Free calculator
Atterberg limits calculator: liquid limit, plastic limit and PI
Enter each liquid limit trial as a blow count and water content, and each plastic limit trial as a water content. The calculator fits water content against log10 of blows, reads the liquid limit at 25 blows, averages the plastic limit, and plots the result on the plasticity chart.
By ProofGrit Editorial TeamLast reviewed:
Results update as you typeUnits: %, blowsRuns in your browser. Nothing is saved.
Key takeaways
- Use at least three liquid limit trials at different blow counts.
- PL is reached when a rolled thread crumbles at about 1/8 in (3.2 mm).
- If PL ≥ LL or PL cannot be found, report non-plastic (NP).
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How it's calculated
How is the result calculated?
LL = water content at 25 blows on the least-squares line of w vs log10(blows); PL = average of trials; PI = LL − PL, or NP.
- 01Flow curve: least-squares straight line of water content (%) against log10(number of blows)
- 02Liquid limit (LL) = the water content on that line at 25 blows
- 03Plastic limit (PL) = average water content of the plastic limit trials, where the thread crumbles at about 1/8 in (3.2 mm)
- 04Plasticity index PI = LL − PL
- 05If the PL cannot be determined, or PL ≥ LL, report the soil as non-plastic (NP)
Rounding: moisture content, liquid limit, plastic limit and plasticity index to 0.1. Full precision is kept between steps.
FAQ
Frequently asked questions about the atterberg limits calculator
How many liquid limit trials are needed?
The calculator needs at least three trials at different blow counts to fit the flow curve. Spread them on both sides of 25 blows for a reliable reading.
Why is the blow count on a log scale?
Water content plots close to a straight line against the logarithm of blows, so the fit uses log10(blows) and reads the line at 25.
When is a soil non-plastic?
When the plastic limit cannot be determined, or when it is equal to or greater than the liquid limit. The result is reported as NP.
What does R² tell me?
How closely the trials follow a straight flow curve. A low value suggests a trial worth rechecking before you report.
Where do the water contents come from?
From oven-dry moisture content of each trial specimen. The soil moisture content calculator does that arithmetic.
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- Standard vs Modified Proctor: ASTM D698 and D1557 comparedHow Standard (D698) and Modified (D1557) Proctor differ in rammer, drop and layers, why Modified gives a higher MDD, and why the right reference matters.
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- Standards indexEvery standard explained on this site, with plain-English purpose.
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Sources and further reading
- ASTM D4318, liquid limit, plastic limit and plasticity index (linked edition marked withdrawn; confirm the applicable current edition)ASTM International
- ASTM D2487, classification of soils for engineering purposes (USCS)ASTM International
- ASTM D2216, laboratory water (moisture) content of soil and rock by massASTM International
Standards are summarized in our own words. Read the current edition from the publisher before you test.
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