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CPT Relative Density Calculator

Estimate relative density of granular soils from CPT cone resistance using Baldi et al. (1986) correlation with compactness classification.

Last updated 10 November 2026

About this tool

Estimate relative density (Dr) of sands from CPT cone resistance. Uses Baldi et al. (1986) correlation for normally consolidated uncemented quartz sands.

Includes Terzaghi & Peck compactness classification: very loose, loose, medium dense, dense, and very dense.

How to use this tool

Enter cone resistance (qc) — in MPa, from CPT field data.
Enter depth and GWL — to compute effective vertical stress.
Review Dr result — relative density (%) with Terzaghi & Peck compactness classification.

Technical information

Dr = (1/2.41) × ln(qc / (157 × σ'v0^0.55))

Baldi et al. (1986) correlation developed for normally consolidated uncemented quartz sands. qc in kPa, σ'v0 in kPa.

Limitations

Valid for normally consolidated uncemented quartz sands only.
Overconsolidation, aging, and cementation cause overestimation of relative density.
Correlation coefficients are sand-specific — different sand types may require different calibrations.

Revision history

10 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)

Depth (m)

Groundwater level (m)

Unit weight above GWL (kN/m³)

Saturated unit weight (kN/m³)

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