Rock Quality Designation Calculator

Rating rock mass from a drill core

Rock Quality Designation, developed by Don Deere in 1964, remains one of the most widely used indices for assessing rock mass quality from drill core samples - it is a simple percentage that correlates strongly with how fractured, and therefore how mechanically weak, a rock mass is likely to be.

Worked example

For a 100cm core run containing 85cm of sound core pieces (each individually longer than 10cm):

RQD = 85 / 100 x 100 = 85.0% (Good)

RQD Range	Rock Mass Quality
Under 25%	Very poor
25% - 50%	Poor
50% - 75%	Fair
75% - 90%	Good
90% - 100%	Excellent

Only pieces of sound, unweathered core longer than 10cm along the core axis count toward the RQD numerator - heavily fractured intervals naturally break into shorter fragments during drilling, which is precisely the mechanism that makes RQD a useful proxy for rock mass integrity.