Atterberg Limits Plasticity Interactive Calculator
Calculate plasticity index, liquidity index, consistency index, activity number, natural moisture content, and plastic limit from common geotechnical laboratory inputs.
Soil behavior changes at PL and LL
The plastic limit and liquid limit define the moisture range where clayey soil behaves plastically.
Consistency from LI: Soft => CH
Plasticity chart with real-time sliders
Casagrande Plasticity Chart
The red point is driven by the sliders on the right. It reads LL, PL, and natural moisture content w, then plots PI = LL − PL.
The natural moisture content is between PL and LL, so the soil is in the plastic range.
Position of natural moisture content relative to the plastic range.
Core Atterberg relationships
Plasticity Index
Measures the moisture range over which the soil remains plastic.
Liquidity Index
Normalizes natural moisture content between plastic and liquid limits.
Consistency Index
Complements liquidity index; for consistent inputs, CI + LI = 1.
Activity Number
Relates plasticity to clay-size fraction to infer mineral activity.
A-line
Separates clay-like behavior above the line from silt/organic-like behavior below the line.
U-line
Upper empirical boundary; points above this line should be checked for data quality or unusual material.
Theory & practical applications
Atterberg limits are used for fine-grained soil classification, earthwork control, preliminary foundation assessment, shrink–swell screening, and consistency interpretation. PI is commonly used with the Casagrande plasticity chart, LI/CI help interpret in-situ water content relative to the lab limits, and activity number supports clay-mineral behavior screening.
| Index | Common interpretation | Engineering note |
|---|---|---|
| PI | Plasticity range | Higher PI often indicates greater clay influence and shrink–swell concern. |
| LI | Position between PL and LL | LI near or above 1 suggests very soft to liquid behavior. |
| CI | Relative consistency | Higher CI indicates stiffer condition relative to LL. |
| Activity | PI normalized by clay fraction | High values suggest active clay minerals and expansive potential. |
Frequently asked questions
What inputs are needed for the basic PI calculation?
Enter liquid limit and plastic limit. The calculator computes PI as LL minus PL.
Can this replace laboratory testing?
No. This calculator interprets measured or assumed values. Use project-specific laboratory results for design decisions.
How is USCS classification estimated here?
The estimate compares LL with 50 and checks whether PI plots above or below the A-line, PI = 0.73(LL − 20).
What does an activity number greater than 1.25 imply?
It usually indicates an active clay, often associated with higher swelling potential and greater sensitivity to moisture changes.