Weather you want to use a drill press or a hand held drill, the drill bit must be secured with a drill chuck. A drill chuck has three clamps located inside the chuck housing. As you turn the key on the side of the drill chuck, the three clamps close in and tighten around the drill bit. A drill chuck can fit a specific range of drill bits. However, when measuring a drill chuck, the idea is to determine how large a drill bit you can place inside the drill chuck.
Disconnect the drill from its power source if the drill is electric. If the drill is battery operated, remove the battery. This prevents possible injury. In some cases, hand held drills have a "keyless" chuck. If this is the case, do not disconnect the drill from its power source.
Loosen the chuck with the chuck key. The key fits inside a hole on the chuck. There are splines on the key that mesh with splines on the drill chuck. Rotate the key counterclockwise until the key no longer has the ability to rotate. This means the chuck is in its wide-open position. If the drill has a keyless chuck, grip the chuck with one hand, and run the drill in reverse until the chuck reaches its wide-open position.
Look at the opening in the hole of your drill chuck. Place different drill bits into the drill chuck. Graduate the size of the rill bits until you reach the first size that will not fit into the drill bit. Go back down to the previous drill bit. The size of the bit is printed on the drill bit. This is the largest size you can use in that drill.