A chuck key is a tool used to tighten and loosen the chuck part of a drill in order to hold or release a drill bit.

Chuck keys come in a variety of sizes, but the most common are 6, 8 ,10 and 13mm sizes. The size denotes the diameter of the chuck key at the point just below the gear head.

Many modern drills, especially cordless drills, don't require a chuck key as they have a fast-release mechanism which is hand operated.