Backset Explained: Why 2-3/8" and 2-3/4" Are Not Interchangeable
, by United Doors and Locks, 5 min reading time
Backset is the distance from the door edge to the centre of the bore. Get it wrong and the latch will not reach the strike. How to measure it and which to order.
Backset is the distance from the edge of the door to the centre of the bored hole that the lock sits in. There are two standard sizes in North America, they differ by three-eighths of an inch, and that three-eighths is the difference between a lock that works and a lock that goes back in the box.
The two standards
- 2-3/8 inches — the residential standard. Common on interior doors in houses and light apartment work.
- 2-3/4 inches — the commercial standard. Almost every commercial lockset, and the ANSI strike prep that goes with it, is built around this dimension.
If you are working on a commercial opening and you have no information at all, 2-3/4 is the better assumption. But assumption is not measurement, and measuring takes ten seconds.
How to measure it
Open the door. On the face of the door, measure from the edge of the door — the latch edge, not the hinge edge — to the centre of the large bored hole. Not to the edge of the hole, and not to the edge of the rose or trim, which can overhang the bore by a quarter inch or more and will give you a wrong answer.
Two other dimensions you will be asked for at the same time:
- Cross bore: the large hole through the door face, 2-1/8 inches on virtually all commercial preps.
- Edge bore: the hole in the door edge that the latch slides through, 1 inch standard.
- Door thickness: 1-3/4 inches on commercial doors, 1-3/8 on lighter interior doors.
Those four numbers describe the prep completely.
What goes wrong when the backset is wrong
Too short a backset in too large a prep. The latch does not project far enough past the door edge to enter the strike, or it enters at an angle and drags. The door will not stay shut, and no amount of strike adjustment fixes it.
Too long a backset. The lock sits further from the edge than the prep allows, the latch tube fouls the edge bore, and the trim no longer aligns. On a lever handle, an over-long backset also moves the lever inboard, which can put a knuckle-width of clearance between the lever and the frame stop.
The strike does not match. Backset determines where the latch arrives at the frame. A 2-3/4 lock in a frame prepped for 2-3/8 will present the latch three-eighths of an inch off the strike opening. This is the most common form of the mistake, because people check the door and forget the frame.
Adjustable and dual backset hardware
Some hardware removes the decision. Adjustable-backset latches fit either prep by repositioning the latch tube, which is why the DB2000 Series Grade 2 deadbolt is specified with an adjustable 2-3/8 or 2-3/4 backset — useful when you are ordering for a mixed building without a full survey. Codelocks supply a dual backset latchbolt for the same reason.
Fixed-backset hardware is not a shortcoming. The DB1000 Series Grade 1 deadbolt is built to a fixed 2-3/4 inch backset because it is a heavy duty commercial product and 2-3/4 is the commercial prep. Fixed parts have fewer moving pieces to loosen.
Cylindrical locks and levers
Cylindrical locksets are the ones where backset matters most, because the whole mechanism is positioned by the bore. The LC2600 Series and LC2400 Series are supplied with a 2-3/4 inch latch as standard, matching the ANSI strike. Order the latch to suit the prep you measured, not the prep you expect.
Mortise locks measure differently
A mortise lock has no bore, so its backset is quoted from the door edge to the centre of the spindle — typically 2-3/4 inches on commercial locks such as the ML1300 Series. What matters more on a mortise lock is the pocket dimensions and the face plate size, since those are what the door is routed for.
Narrow stile aluminium doors
Storefront and glass entrance doors are a separate case. The stile is too narrow for a 2-3/4 backset, so narrow stile hardware uses much smaller backsets — commonly 31/32 or 1-1/8 inches — with deadlatches designed for the profile. Codelocks make narrow stile locks such as the CL460 and CL465 for exactly these doors. Do not attempt to fit standard commercial hardware to a narrow stile door; the stile will not carry it.
The habit worth forming
Measure the door and the frame, write both down, and photograph the prep with a tape measure in the shot. Backset errors are almost always memory errors made at a desk, not measurement errors made at the door.
If you are ordering for a building you have not surveyed, send us what you do know — door thickness, age, and whether the openings are commercial or residential prep — and we will flag which lines need a measurement before they ship.