To drill a hole for a door knob, you will need a hole saw or spade bit. The size of the hole saw you need depends on the type of lockset you are installing. Here are some key points from the search results:

• The standard size hole saw for a door knob is 2 1/8 inches.
• According to one source, a 1-inch hole saw or spade bit is recommended for drilling the hole for a doorknob.
• Another source recommends using a 2 1/8-inch hole saw to bore the doorknob hole through the face of the door.
• A door can be prepped for one of three different types of latches: drive-in latches, rounded corner faceplate latches, or square corner latches.
• If you want faceplates but your door is prepped for drive-in latches, you can easily prep your door for these using a 1-inch chisel.

Based on this information, it seems that a 2 1/8-inch hole saw is the most commonly used size for drilling a hole for a door knob. However, a 1-inch hole saw or spade bit may also work, depending on the type of lockset you are installing.