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Fracture Frequency Calculator

Calculate fracture frequency and average fracture spacing from drill core to assess rock mass quality using ISRM classification.

Last updated 11 February 2026

About this tool

Calculate fracture frequency and average fracture spacing from drill core measurements to assess rock mass quality and discontinuity characteristics.

This calculator helps geotechnical engineers and engineering geologists quantify fracture intensity in rock cores for rock mass classification and engineering design.

How to use this tool

Enter core length. Input the total length of core being analyzed (in meters).
Enter total fractures. Count and input the total number of natural fractures observed in the core.
Review results. The calculator provides fracture frequency, average spacing and rock quality classification.

What is fracture frequency?

Fracture frequency (FF) is the number of natural fractures per unit length of core, typically expressed as fractures per meter.

It is a key parameter for characterizing rock mass discontinuities and is used in rock mass classification systems including RMR, Q-system, and GSI.

Rock quality classification

Very Wide (>2m): Very good quality, minimal fracturing
Wide (0.6-2m): Good quality, low fracturing
Moderate (0.2-0.6m): Fair quality, moderate fracturing
Close (0.06-0.2m): Poor quality, high fracturing
Very Close (<0.06m): Very poor quality, intense fracturing

Limitations

Only natural fractures should be counted. Mechanical breaks must be excluded.
Fracture frequency is a 1D measurement along the borehole and may not represent true 3D fracture density.
Orientation bias may occur if fractures are sub-parallel to borehole.
Results should be combined with other parameters (RQD, weathering, strength) for complete rock mass assessment.

Technical information

Fracture Frequency (FF): FF = Total Number of Fractures / Core Length (fractures/m)

Average Spacing: Spacing = Core Length / Total Number of Fractures (m)

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.

Core length (meters)

Total length of core run being analyzed.

Total fractures (count)

Number of natural fractures observed. Do not count mechanical breaks.