About this tool Calculate Plasticity Index and classify fine-grained soils on an interactive Casagrande plasticity chart. Enter liquid limit and plastic limit for individual samples, or upload an AGS file to auto-plot every LLPL sample.
The chart plots the A-line (PI = 0.73 × (LL - 20)) and classifies each sample into BS 5930 plasticity groups: CL, CI, CH, CV, CE for clays and ML, MI, MH, MV, ME for silts. Results update in real time as you enter data.
Standards: BS 1377-2 (Atterberg limit testing), BS 5930:2015 (plasticity classification and Casagrande chart). AGS file support follows AGS4 format (LLPL group).
How to use this tool 1. Enter LL and PL manually — or drag and drop an AGS file to auto-populate all LLPL samples.
2. View the plasticity chart — samples are plotted against the A-line with BS 5930 classification zones.
3. Read the classification — PI, plasticity group (e.g. CH), and position relative to the A-line.
Technical information PI = LL - PL
A-line: PI = 0.73 × (LL - 20)
BS 5930 plasticity groups: Low (LL < 35), Intermediate (35–50), High (50–70), Very high (70–90), Extremely high (>90). Samples above the A-line are clays (C); below are silts (M).
Limitations The A-line classification works well for most UK and European soils but may not apply to tropical residual soils, volcanic ash soils, or highly organic materials which can plot in unusual positions on the chart.
AGS file support reads the LLPL group. Files must be valid AGS4 format. If LLPL data is missing or malformed, samples will not be plotted.
Plasticity Index alone does not fully characterise a fine-grained soil. Activity (PI / clay fraction), liquidity index, and mineralogy all affect engineering behaviour.
Revision history 1 February 2026: Content update and publish
Disclaimer This tool is provided for educational and general information purposes only. It is not a substitute for professional engineering advice, design or verification.
Diggy and its contributors are not licensed engineering consultants and no results generated by this tool should be used directly for construction, design or safety-critical decisions.
All values and outputs are based on published empirical correlations and should be independently checked and confirmed by a qualified geotechnical engineer before use.
By using this tool, you accept full responsibility for how you interpret and apply the information provided.
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