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Point Load Index Converter

Converts an ISRM point-load test failure load into the point-load strength index Is, the size-corrected Is₅₀, and an estimated uniaxial compressive strength (UCS). Geologists and geotechnical engineers use it to rate intact rock strength quickly in the field or core shed without a full UCS press.

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How to use this calculator

Enter Values

Enter your values and press Calculate to see results.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Enter the failure load P in kN and the core (specimen) diameter D in mm for a diametral test.
  2. Optionally set the UCS conversion factor k (default 22) to match your rock type.
  3. Read Is₅₀ (the standard reference index) and the estimated UCS; use them for logging and preliminary strength classification.

How it works

For a diametral test the equivalent core diameter squared is De² = D². The uncorrected index is Is = P / De² with P converted to newtons and De² in mm², which gives megapascals directly.

Because Is varies with specimen size, ISRM normalises it to a 50 mm reference core: Is₅₀ = Is × (De/50)^0.45. UCS is then estimated as UCS ≈ k × Is₅₀, where k is commonly 20–25 (often 22–24) and depends on the rock. For strong, brittle rock the higher end applies; always confirm against direct UCS tests where possible.

Worked example

50 mm core, 15 kN failure load. De² = 50² = 2500 mm². Is = 15,000 N / 2500 mm² = 6.0 MPa. The size-correction factor (50/50)^0.45 = 1, so Is₅₀ = 6.0 MPa. With k = 22, estimated UCS ≈ 22 × 6.0 = 132 MPa — a strong rock (ISRM R4).

Common mistakes

  • Entering the load in newtons instead of kN — the field expects kN and converts internally.
  • Forgetting the size correction on cores that aren't 50 mm; Is and Is₅₀ differ for any other diameter.
  • Treating the estimated UCS as exact — the conversion factor k is rock-dependent and can vary from about 20 to 25.

Frequently asked questions

What is the point-load strength index Is₅₀?

It is the ISRM point-load index corrected to a standard 50 mm equivalent core diameter, so results from different specimen sizes can be compared and used to estimate UCS.

Why size-correct to 50 mm?

The raw index depends on specimen size. ISRM standardises to a 50 mm reference core using Is₅₀ = Is × (De/50)^0.45 so the value is repeatable regardless of the core you tested.

What UCS factor should I use?

A factor of 22 is a common default. The real factor typically lies between about 20 and 25 (often 22–24) and depends on rock type, so calibrate it against direct UCS tests for your material.

Is this a diametral or axial test?

This converter uses the diametral test, where the load is applied across the core diameter and De² = D². Axial and irregular-lump tests use different equivalent-diameter formulas.

Can I use it for weak or anisotropic rock?

Point-load testing is intended for rock stronger than roughly 25 MPa and can be unreliable for weak, weathered or strongly anisotropic material — treat those results with caution and prefer direct UCS testing.

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