RQD Calculator: Rock Quality Designation Online
This free RQD calculator determines Rock Quality Designation using two standard methods recognized in geotechnical practice. Rock Quality Designation is a fundamental measure of rock mass quality from borehole core data and serves as an input parameter for both the Rock Mass Rating (RMR) system and the Q-System. For more geotechnical assessment tools, visit our Rock Mass Tools hub.
Calculate RQD
Choose your measurement method. Results update in real time.
Total Core Run Length (mm)
Enter the total length of the core run in millimeters.
Core Piece Lengths (mm)
Enter the length of each intact core piece in millimeters. Only natural breaks count; exclude mechanical breaks from drilling.
Pieces ≥ 100 mm: 0 mm | Total pieces: 0 mm
Volumetric Joint Count (Jv)
Number of joints per cubic meter of rock mass, measured from exposed face mapping or scanline surveys. Formula: RQD = 115 - 3.3 × Jv (capped between 0 and 100%).
What is RQD?
Rock Quality Designation (RQD) is a quantitative index of rock mass quality developed by Don U. Deere at the University of Illinois in 1967. It was originally proposed as a simple, reproducible measurement that could be obtained during routine core logging to provide an objective assessment of rock mass fracturing. RQD measures the percentage of intact core pieces that are 100 mm (10 cm) or longer relative to the total length of the core run.
Despite its simplicity, RQD has become one of the most universally adopted parameters in geotechnical engineering. It serves as a direct input to the two major rock mass classification systems: it is Parameter 2 in Bieniawski's RMR system (contributing up to 20 points to the total rating) and the numerator of the first quotient in Barton's Q-System formula. RQD is routinely reported in borehole logs worldwide and is required by most geotechnical investigation standards including ISRM, ASTM D6032, and Eurocode 7.
How RQD is Measured
The standard method for measuring RQD requires NX-size (54.7 mm diameter) or larger diamond drill core. The geologist examines each core run and identifies all intact pieces of core that are 100 mm or longer, measuring along the centerline of the core. Only natural fractures are counted as breaks; mechanical breaks caused by the drilling process, identified by fresh, rough surfaces that fit back together, are ignored and the pieces on either side are treated as a single intact piece.
The formula for the core-based method is: RQD = (sum of lengths of intact pieces ≥ 100 mm / total core run length) x 100%. For example, if a 1500 mm core run contains pieces measuring 250, 80, 350, 40, 120, and 300 mm, the qualifying pieces are 250, 350, 120, and 300 mm (total 1020 mm). The RQD would be (1020/1500) x 100 = 68%, classified as Fair quality.
When borehole core is not available, Palmstrom (1982) proposed an alternative method based on the volumetric joint count Jv, which counts the number of joints per cubic meter observed on exposed rock faces. The empirical correlation RQD = 115 - 3.3 Jv provides an estimate of RQD from surface mapping data, with the result capped between 0 and 100%. This method is particularly useful during preliminary investigations when only surface exposures are available.
RQD Classification Table
| RQD (%) | Quality Class | Description |
|---|---|---|
| 90 – 100 | Excellent | Very few fractures. Rock mass is essentially intact with widely spaced discontinuities. |
| 75 – 90 | Good | Few fractures. Rock mass has 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 core pieces, frequent joint intersections. |
| 0 – 25 | Very Poor | Heavily fractured or crushed. Most core pieces shorter than 100 mm. |
RQD Limitations
While RQD is widely used and easy to measure, it has several recognized limitations that engineers should understand. First, RQD is insensitive to joint spacing when spacing exceeds about 110 mm, because all pieces longer than 100 mm contribute equally regardless of their actual length. A core run with all pieces exactly 110 mm long and one with all pieces 500 mm long would both yield RQD = 100%, despite representing very different rock mass conditions.
Second, RQD depends on the orientation of the borehole relative to the joint sets. A borehole drilled parallel to the dominant joint set will intersect few joints and produce a high RQD, while a borehole perpendicular to the same joints will intersect many and produce a lower RQD. For this reason, directional bias should be considered when interpreting RQD values, and multiple borehole orientations are recommended for comprehensive assessment.
Third, RQD does not account for joint condition, filling, roughness, or water pressure, all of which significantly affect rock mass behavior. RQD should therefore be used as one component of a multi-parameter classification system such as RMR or Q, not as a standalone measure of rock mass quality for engineering design.
Finally, the 100 mm threshold is arbitrary. Some authors have proposed modified RQD calculations using different threshold values (for example, RQD at 200 mm for massive rock), but the standard 100 mm threshold remains the accepted convention in practice and is the value used in both RMR and Q-System parameter tables.
Frequently Asked Questions
RQD is calculated as the sum of the lengths of all intact core pieces equal to or greater than 100 mm divided by the total core run length, multiplied by 100 to express the result as a percentage. Only naturally broken pieces count; mechanical breaks from drilling are excluded and the adjacent pieces are treated as one continuous piece. An alternative formula using the volumetric joint count is RQD = 115 - 3.3 Jv, where Jv is the number of joints per cubic meter measured from exposed rock faces.
RQD values of 90 to 100% are classified as Excellent, indicating very high quality rock with few natural fractures. Good rock quality falls between 75 and 90%. Fair quality ranges from 50 to 75%, Poor from 25 to 50%, and Very Poor is below 25%. These classification boundaries are universally adopted in geotechnical practice and referenced in both the RMR and Q-System classification methods.
Yes, RQD is the second of six parameters in the Bieniawski 1989 Rock Mass Rating system. RQD percentage values are converted to RMR ratings as follows: 90-100% receives 20 points, 75-90% receives 17 points, 50-75% receives 13 points, 25-50% receives 8 points, and less than 25% receives 3 points. Our calculator includes a direct link to transfer your calculated RQD value into the RMR calculator for a seamless workflow.