<p>This paper proposes a new brittleness index (BI) based on the rock’s pre-peak fracture energy and post-peak stress–strain relations to determine rock brittleness. A series of uniaxial and triaxial compression tests were conducted on specimens of Hawkesbury sandstone, Massangis limestone, Chassagne Beau limestone, Tuffeau limestone, and Alvand A granite using the lateral strain-controlled loading method. The BI accounts for the shape of the stress–strain curves before and after the peak and considers the amount of residual strength. The results indicate that rock brittleness increases with peak strength and decreases with increasing confining pressure. Confining pressure has a significant impact on rock brittleness, with a greater effect observed in limestone rocks compared to sandstone and granite. The experimental results suggest a classification system for rock brittleness, categorizing it into 6 classes ranging from very brittle to very ductile. The BI adequately factors in the effect of confining pressure on the ductile-brittle transition of rocks and the residual strength that is observed in confined tests, making it applicable to a broader range of rock types.</p>