Calculate Rock Quality Designation instantly using the core piece method (ISRM / ASTM D6032) or Palmstrom's volumetric joint count (Jv) method. Get your RQD%, quality class, and a full per-piece breakdown — then send the result straight into the RMR or Q-System calculator.
Full breakdown of every value used in the calculation above — useful for QA, field logs, and report appendices.
Rock Quality Designation (RQD) is a quantitative index of rock mass quality introduced by Don U. Deere at the University of Illinois in 1967. It was designed as a simple, repeatable measurement that any geologist could take during routine core logging, without needing specialized equipment beyond a measuring tape. RQD expresses, as a single percentage, how much of a drilled core run consists of reasonably intact rock versus closely fractured material.
Despite its simplicity, RQD became one of the most widely adopted parameters in geotechnical engineering. It is reported on nearly every borehole log worldwide and is required by major investigation standards, including ISRM suggested methods, ASTM D6032, and Eurocode 7. Its real importance, though, comes from its role as a direct input into the two dominant rock mass classification systems: it is Parameter 2 in Bieniawski's RMR system, worth up to 20 of the 100 total points, and it forms the numerator of the first term in Barton's Q-System formula, RQD divided by Jn.
The standard core method requires NX-size (54.7 mm diameter) or larger diamond drill core. A geologist examines each core run and measures every intact piece of core that is 100 mm (10 cm) or longer along its centerline. Only breaks caused naturally — by joints, bedding planes, or faults already present in the rock — are counted as fractures. Breaks caused by the drilling process itself are identified by a fresh, rough, mechanically-fitting surface, and are ignored; the two pieces on either side of a mechanical break are treated as one continuous piece for measurement purposes.
The core formula is straightforward: RQD = (sum of intact piece lengths ≥ 100 mm ÷ total core run length) × 100. For example, a 1500 mm core run containing pieces of 250, 80, 350, 40, 120, and 300 mm has four qualifying pieces (250 + 350 + 120 + 300 = 1020 mm). RQD = (1020 / 1500) × 100 = 68%, which falls in the Fair quality class.
When drill core isn't available — for example during early-stage surface mapping — Palmstrom (1982) proposed estimating RQD from the volumetric joint count Jv, the number of joints per cubic meter visible on an exposed rock face. The correlation RQD = 115 − 3.3 × Jv gives a reasonable estimate, with results capped at 0% and 100%. This method trades precision for speed and is best treated as a preliminary estimate, not a substitute for core logging where core is available.
| RQD (%) | Quality Class | Description |
|---|---|---|
| 90 – 100 | Excellent | Very few fractures; rock mass is essentially intact with widely spaced discontinuities. |
| 75 – 90 | Good | Few fractures; moderate joint spacing with mostly intact core. |
| 50 – 75 | Fair | Moderate fracturing; noticeable jointing with some short core pieces. |
| 25 – 50 | Poor | Significant fracturing; many short pieces and frequent joint intersections. |
| 0 – 25 | Very Poor | Heavily fractured or crushed; most pieces shorter than 100 mm. |
Once RQD is known, it converts directly to an RMR parameter score using Bieniawski's 1989 rating table:
| RQD (%) | RMR Rating (max 20) |
|---|---|
| 90 – 100 | 20 |
| 75 – 90 | 17 |
| 50 – 75 | 13 |
| 25 – 50 | 8 |
| < 25 | 3 |
Use the "Use this RQD in the RMR Calculator" link above the result box to carry your value straight into the full Rock Mass Rating calculator without re-entering it.
RQD is fast and reproducible, but engineers should be aware of four well-documented limitations before relying on it alone.
Insensitive above 100 mm spacing. RQD treats every piece 100 mm or longer identically, regardless of how much longer it is. A core run made entirely of 110 mm pieces and one made entirely of 500 mm pieces both score RQD = 100%, despite representing very different block sizes and very different rock mass behavior.
Directional bias. RQD depends heavily on the angle between the borehole and the dominant joint set. A hole drilled parallel to the main joint set intersects few of those joints and reads artificially high; a hole drilled perpendicular to the same joints intersects many and reads lower. Multiple borehole orientations are recommended wherever the joint pattern is anisotropic.
No joint condition data. RQD says nothing about joint roughness, infill material, weathering, or water pressure — all of which strongly affect how a rock mass actually behaves under load. This is precisely why RQD is used as one input among several in RMR and the Q-System, not as a standalone design parameter.
Arbitrary threshold. The 100 mm cutoff was chosen for practicality, not derived from rock mechanics theory. A few researchers have proposed alternative thresholds for special cases (such as 200 mm for very massive rock), but 100 mm remains the universal convention used in both RMR and Q-System tables, and is the value this calculator applies.
Based on Deere 1967 (core method) and Palmstrom 1982 (volumetric Jv method). For preliminary assessment — qualified geotechnical review required for final design.