Zerdly

Rock Strength Unit Converter

Converts a rock uniaxial compressive strength (UCS) value between MPa, kPa and psi, and returns the ISRM strength grade (R0–R6). It is a quick reference for geologists, geotechnical engineers and drillers reading strength values from different logs and standards.

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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 rock strength in MPa.
  2. Read the equivalent values in kPa and psi.
  3. Check the returned ISRM grade (R0 extremely weak to R6 extremely strong) for classification.

How it works

The conversions are exact unit definitions: 1 MPa = 1000 kPa and 1 MPa = 145.037738 psi. Enter the value in MPa and the tool echoes it and reports the other two units.

The ISRM (1978) strength grade is read straight from the MPa value: R0 <1, R1 1–5, R2 5–25, R3 25–50, R4 50–100, R5 100–250, R6 >250 MPa. These grades map field/lab strength onto the standard rock-description scale.

Worked example

75 MPa sandstone. 75 MPa × 1000 = 75,000 kPa. 75 MPa × 145.037738 ≈ 10,877.8 psi. A value of 75 MPa falls in the 50–100 MPa band, so the ISRM grade is R4 — strong rock.

Common mistakes

  • Confusing kPa with MPa — a factor of 1000 difference that misplaces the strength grade entirely.
  • Using 145 psi/MPa loosely; the precise factor is 145.037738 psi per MPa.
  • Reading the grade off shear or tensile strength — the ISRM R-grades apply to uniaxial compressive strength.

Frequently asked questions

How many psi is 1 MPa?

1 MPa equals 145.037738 psi. So, for example, 100 MPa ≈ 14,503.8 psi.

How many kPa in a MPa?

1 MPa = 1000 kPa. Multiply MPa by 1000 for kPa, or divide kPa by 1000 for MPa.

What are the ISRM strength grades?

R0 <1 (extremely weak), R1 1–5 (very weak), R2 5–25 (weak), R3 25–50 (medium strong), R4 50–100 (strong), R5 100–250 (very strong), R6 >250 MPa (extremely strong).

Is R-grade the same as UCS?

The R-grade is a band based on the UCS value. It describes the strength class rather than the exact number, so two rocks with different UCS can share the same grade.

Which strength value do I enter?

Enter the uniaxial (unconfined) compressive strength in MPa — the value from a UCS test or estimated from a point-load index.

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Tip: Enter any known values to calculate the remaining results.

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