Rock Compressive Strength Calculator
This calculator determines the unconfined compressive strength (UCS) of rock from laboratory test data. UCS is calculated by dividing the peak axial load at failure by the original cross-sectional area of the test specimen, per ASTM D7012 Standard Test Methods for Compressive Strength and Elastic Moduli of Intact Rock Core. Enter the failure load and sample dimensions to compute UCS in MPa and psi, and receive a rock strength classification based on ISRM criteria.
UCS formula (ASTM D7012)
UCS (MPa) = F(peak) (N) / A (mm2) A = pi x (d/2)2
Where F(peak) is the peak compressive load in Newtons, A is the original cross-sectional area in mm2, and d is the specimen diameter in mm. 1 MPa = 1 N/mm2 = 145.038 psi. Source: ASTM D7012-14 (2014), ISRM Suggested Methods (1978, updated 2007).
ISRM strength classification
| UCS (MPa) | Class | Description | Example Rock |
|---|---|---|---|
| < 1 | R0 | Extremely weak | Stiff fault gouge |
| 1-5 | R1 | Very weak | Soft chalk, highly weathered rock |
| 5-25 | R2 | Weak | Coal, siltstone |
| 25-50 | R3 | Medium strong | Limestone, sandstone |
| 50-100 | R4 | Strong | Sandstone, marble |
| 100-250 | R5 | Very strong | Granite, gneiss |
| > 250 | R6 | Extremely strong | Fresh basalt, quartzite |
Rock compressive strength calculator: frequently asked questions
What is unconfined compressive strength (UCS) of rock?
Unconfined compressive strength (UCS), also called uniaxial compressive strength, is the maximum axial compressive stress a rock specimen can sustain before failure when no confining pressure is applied. It is the most commonly used measure of rock strength, expressed in megapascals (MPa) or pounds per square inch (psi).
How is UCS measured and calculated?
UCS is calculated as: UCS (MPa) = Peak Load (N) / Cross-sectional Area (mm2). The test is performed on a cylindrical core sample with a length-to-diameter ratio of 2:1 to 2.5:1, loaded axially to failure in a compression testing machine, per ASTM D7012 or ISRM standards.
What are typical UCS values for common rock types?
Typical UCS values: granite 100-250 MPa; basalt 100-350 MPa; limestone 30-250 MPa; sandstone 20-170 MPa; shale 5-100 MPa; coal 5-50 MPa; soft chalk 2-25 MPa. These ranges reflect natural variability in mineralogy, texture, and weathering.
How does UCS relate to rock quality designation (RQD)?
UCS and RQD are both rock mass characterisation parameters but measure different properties. UCS measures intact rock strength; RQD (Rock Quality Designation) measures the degree of fracturing of the rock mass from drill core recovery. Both are used together in rock mass classification systems such as the RMR (Bieniawski) and Q-system (Barton).
What is the point load strength index and how does it relate to UCS?
The point load strength index (Is50) is a simpler field index test. The correlation UCS approximately equals 22 x Is50 is widely used but with caution, as the correlation coefficient varies by rock type. Direct UCS testing per ASTM D7012 is always preferred for design purposes.
Official sources
- ASTM International: ASTM D7012-14: Standard Test Methods for Compressive Strength and Elastic Moduli of Intact Rock Core Specimens.
- International Society for Rock Mechanics (ISRM): ISRM Suggested Methods for Rock Characterisation.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.