ToolsRock MechanicsRock Strength Testing

Point Load to UCS Converter

Convert point load index Is(50) to uniaxial compressive strength using size-corrected ISRM method. Supports diametral, axial, and irregular lump tests with rock-type-specific k factors.

Last updated 19 May 2026

About this tool

Convert point load test results to estimated uniaxial compressive strength (UCS). Enter the failure load and specimen dimensions — the tool calculates the point load index, applies the ISRM size correction to 50mm equivalent diameter, and estimates UCS using rock-type-specific k factors.

Three test types are supported: diametral (core), axial, and irregular lump. Ten rock types are included with published k factors ranging from 15 (chalk) to 25 (basalt, gneiss).

This is the standard field method for estimating rock strength without laboratory UCS testing. Per ISRM Suggested Methods and ASTM D5731.

How to use this tool

1. Select the test type — diametral for core, axial or irregular for field specimens.

2. Enter failure load and dimensions — load in kN, dimensions in mm.

3. Select the rock type — this determines the k factor. Use 'General' (k=23) if unsure.

Technical information

Is = P / De²

Is(50) = Is × (De / 50)^0.45 — ISRM size correction

UCS = k × Is(50)

k factors are empirical and vary by source. ISRM suggests a general range of 20–25. Always verify against site-specific correlations where available. Standard: ISRM Suggested Methods, ASTM D5731.

Limitations

The k factor varies significantly between rock types and even within the same rock formation. Published values are indicative — site-specific calibration against UCS tests is recommended for design use.

The size correction exponent (0.45) is an approximation. ISRM recommends determining the exponent from a suite of tests at different sizes where possible.

Point load tests on anisotropic rocks (e.g. schist, slate) are sensitive to loading direction relative to foliation. Results should note the loading orientation.

Revision history

19 May 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.

Test type

Failure load (kN)

Core diameter (mm)

Common: NQ = 47.6mm, HQ = 63.5mm, PQ = 85.0mm

Rock type (for k factor)

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