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Atterberg Limits and Plasticity Index Calculator

Atterberg limits are laboratory index values used to describe how the consistency of a fine-grained soil changes with water content. The liquid limit, LL, marks the boundary used between the plastic and liquid consistency states. The plastic limit, PL, is the lowest water content at which the soil remains plastic under the specified procedure. For valid numeric limits, plasticity index is the width of that plastic range: PI equals LL minus PL. GeoMiner performs that subtraction, calculates the published plasticity-chart reference lines, and optionally positions a natural water content relative to LL and PL. It does not run the laboratory test, validate a specimen, or replace the current method required by a project, regulator, laboratory, or authority. Enter results only after representative sampling, preparation, moisture-content determinations, apparatus checks, repeated trials, method acceptance, and reporting decisions have been completed under the governing procedure. The cited FHWA laboratory workbook and LTPP protocol describe liquid-limit and plastic-limit workflows based on AASHTO T 89 and T 90. In the multipoint liquid-limit workflow, the flow curve is used to determine the moisture content corresponding to 25 blows. Plastic limit is determined through its separate procedure. The official FHWA worksheet preserves container and moisture calculations before LL, PL, and PI are reported. This calculator intentionally begins with valid reported LL and PL values rather than recreating those laboratory quality controls. The cited procedures report LL, PL, and PI to the nearest whole number. GeoMiner accepts decimals so users can review arithmetic without silently discarding information, but the required reporting precision remains controlled by the governing method. If PL is equal to or greater than LL, or if a required limit cannot be determined, the cited workflow reports NP. GeoMiner therefore displays NP and does not invent a zero or negative PI or plot a chart point. A numeric result uses PI = LL - PL. For the worked example LL = 40 percent and PL = 20 percent, PI = 20 percentage points. The USCS plasticity chart plots liquid limit on the horizontal axis and plasticity index on the vertical axis. Its A-line is horizontal at PI = 4 through LL = 25.5, then follows PI = 0.73 LL - 20 . At LL = 40, the A-line value is 14.6, so the worked PI of 20 plots ab

Verification resources

Cross-check terminology, classification, methods, and safety with these authoritative external resources.

  • undefined. Defines liquid limit, plastic limit, and plasticity index; describes the 25-blow liquid-limit result; and states the non-plastic reporting rule when a limit cannot be determined or PL is at least LL.
  • undefined. Documents the AASHTO T 89 and T 90 laboratory workflow, representative sampling, whole-number reporting, and the NP rule.
  • undefined. Provides the official laboratory worksheet linking moisture-content determinations to LL, PL, and PI equals LL minus PL.
  • undefined. Publishes the Unified Soil Classification System plasticity chart, including the horizontal and diagonal A-line segments, the vertical and diagonal U-line segments, the LL equals 50 division, and fine-grained chart regions.
  • undefined. Publishes the piecewise USCS plasticity-chart A-line and U-line equations and reinforces that the chart is one part of soil classification.

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Common questions

How is plasticity index calculated?
For valid laboratory limits, plasticity index is liquid limit minus plastic limit: PI = LL - PL. Keep all values on the same water-content basis and follow the current governing laboratory method.
What happens when plastic limit is equal to or greater than liquid limit?
The cited FHWA procedures report the soil as NP rather than a numeric plasticity index when PL is equal to or greater than LL, or when a required limit cannot be determined.
What are the A-line and U-line equations?
The USCS chart draws the A-line horizontally at PI = 4 through LL = 25.5, then as PI = 0.73(LL - 20). The U-line is vertical at LL = 16 to its diagonal, then follows PI = 0.9(LL - 8). A point outside the U-line should trigger review.
Does the chart result give a complete USCS soil classification?
No. The chart position alone cannot distinguish inorganic from organic soil and does not supply gradation, percent fines, or the other evidence needed for a complete USCS group symbol and name.
What do liquidity index and consistency index mean?
When natural water content is entered, LI = (w - PL) / PI and Ic = (LL - w) / PI. They position the water content relative to the plastic range but do not predict design performance.

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