Article

Door Thickness Standards: 1-3/8 vs 1-3/4

Updated October 11, 2026

The two numbers that cover 95% of US homes

Interior doors are usually 1-3/8 in thick. Exterior doors are usually 1-3/4 in. If your door matches one of these, nearly every residential lock fits out of the box. Anything else — 1-1/2 in, 2 in, or thicker — needs a thick-door kit or a commercial lock.

How to measure it right

Hook calipers on the slab edge at three points: top, middle, and bottom. Use the largest reading. Exclude weatherstrip, trim, and paint buildup. A door that reads 1.78 in at the middle but 1.75 in elsewhere is a 1-3/4 in door with a high spot, not a thick door.

What thickness decides

Standard 1-3/8 to 1-3/4 in

Stock deadbolts, knobs, levers, and retrofit smart locks fit. Check the thickness checker to confirm, then run the three-dimension fit before ordering.

Over 1-3/4 in

Order the manufacturer’s thick-door kit (longer screws, extended tailpiece, longer connecting bolts). Retrofit smart locks are the first thing to fail on thick doors because the interior escutcheon cannot clamp — verify with the smart-lock fit checker.

Under 1-3/8 in

Common on closet and-linear hollow doors. Most deadbolts will not clamp; use passage or privacy function, or replace the slab.

Pair this with the four prep readings and the compatibility checker for a complete pre-purchase screen.