About this tool Calculate the Q-system rock mass quality index (Barton et al 1974, updated NGI 2015). Enter RQD and select values for the five Q-system parameters — Jn, Jr, Ja, Jw, and SRF — from built-in lookup tables.
The Q value is the product of three ratios: RQD/Jn (block size), Jr/Ja (inter-block shear strength), and Jw/SRF (active stress). Values range from 0.001 to 1000, spanning nine rock mass quality classes.
The Q-system is the global standard for tunnel support design and is widely used for rock mass characterisation in mining, slopes, and foundations. Per NGI (2015) and Barton et al (1974).
How to use this tool 1. Enter RQD — Rock Quality Designation from core logging (0–100%). Use the Core Recovery Calculator if needed.
2. Select Jn, Jr, Ja, Jw, SRF — from the dropdown lists. Each shows the standard description and numeric value.
3. Read the Q value and class — plus the three component ratios that describe block size, shear strength, and active stress.
Technical information Q = (RQD / Jn) × (Jr / Ja) × (Jw / SRF)
RQD/Jn represents relative block size. Jr/Ja represents inter-block shear strength. Jw/SRF represents the effect of water and stress.
Nine quality classes from Exceptionally Poor (Q < 0.01) to Exceptionally Good (Q > 400). Parameter tables follow NGI (2015) — the most current version of the system.
Limitations The Q-system was developed primarily for tunnel design. Application to slopes and foundations requires additional consideration of orientation effects not captured by Q alone.
Parameter selection requires engineering judgement and field experience. Different practitioners may assign different values for the same rock mass — always document your reasoning.
For support design, use the full Q-system chart (De vs Q) from NGI. This tool provides the Q value only, not the support recommendation.
Revision history 2 June 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.
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