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Liquid Limit Calculator

Calculate liquid limit from cone penetration test data per BS 1377-2. Enter penetration and moisture content data points — the tool fits a regression line and finds LL at 20mm.

Last updated 9 June 2026

About this tool

Calculate liquid limit (LL) from cone penetration test data per BS 1377-2. Enter multiple data points of cone penetration (mm) and moisture content (%). The tool performs a linear regression on a semi-log plot and determines the moisture content at 20mm penetration.

The cone penetrometer method is preferred over the Casagrande cup method in BS 1377-2 as it is less operator-dependent and gives more reproducible results.

How to use this tool

1. Enter cone penetration and moisture content pairs — minimum 2 points, ideally 4+ spanning the 15–25mm range.

2. Add rows as needed — more points improve the regression fit.

3. Check R² value — should be >0.90 for a reliable result.

Technical information

Regression: w = a + b × ln(penetration)

LL = a + b × ln(20)

Standard: BS 1377-2:1990, Clause 4.3 (cone penetrometer method). At least four determinations are required, with penetrations in the range 15–25mm.

Limitations

The tool uses a log-linear regression. BS 1377-2 requires at least four test points with penetrations spanning 15–25mm. Fewer points or a narrow range reduces reliability.

An R² below 0.90 indicates poor data quality — check for outliers, specimen preparation issues, or cone damage.

For soils with unusual plasticity behaviour (e.g. volcanic ash, organic soils), the cone method and Casagrande method may give different results.

Revision history

9 June 2026: Initial release

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.

Spotted an error? Let us know.

Cone penetration test data

Cone penetration (mm)Moisture content (%)

Enter at least 2 data points. Typical cone penetrations are 15–25 mm.

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