ToolsFabric SurveyConcrete Testing

In-Situ Strength Estimator

Free online in-situ concrete strength estimator per BS EN 13791:2019. Corrects core results and estimates characteristic compressive strength.

Last updated 17 March 2026

About this tool

Estimate the characteristic in-situ compressive strength of concrete from core test results using BS EN 13791:2019 Approach A and Approach B.

The tool applies correction factors for length/diameter ratio, core diameter, reinforcement presence, and moisture condition, then determines fck,is and assesses it against the specified strength class.

Designed for structural engineers assessing existing concrete structures where core samples have been taken.

How to use this tool

Enter core results — comma-separated compressive strengths in MPa (minimum 3 cores).
Set core parameters — diameter, length/diameter ratio, whether reinforcement is present, and moisture condition at test.
Enter specified strength class — the characteristic cylinder strength (e.g. 30 for C30/37).
Review results — the tool shows corrected core strengths, Approach A and B calculations, the estimated fck,is, and an adequacy assessment.

Technical information

Correction factors (legacy BS 1881-120 / ASTM C42 practice):

L/D ratio — 1.0→0.87, 1.25→0.93, 1.5→0.96, 1.75→0.98, 2.0→1.00 (interpolated)
Diameter — 50mm→0.97, 75mm→0.99, 100mm→1.00, 150mm→1.02 (approximate)
Reinforcement — ×1.06 if cores contain rebar
Moisture — ×0.96 for air-dry cores (ACI 214.4R)

BS EN 13791:2019 characteristic strength

Approach A — min(mean − k₁×s, lowest + 4)
Approach B — lowest + 4
fck,is — min(Approach A, Approach B)

Limitations

Correction factors follow legacy BS 1881-120 / ASTM C42 practice. BS EN 12504-1:2019 removed these corrections — consult your project specification.
Diameter correction factors are approximate and may vary between sources.
Core results should be interpreted by a qualified structural engineer in the context of the full investigation.

Revision history

17 March 2026: Initial release

Disclaimer

This tool is provided for educational and general information purposes only. It is not a substitute for professional engineering advice, design or verification.

Diggy and its contributors are not licensed engineering consultants and no results generated by this tool should be used directly for construction, design or safety-critical decisions.

All values and outputs are based on published empirical correlations and should be independently checked and confirmed by a qualified geotechnical engineer before use.

By using this tool, you accept full responsibility for how you interpret and apply the information provided.

Spotted an error? Let us know.

Core results (MPa, comma-separated)

Minimum 3 core results required

Core diameter (mm)

Length/diameter ratio

Cores contain reinforcement

Moisture condition at test

Specified strength class (MPa)