About this tool Calculate California Bearing Ratio from laboratory test loads at 2.5mm and 5.0mm penetration per BS 1377-4. Enter the measured loads and the tool divides by the standard reference loads (13.24 kN and 19.96 kN) to give CBR at both penetrations.
The design CBR is automatically selected — normally the 2.5mm value governs, but if the 5.0mm value is higher, the tool flags this and recommends re-testing per BS 1377-4 procedure.
How to use this tool 1. Enter test load at 2.5mm penetration (kN)
2. Enter test load at 5.0mm penetration (kN)
3. Read the design CBR — the tool selects the governing value and flags if re-testing is needed.
Technical information CBR (%) = (test load / standard load) × 100
Standard loads per BS 1377-4: 13.24 kN at 2.5mm, 19.96 kN at 5.0mm. Report 2.5mm value unless 5.0mm is higher — then re-test and report confirmed higher value.
Limitations Laboratory CBR is measured on remoulded or undisturbed samples at specific moisture and density conditions. Results may not represent field equilibrium conditions.
For pavement design, use the design CBR from a suite of tests (typically the lower quartile or 10th percentile of results), not a single test value.
Revision history 29 September 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.
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