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How to Measure Door-Lock Prep: Thickness, Bore, and Backset

Updated October 11, 2026

Why four readings decide everything

Every replacement lock must match the holes already in the door. Drill first and ask later, and the door is ruined. Take these four readings with calipers and a tape measure before opening any product page.

1. Door thickness

Calipers on the slab edge at the top, middle, and bottom. Keep the largest reading and exclude trim and weatherstrip. Stock residential locksets cover 1-3/8 to 1-3/4 in. Anything thicker needs a thick-door kit with a longer spindle and screws.

2. Face bore

Tape across the widest point of the large face hole. The US residential standard is 2-1/8 in, with about 1/16 in of tolerance. Older doors at 1-1/2 in cannot take a modern deadbolt without re-drilling.

3. Edge bore

The small latch hole through the door edge is 1 in on standard prep. Measure it separately: a correct face bore with an odd edge bore still means an adapter or a new latch.

4. Backset

Hook the tape on the door edge and read to the center of the bore hole: 2-3/8 or 2-3/4 in. Never measure to the trim edge. When the reading sits between standards, buy an adjustable latch.

Photograph everything

One photo per reading plus a wide shot of hinges and strike. A locksmith can quote accurately from four photos; a store cannot take back a drilled door.