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SPT N-value Calculator

Calculate corrected SPT N₆₀ values with energy and overburden corrections for geotechnical site investigations.

Last updated 11 February 2026

About this tool

Calculate corrected SPT N₆₀ values with energy and overburden corrections for geotechnical site investigations.

This calculator normalises Standard Penetration Test (SPT) blow counts to 60% hammer energy efficiency, allowing for consistent comparison across different testing equipment and site conditions.

How to use this tool

Enter your measured N-value. Input the raw blow count obtained from your field SPT testing.
Select hammer type. Choose the hammer type used in your test. This will set the typical energy efficiency.
Adjust efficiency if known. If you have measured energy efficiency values, you can manually adjust the percentage.
Apply overburden correction (optional). For shallow depths (typically <10m), enable overburden correction and enter the depth.

What is N₆₀?

N₆₀ is the SPT blow count normalised to 60% hammer energy efficiency. It provides a standardised value for comparing SPT results across different equipment and testing procedures.

The 60% reference is a standardised energy ratio adopted for consistent comparison of SPT results worldwide. Actual field equipment may deliver different energy levels, requiring correction factors.

Limitations

This calculator uses simplified correction factors and may not account for all site-specific conditions.
Overburden correction assumes typical soil unit weights and may require adjustment for unusual conditions.
Soil classification is approximate and should be verified with laboratory testing.

Technical information

Energy Correction Factor (CE): CE = Er / 60, where Er is the measured energy ratio (%)

Overburden Correction Factor (CN): Based on Liao & Whitman (1986): CN = √(Pa / σ'v), where Pa = 100 kPa and σ'v is the effective overburden stress. Maximum value capped at 2.0.

Corrected N-value: N₆₀ = N × CE × CN

Revision history

11 February 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.

Measured N-value

Hammer Type

Hammer Efficiency (%)

Typical range: 45-100%. Adjust if known.

Apply overburden correction