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CPT-SPT Correlation

Convert between CPT cone resistance and SPT N-value using soil-type-dependent qc/N ratios per Robertson et al. (1983). Free geotechnical calculator.

Last updated 3 November 2026

About this tool

Convert between CPT cone resistance (qc) and SPT N-value using soil-type-dependent qc/N ratios. Based on Robertson et al. (1983) — the most widely used CPT-SPT correlation in practice.

Useful when you have CPT data but need SPT-based design parameters, or vice versa. The ratio depends on mean grain size (D50) and soil behaviour type.

How to use this tool

1. Enter qc or N-value — whichever you have.

2. Select or enter the qc/N ratio — based on soil type. Typical values shown in the reference table.

Technical information

qc/N ratios per Robertson et al. (1983): clay 1-2, silt 2-3, sand 3-6, gravel 5-8. Ratio increases with grain size.

Limitations

The qc/N ratio is highly variable and soil-type dependent. It should not be used as a substitute for direct testing.

The correlation was developed for uncemented, relatively uniform soils. Cemented soils, residual soils, and fills may not follow published ratios.

Revision history

3 November 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.

Cone resistance, qc (MPa)

Sleeve friction, fs (MPa)

Pore pressure, u2 (MPa)

Net area ratio (a)

Depth (m)

Groundwater level (m)

Unit weight above GWL (kN/m³)

Saturated unit weight (kN/m³)

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