RQD Calculator

Rock Quality Designation (RQD) is a modified core-recovery percentage introduced by Deere (1964).

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Enter Values

Total core run length

Sum of intact (sound) pieces ≥ 100 mm

Total core recovered (mm) — optional, for recovery %

Enter your values and press Calculate to see results.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

Enter the total length drilled in the core run (the length of the run interval), in millimetres.
Enter the summed length of all naturally intact pieces that are 100 mm or longer, measured along the core axis — exclude pieces broken by handling or drilling.
Optionally enter the total length of core recovered to also get the core recovery percentage.

How it works

Rock Quality Designation (RQD) is a modified core-recovery percentage introduced by Deere (1964). It is the sum of the lengths of intact, sound core pieces 100 mm (4 in) or longer, divided by the total length of the core run, expressed as a percentage: RQD = (Σ pieces ≥ 100 mm ÷ total run) × 100. Only pieces of hard, sound rock are counted; fractures caused by drilling or handling are ignored (the pieces are fitted back together), and highly weathered material is excluded.

The result is commonly grouped using Deere's descriptive scale: 0–25 % Very poor, 25–50 % Poor, 50–75 % Fair, 75–90 % Good, and 90–100 % Excellent. RQD is also a rating input to rock-mass classification systems such as RMR and the Q-system. Core recovery, when the recovered length is entered, is simply the total length of core brought up divided by the length drilled — always equal to or greater than RQD for the same run.

Worked example

A 1.5 m core run with 950 mm of sound pieces. You log a 1,500 mm core run. Measuring along the core axis, the intact (naturally sound) pieces that are 100 mm or longer sum to 950 mm. RQD = (950 ÷ 1,500) × 100 = 63.33 %, which is 'Fair' rock mass quality on the Deere scale. If 1,350 mm of core was recovered from that 1,500 mm run, core recovery = (1,350 ÷ 1,500) × 100 = 90 %.

Common mistakes

Counting every recovered piece instead of only sound pieces 100 mm or longer — RQD ignores short and mechanically-broken fragments, so it is not the same as core recovery.
Measuring piece lengths on the curved outer surface rather than along the central axis of the core, which overstates RQD.
Using the wrong run length — RQD is per core run, so divide by the actual interval drilled, not the whole borehole depth or the recovered length.

Frequently asked questions

What is the 100 mm threshold in RQD?

Only intact core pieces that are at least 100 mm (about 4 inches) long, measured along the core axis, are summed for RQD. Shorter pieces, and any pieces broken by drilling or handling rather than by natural fractures, are excluded. This threshold is the standard Deere/ISRM definition.

Is RQD the same as core recovery?

No. Core recovery is the total recovered core length divided by the length drilled. RQD is stricter — it counts only sound pieces 100 mm or longer. RQD is therefore always less than or equal to core recovery for the same run, and a low RQD with high recovery signals closely-fractured but present rock.

How is the RQD percentage classified?

Deere's descriptive scale is: 0–25 % Very poor, 25–50 % Poor, 50–75 % Fair, 75–90 % Good, and 90–100 % Excellent rock mass quality. These bands feed into wider classification systems such as RMR and the Q-system, but should not replace a full geotechnical assessment.

Tip: Enter any known values to calculate the remaining results.

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