How to Measure Door Knob Backset Before You Order a Set

Door knob backset is the distance from the edge of the door to the center of the big hole the knob's spindle passes through. On residential doors in the US it is usually one of two standard sizes: 2-3/8 in (about 60 mm) or 2-3/4 in (about 70 mm). If your door is already drilled, the hole decides: order the backset you measure, not the one you would prefer. On a new, undrilled door you get to choose, but the choice has to suit the hardware you picked, the door's construction, the width of its stile and the manufacturer's template.

Why a 3/8-inch difference stops a set from working

A tubular latch is a short cylinder that slides into the small hole in the door edge. At its inner end sits a square or slotted hub, and the knob's spindle passes through that hub to pull the latch back. The latch is built so the hub lands exactly at the center of the big hole. Put a fixed-size latch made for 2-3/4 in into a door drilled at 2-3/8 in and the hub sits 3/8 in past the center of the hole, where the spindle cannot reach it. The reverse leaves the hub short of center. Either way the spindle misses the hub, and the set cannot go together properly.

That is why backset is a dimension you confirm before ordering, not one you can fix with a screwdriver. Some latches adjust between the two sizes; Rocky Mountain Hardware describes its passage sets as including adjustable latch mechanisms. Others are made in one size, and you name the size you need when you order. Emtek's product pages, for example, say the standard latch is for 2-3/8 in backset and to specify 2-3/4 in if required. Read the listing for the set you want, because assuming either behavior is how the wrong latch arrives.

Measure it three ways, then confirm against the door

On a properly prepared door the three methods should agree. Use the one that fits what is on your door now, and compare it with another when you can.

• Hardware still on the door: hold a tape flat on the door face at knob height, with its end flush with the door edge, and read to the center of the spindle or bore. Do not assume the decorative plate is centered on it: roses and sideplates can be asymmetric or offset. Do not hook the tape around the edge, and do not measure from the door frame.
• Knob removed, hole visible: measure from the door edge to the center of the big hole. The cross bore on most sets is 2-1/8 in across, so its center sits a little over an inch in from each side of the hole.
• Latch in your hand: measure from the front of the latch faceplate to the center of the hub. If the latch is adjustable, measure it in its installed setting, and always confirm the number against the door preparation, because the door decides what fits.

Reading the tape is where people stumble, since 3/8 and 3/4 look close together on a busy ruler. On a tape marked in sixteenths, 2-3/8 in is the sixth small mark past 2, and 2-3/4 in is the twelfth. If your reading is uncertain, measure again and check it another way. Do not round an unclear measurement to a standard size until you have confirmed it against the hardware and the door preparation. Longer backsets exist as special orders, and old houses can surprise you, but a misread tape is the likelier cause.

Which one your doors probably have

Interior doors in houses are more often drilled at 2-3/8 in, and 2-3/4 in turns up more often on exterior doors and in commercial buildings. That is a tendency, not a rule. Builders and door shops drill whatever the hardware on the job called for, so two bedroom doors in the same house can differ if one was replaced. If you are buying for a whole house, I would measure every door you are changing, not one and multiply. Write the backset next to each door on your list, along with which way it swings.

While you are at it, note the door thickness. Many interior doors are 1-3/8 in thick and exterior doors are commonly 1-3/4 in, and sets are made to fit a range. Thickness ranges vary by maker and by model, so read the listing for the specific set rather than assuming a range applies across a brand. A door outside the range a set lists needs a different set or a thick-door option from the maker.

On a new door, choose for the hardware and the frame

When the door has not been drilled, you pick, and the first step is the set you want. Check which backsets it is made for and follow the manufacturer's template. A longer backset can leave more room for your knuckles as you turn a larger knob, which is a reason to consider one, but it is not a rule for every door or every set.

The other limit is the width of the door's edge frame. On a paneled door, the bore has to sit in the solid stile beside the panel. On a door with glass or a raised panel close to the edge, a longer backset can push the round plate onto the panel molding, which is where the shorter backset earns its keep. Hollow-core doors have a related limit: the bore sits in a solid lock block inside the edge, so check that the backset you choose lands inside it. If you are hanging new doors, tell the door supplier your backset before they drill.

Entry doors: measure the latch and the deadbolt separately

On a front or back door with a tubular latch and a separate deadbolt above it, the two commonly share a backset so the knob and the deadbolt line up vertically. Commonly is not always, so when you replace one of the two, measure each existing bore's actual backset. Do not order from the other component's number alone. When you shop entry door sets or deadbolt sets, check the backset options on each product against the number on your door.

Mortise locks play by different numbers

A mortise lock is not a tubular latch in a round hole. It is a rectangular lock body set into a deep pocket cut into the door edge, and the backset is set by that body. Backset alone does not establish that a mortise lock will fit. Also confirm the lock body dimensions, the function, the spindle and cylinder locations, the faceplate, the handing and compatible trim, using the drawing for the specific lock. Emtek's mortise entry sets, for example, are offered in 2-1/2 in or 2-3/4 in backsets, and Emtek asks for the function, the backset and the handing on every mortise lock order. To start, measure from the faceplate to the center of the spindle and compare it with the maker's drawing.

Older houses with original mortise locks on the interior doors can measure to sizes that match neither tubular standard. I would not assume a modern tubular set will drop into those doors. Replacing a mortise lock with a tubular set means filling the pocket and boring the door, which is carpentry, not a hardware swap. When the numbers you measure do not match anything on the listing, send them to us through our contact page before ordering, along with the door thickness and a photo of the latch edge.

For interior doors that are already bored to one of the two standards, the choice is simple once the tape is read: pick the design you like from our interior door sets, and order it in the backset your doors already have.