Atterberg Limits Calculator
The one-point liquid limit = water content at N blows × (N ÷ 25)^0.121; the plastic limit is the water content of the rolled thread; plasticity index = LL − PL, per ASTM D4318. At 25 blows the example masses give LL 30.4%, PL 25.0% and PI 5.4%. Moisture: Soil Moisture Content Calculator.
How it's calculated: wN = (wet − dry) ÷ (dry − container) × 100; LL = wN × (N ÷ 25)^0.121; PL = thread water content; PI = LL − PL.
Result appears here
Fill in the fields and press Calculate.
Formula, calculation, assumptions and sources
Moisture at N blows, wN (%) = (LL wet − LL dry) ÷ (LL dry − LL container) × 100
Liquid limit (one-point method) = wN × (N ÷ 25)^0.121
Plastic limit (%) = (PL wet − PL dry) ÷ (PL dry − PL container) × 100
Plasticity index = Liquid limit − Plastic limit
Run the calculation to see each step with your own numbers.
Liquid limit test at N = 25 blows; plastic limit from a rolled thread.
wN = (45 − 38) ÷ (38 − 15) × 100 = 30.4%; LL = 30.4 × (25/25)^0.121 = 30.4%
PL = (30 − 27) ÷ (27 − 15) × 100 = 25.0%
PI = 30.4 − 25.0 = 5.4%
- The one-point liquid-limit method (ASTM D4318 Annex A1) is used — valid for a blow count roughly between 20 and 30; outside that range the multi-point method is more reliable.
- The plastic limit is the water content of a 1/8-in (3.2 mm) diameter soil thread at the point it just crumbles — entered here as a single wet/dry mass pair rather than an average of multiple trials.
- A negative plasticity index (PL measured higher than LL, or the soil won't roll to a thread) indicates a non-plastic soil — report PI as 'NP' by convention rather than the negative number.
- Why 25 blows as the reference?
- ASTM D4318 defines the liquid limit as the water content at exactly 25 blows of the Casagrande cup. The one-point method's (N/25)^0.121 term corrects a test run at a different blow count back to that 25-blow reference, instead of requiring a full multi-point flow curve.
- What does the plasticity index tell you?
- It's the water-content range over which the soil behaves plastically (moldable but not fluid) — the range between the liquid limit and plastic limit. A higher PI generally means more clay content and more shrink-swell / compressibility concern; PI near 0 or negative means a non-plastic (silty/sandy) soil.
- Can I use this for USCS classification?
- The LL and PI here are exactly what the USCS plasticity chart (A-line) uses to classify fine-grained soil as clay or silt, high or low plasticity — but this calculator returns the limits only, not the classification symbol itself.
- How do you calculate the plasticity index?
- Subtract the plastic limit from the liquid limit. A liquid limit of 30.4% and a plastic limit of 25.0% give a PI of 5.4%. A negative result means the soil is non-plastic and is reported as NP.
- How is the one-point liquid limit calculated?
- Measure the water content at the blow count N and multiply by (N ÷ 25)^0.121. At 25 blows the factor is 1; at 30 blows the same 30.4% water content gives a liquid limit of about 31.1%.
Test method & formulas
ASTM D4318, 'Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils' — the one-point liquid-limit equation is Annex A1's wN × (N/25)^0.121.
About the Atterberg Limits Calculator
The Atterberg Limits Calculator applies ASTM D4318: the liquid limit via the one-point method (a Casagrande-cup blow count with a moisture-content correction), the plastic limit from the water content of a soil thread rolled to 1/8-in diameter, and the plasticity index as their difference. These three numbers classify a fine-grained soil's consistency and are central to the Unified Soil Classification System (USCS).
What you enter
- Blow count N (liquid limit) — Valid for the one-point method over roughly N = 20–30; 25 is the calibration point where no correction is applied.
- LL container mass
- LL container + wet soil
- LL container + dry soil
- PL container mass
- PL container + wet soil
- PL container + dry soil
What you get back
- Liquid limit (LL) — %
- Plastic limit (PL) — %
- Plasticity index (PI = LL − PL) — %
The formula, a worked example, the assumptions behind it and the sources are shown in full below the calculator.
Related calculators
- Soil Moisture Content Calculator — Water content of a soil sample as a percentage of its dry mass, from the container, wet-soil and dry-soil masses.
- Specific Gravity of Soil Calculator — Specific gravity of soil solids from a pycnometer test, using the dry-soil and water masses.
- California Bearing Ratio (CBR) Calculator — California Bearing Ratio of a subgrade or base sample at 2.5 mm and 5.0 mm penetration, against the ASTM standard loads.
- Soil Permeability Calculator — Coefficient of permeability (hydraulic conductivity) of a soil sample by the falling-head or constant-head method.
Related articles
- What Are Atterberg Limits? — Atterberg limits describe how a fine-grained soil's behavior changes with moisture — from brittle solid to plastic to liquid-like — and they underpin soil classification.
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