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SPT Relative Density Calculator

Estimate relative density and compactness classification from corrected SPT N60 values using Terzaghi & Peck (1948) or Skempton (1986) correlations.

Last updated 23 March 2026

About this tool

Estimate the relative density (Dr) and compactness classification of granular soils from corrected Standard Penetration Test (SPT) N60 values.

This calculator provides two well-established methods: the classic Terzaghi & Peck (1948) table classification, and the Skempton (1986) formula for normally consolidated sands.

Relative density is a key parameter for assessing the in-situ state of granular soils and is used in liquefaction analysis, settlement estimation, and foundation design.

How to use this tool

Enter corrected N60 - input the SPT N60 value from field testing or your energy correction calculator.
Select correlation method - choose Terzaghi & Peck for a table classification or Skempton for a calculated Dr value.
Review the result - the tool outputs the compactness classification (Very Loose to Very Dense) and the estimated relative density range or value.

What is relative density?

Relative density (Dr) describes how tightly packed a granular soil is compared to its loosest and densest possible states. It is expressed as a percentage from 0% (loosest) to 100% (densest).

It is one of the most important state parameters for sands and gravels, influencing:

Shear strength and friction angle
Compressibility and settlement behaviour
Liquefaction susceptibility
Pile driveability and capacity

Technical information

Terzaghi & Peck (1948) - the original and most widely used classification for granular soils:

N60 = 0-4: Very Loose (Dr 0-15%)
N60 = 4-10: Loose (Dr 15-35%)
N60 = 10-30: Medium Dense (Dr 35-65%)
N60 = 30-50: Dense (Dr 65-85%)
N60 > 50: Very Dense (Dr 85-100%)

Skempton (1986) - for normally consolidated clean sands:

Dr = sqrt(N60 / 60) x 100 (%)

Based on the relationship N60/Dr squared approx 60 for normally consolidated, moderately compressible, clean quartz sands.

Limitations

Applies to granular soils only - not valid for clays or cemented materials.
Skempton's formula assumes normally consolidated clean sand - overconsolidated or silty sands may give misleading results.
Energy and overburden corrections must be applied before using N60.
Correlations are empirical - laboratory testing (e.g. maximum/minimum density tests) should confirm values for critical projects.

Revision history

23 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.

Corrected N-value (N₆₀)

Correlation method