How to Use the Door Lock Backset Checker
Before buying a new door lock or replacing a door, the backset is the first measurement to get right. It's the distance from the edge of the door to the center of the hole where the knob or deadbolt sits, and if that distance doesn't match your new lock's latch length, the lock either won't fit or your door will need to be re-bored.
In the U.S., most residential doors use a 2-3/8 inch backset as the standard, which is what nearly all prehung interior and exterior doors ship with. Older homes, wider doors, and some commercial-grade doors instead use a 2-3/4 inch backset. Many modern locksets come with an adjustable latch that can switch between both, which is worth checking before you buy.
Manufacturers can vary slightly, so treat this as a starting guide rather than a guarantee. If your door already has a knob or lock installed, the most reliable way to confirm your exact backset is to measure it yourself with a tape measure, from the door's edge to the center of the existing hole.
Frequently Asked Questions
The backset is the distance from the edge of the door to the center of the hole for the knob or deadbolt. It has to match your lock's latch length, or the lock won't fit the door properly.
If a knob or lock is already installed, measure from the edge of the door to the center of that hole with a tape measure. For a new prehung door, most U.S. manufacturers default to a 2-3/8 inch backset unless specified otherwise.