI shows where the natural (in-situ) water content sits within the plastic range. LI ≤ 0 means the soil is at or below its plastic limit (stiff, brittle); LI near 1 means it is close to its liquid limit (very soft, sensitive); values in between indicate a workable plastic state.

Is this a substitute for a formal soil classification report?

No. This is a guidance/estimate tool for the arithmetic of Atterberg limits and plasticity-chart placement only. Final classification, index values and any design use must be confirmed against the governing standard (AS 1289, ASTM D4318 or BS 1377) and signed off by a competent geotechnical professional.

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