Ground engineering · Rock mechanics
Point load index (rock) calculator
Is and size-corrected Is(50) for diametral or axial/block tests, AS 1726 strength term and estimated UCS.
Standard: AS 4133.4.1 · size correction (De/50)^0.45 · AS 1726 strength terms
Open the live calculator → All ground engineering calculators
Inputs in the worked example
The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.
| Input | Value | Unit |
|---|---|---|
| Test | Diametral (core) | |
| Failure load P | 6.4 | kN |
| Core diameter / platen separation D | 63 | mm |
| Width W (axial / block) | 60 | mm |
| UCS / Is(50) factor K | 20 |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| Equivalent core diameter De | 63.0 | mm |
| Uncorrected Is | 1.61 | MPa |
| Size correction F | 1.110 | |
| Is(50) | 1.79 | MPa |
| Strength term (AS 1726) | high | |
| Estimated UCS = K·Is(50) | 36 | MPa |
- ✓ Size within 30–85 mm (correction reliable)
Method, formulas and assumptions
- Diametral De² = D²; axial/block De² = 4WD/π
- Is = P/De²; Is(50) = (De/50)^0.45 · Is
- UCS ≈ K·Is(50); K commonly 15–25 but site-specific: calibrate with UCS tests
Related ground engineering calculators
- Rock wedge stability (3-D, Hoek & Bray) with stereonet — Tetrahedral wedge formed by two discontinuities, the slope face and the upper ground surface. Geometry by…
- Rock slope kinematic analysis (planar, wedge, toppling) — Discontinuity sets mapped in a rock cut are tested against the slope face. Planar sliding needs a set…
- Rock Mass Rating (RMR89) — Ratings for UCS, RQD and spacing from values, plus joint condition, groundwater and orientation adjustment…
- Q-system & support — Q = (RQD/Jn)(Jr/Ja)(Jw/SRF), equivalent dimension De = span/ESR, Q–RMR correlation and unsupported span.
- Hoek–Brown rock mass strength (GSI) — mb, s, a from GSI, mi and D; rock mass UCS, tensile strength, global strength and modulus.
- Joint shear strength (Barton–Bandis) — Peak shear strength τ = σn tan(φr + JRC log10(JCS/σn)) and equivalent friction angle.
- Block sliding / toppling criteria — Mode of a block of width b and height h on a plane of dip ψ with friction φ: stable, sliding, toppling or…
- Rock bolt pattern for a loose block / wedge — Bolt load from suspending a loosened zone of height h over the bolt tributary area, with bolt length guidance.
For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.