Simple calculator to convert axial point load strength index (Is(50)) results to uniaxial compressive strength (UCS) using a correlation factor.
This handy calculator takes your axial Point Load Test (PLT) results—often reported as Is(50)—and converts them into an estimated Uniaxial Compressive Strength (UCS). It uses a straightforward correlation factor to bridge the gap between quick field tests and traditional lab-based UCS measurements.
PLT is a brilliant on-the-spot test for rock strength, particularly when you need a rapid check without hauling cores back to the lab for full UCS testing. The correlation gives you a ballpark UCS figure—ideal for early design decisions, ongoing site evaluations, or when you simply don't have enough time or budget for lab work.
Our free tool does the maths for you, meaning you can pop in your Is(50) results and walk away with a concise UCS estimate in seconds, no complicated spreadsheets required.
The correlation between Point Load Strength Index and UCS is typically expressed as:
UCS = K × Is(50)
Where:
UCS = Uniaxial Compressive Strength in MPa
Is(50) = Point Load Strength Index (size-corrected to 50mm) in MPa
K = Correlation factor (typically between 20-25 for most rock types)
The correlation factor (K) can vary depending on rock type, with sedimentary rocks typically having lower values than igneous or metamorphic rocks. For general purposes, a value of 24 is often used when specific correlations for the rock type are not available.
The PLT to UCS Correlation Calculator is particularly valuable for:
For the most reliable results when using PLT to UCS correlations:
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