RQD Calculator: Rock Quality Designation from Core Logging

How It Works

1. Enter Total Core Run Length

   Input the length of the core barrel run in centimeters or feet. This is the distance drilled during one core run, not the amount of core recovered.

2. Enter Core Piece Lengths

   Measure and input the length of each intact core piece. Only natural fractures count as piece boundaries. Fit mechanical breaks back together and measure the combined length as one piece.

3. Calculate RQD

   The calculator sums all pieces 10 cm (4 inches) or longer and divides by the total run length. Result is a percentage from 0% to 100%.

4. Review Classification

   See the RQD percentage and rock quality class: Very Poor (0-25%), Poor (25-50%), Fair (50-75%), Good (75-90%), Excellent (90-100%). Use this for RMR and Q-system input.

Built For

• Geologists logging drill core for geotechnical site investigations and tunnel feasibility studies
• Mining engineers characterizing rock mass quality for stope design and ground support requirements
• Geotechnical engineers computing RMR and Q-system ratings for tunnel support classification
• Dam foundation engineers assessing rock quality below planned structures using core drilling data
• Slope stability engineers evaluating rock mass conditions for cut slope and pit wall design
• Environmental engineers characterizing fractured rock aquifer properties from core drilling programs

Features & Capabilities

RQD = Sum of Pieces >= 10 cm / Run Length

Standard Deere (1964) formula. The 10 cm threshold separates reasonably intact rock from closely fractured material.

Automatic Classification

Maps RQD percentage to standard Deere classification: Very Poor, Poor, Fair, Good, Excellent. Color-coded for quick reference.

Piece-by-Piece Entry

Enter each core piece length individually. The calculator flags which pieces qualify (>= 10 cm) and which do not, so you can double-check your measurements.

Core Recovery Percentage

Also calculates total core recovery (all pieces / run length) alongside RQD. High recovery with low RQD means highly fractured rock.

Jv to RQD Estimate

Converts volumetric joint count (Jv) from outcrop mapping to estimated RQD using RQD = 115 - 3.3 x Jv. Useful for preliminary assessments without core.

PDF Export

Export RQD calculations and classification for core logging reports and geotechnical design documents.

Assumptions

• Core is NX size (54.7 mm / 2.15 in) or larger, as required by the Deere (1964) definition.
• Only naturally occurring fractures are counted — mechanical breaks from drilling are excluded.
• Pieces must be 100 mm (4 in) or longer along the core axis to count toward RQD.
• Core recovery is sufficient to provide a representative sample of the rock mass.

Limitations

• RQD is direction-dependent — a single borehole may not capture all fracture sets.
• Soft or heavily weathered rock may produce low RQD from drilling damage, not actual fracturing.
• Does not capture fracture aperture, filling, roughness, or water condition — use RMR or Q-system for those.
• RQD can jump from 0% to 100% with small changes in fracture spacing near the 100 mm threshold.
• Not applicable to soil, heavily decomposed rock, or non-cored boreholes.

References

• Deere, D.U. (1964) — Technical Description of Rock Cores for Engineering Purposes.
• ISRM Suggested Methods for Quantitative Description of Discontinuities in Rock Masses.
• Bieniawski, Z.T. — Rock Mass Rating (RMR) system incorporating RQD.
• Barton, N. et al. — Q-system for rock mass classification (NGI).

Frequently Asked Questions