Backset and Door Prep Measurements

Why measurements decide the whole purchase

A lock can look right on the shelf and still fail to fit your door. The holes may be the wrong size, the latch may sit too far from the edge, or the faceplate may not match the cut in the door. These issues are common and avoidable, and almost all of them come down to a handful of measurements.

The measurements describe the door as it was prepared, which is what locksmiths call door prep. Someone drilled holes and cut a recess in the edge so that a certain lock could be mounted. A new lock must match that preparation, or the door must be altered. Altering a door is possible, but it means cutting wood, which is not reversible.

Knowing the numbers also helps you talk to a seller or a locksmith. Instead of saying it is a normal door, you can say the backset is a certain distance, the door is a certain thickness and the hole in the face is a certain size. That detail makes advice much more precise.

This page uses everyday language and a ruler, not specialist gear. If any step seems uncertain, a locksmith can measure for you, check that you have authority over the door, and explain the work and the price before starting.

Backset: the distance from the edge to the center

Backset is the distance from the edge of the door to the center of the large hole drilled through its face. It tells you how far the knob, lever or deadbolt sits from the door's edge. The latch or bolt must reach the edge from that hole, so the lock has to be built for the right backset.

Two backset sizes are very common in houses: one a little over two and a quarter inches, usually called two and three eighths, and a longer one a little under three inches, usually called two and three quarters. Many locks include an adjustable latch that can be set to either size, which makes fitting simpler. Some older and specialty doors have other backsets, so do not assume.

To measure, close the door and place a ruler or tape on the edge of the door at the latch. Measure to the center of the hole in the face. If you cannot see the center clearly, measure from the edge to the near side of the hole and add half the hole's width. An error of an eighth of an inch matters less than you might fear, but a mistake of half an inch does.

Deadbolts usually have the same backset as the door's main lock, but not always. Measure each hole separately. Our guide on a plain guide to deadbolt types shows how bolts are arranged.

A short example shows how this plays out. A homeowner buys a new deadbolt because the old one is stiff. At home, the new lock will not seat: the latch pokes out a quarter inch too far from the face, and the bolt will not align with the strike. A simple measurement would have shown that the door has the longer backset, and that the lock needed its latch adjusted before installation. Many fitting problems are of this kind, and a careful ruler beats a hopeful guess.

Another household replaced a lever handle on a door prepared for a knob, and found that the spindle hole in the new handle sat at a different height from the old one. The measurements were fine, but the mechanism was not. Looking at the old hardware before buying, and comparing how the parts connect, avoids that trap.

Cross bore: the big hole through the face

The cross bore is the large round hole drilled through the face of the door, where the lock body sits. In many homes it is about two and one eighth inches across. Measure its diameter with a ruler or a tape, across the widest point of the hole.

Some doors have a smaller cross bore, and some specialty locks need a larger one. If the hole is too small, it can be enlarged with a proper tool. If it is too large, the lock may not seat properly, and a cover plate or adapter may be needed. These are tasks to leave to someone who has the right tools.

The cross bore is the most visible part of the preparation. You see it as the hole behind the knob or deadbolt when the old lock is removed. Take a photograph with a ruler held beside it. That image helps a locksmith identify the door's preparation without a visit.

If your door has no cross bore at all, such as a new unprepared door, the lock installer will drill it. Make sure they use the right size for the lock you chose, and ask them to check the backset before cutting.

• Backset is measured from the door edge to the center of the large hole.
• Cross bore is the diameter of the large hole through the door face.
• Edge bore is the diameter of the smaller hole in the edge for the latch.
• Faceplate size is the size and shape of the plate set into the edge.

Edge bore, latch and faceplate

The edge bore is the smaller hole drilled into the edge of the door, running toward the cross bore. The latch or bolt slides into this hole. A common size is about one inch. Measure its diameter with a ruler, and note whether the hole is clean or ragged.

The faceplate is the plate on the edge of the door that surrounds the latch. It can be round, square cornered or rectangular, and it may be set flush into a shallow recess. A square corner faceplate needs a squared recess, while a round faceplate needs a round one. Fitting a square plate into a round recess leaves gaps, and the reverse forces the wood to be cut.

Measure the width and height of the faceplate, and note the corner style. Some latches come with both round and square plates, so you can match the existing recess. Others come with a drive in latch that has no plate at all, which fits a plain hole.

Also note whether the latch is a spring latch, a deadlatch or a deadbolt. Each interacts with the strike plate on the frame differently, which is why the frame matters too. Our page on how strike plates affect door security explains that side.

Hardware type affects the measurements too. A lever handle needs clearance so that it does not strike the door frame or a nearby wall when it turns. Check the space around the handle on the hinge side and the latch side before choosing a long lever. A handleset, which pairs a grip with a thumb latch, has its own spacing between the grip and the deadbolt, and that spacing must match the holes. Our guide on choosing between knob, lever and handleset locks covers those styles.

Think also about how the door is used. An entry door that is opened and closed hundreds of times needs sturdy preparation, while a closet door asks for much less. Match the quality of the hardware to the amount of use, and be sure that the preparation is clean and square.

Door thickness and why it matters

Most residential doors are close to one and three eighths inches thick for interior doors or one and three quarters inches thick for exterior doors. Locks are made to fit within a range of thicknesses, and some need a thicker door than others. A lock that is too short for the door will not connect, and one that is too long may bind.

To measure, open the door and place a ruler across the edge. Measure at the latch, where the preparation sits. Our guide on door thickness and lock fit goes further and explains what to do when a door falls outside the usual sizes.

If your door is unusually thick or thin, a locksmith can often find hardware to suit or add extensions. Older doors, glass doors and custom doors sometimes need special parts.

Weather can influence thickness slightly. A wooden door that has swollen in humid weather may measure a little more than in dry months. For hardware fit, small changes rarely matter, but they can affect how the door closes, so check the door's operation as well as its size.

If you are replacing a lock set that was installed by a previous owner, do not assume it was fitted well. Holes may be off center, recesses may be deeper than needed and shims may be hiding a mismatch. Measure what is actually there, not what ought to be there. When something looks odd, take a photograph and ask.

Keep in mind how old hardware sits on the door. Paint can build up around a faceplate and make it look thicker than it is. A recess that has been filled and repainted can hide its true size. Run a fingernail along the edge to feel where the plate really sits. These small checks add precision without special tools.

Write the measurements in pencil on a card as you go, and double check each number before you shop. Measuring twice costs a minute. A return trip to the store, or a lock that does not fit, costs an afternoon.

Taking the measurements step by step

Gather a tape measure, a pencil, a notepad and a phone for photographs. Start with the door open and the old lock removed if you can do so safely. If the lock stays in place, measure what you can and photograph the rest.

Write the numbers in order: backset, cross bore diameter, edge bore diameter, faceplate width and height, faceplate corner style, door thickness. Add the door's swing direction and which side the hinges are on. Handing matters for some locks, and our guide on left and right door handing explained shows how to tell.

Take photographs from four angles: the face with the lock, the edge with the latch, the face of the opposite side, and the frame where the latch enters. Include a ruler in at least one photo. If anything looks damaged, such as a split near the hole, photograph that too.

With these notes, you can read the packaging on a new lock and check each number. If something does not match, ask before buying. It is far easier to return a lock before it is installed.

Strike plate and frame measurements

The door is only half the story. The frame has its own preparation, with a recess for the strike plate and holes for the bolt. The strike must line up with the latch and the bolt, or the door will not lock properly. Check the height of the strike relative to the latch, and see whether the bolt enters cleanly.

If a door has dropped slightly on its hinges, the bolt may rub or miss the hole. Adjusting hinges or the strike may fix this. A locksmith can advise whether the issue lies with the lock or the door. Our page on when a door will not latch explains the usual causes.

Screw length in the strike also matters. Long screws that reach the framing behind the jamb add strength, which is why they are recommended by many installers. Our guide on screw length and strike plate security explains this.

When you replace a lock, take a moment to check the frame. A new lock on a worn or cracked jamb will not give the benefit you expect.

Special cases: glass doors, steel doors and sliding units

Not every door follows the usual patterns. A steel or fiberglass door may have a prepared lock block that sets limits on what can be installed. A glass door may use a surface mounted or patch fitting lock with no holes in the glass at all. A sliding door uses a hook or mortise latch built into the frame.

For these doors, measurements differ, and a locksmith with the right experience will serve you better than a guess. Take photographs and note the brand if it appears on the hardware. Our page on sliding glass door lock basics and the one on glass storefront door locks explained cover those types.

Mortise locks require their own measurements, including case size and spindle position. If your door has a long thin plate on the edge, see rim, mortise and cylinder basics to identify it first.

Doors on garages, sheds and gates may have other arrangements again. If in doubt, ask. Do not drill into a door you do not understand.

Finish and weather affect the choice of hardware as well. In humid, salty or storm prone areas such as Central Florida, exterior locks face more moisture than most. Look for finishes made for outdoor use, and keep the faceplate and strike clean. Our page on lock finishes and humid air offers more detail.

Finally, keep the original packaging or the instruction sheet of any lock you install. It often contains a template for the holes, a note of the backset range and the fit range for door thickness. When you need parts or help later, those details save a lot of guessing.

When to hand it to a locksmith

If measurements are confusing, the door is damaged, or a previous installer left odd holes, a locksmith can save time and money. They will check that you have authority over the door, take the measurements, explain the options and the price before beginning, and test the new lock with you.

Describe your goal. Do you want to replace a worn knob, add a deadbolt, convert to a keypad, or match several doors to one key? The right hardware depends on the goal as well as the measurements.

Share your notes and photographs when you call. A locksmith who knows the backset, bore and thickness can recommend suitable products and flag any need for adjustment. Our services page on lock replacement and repair gives a sense of the work.

Keep your measurements after the job. Write them on a card and store it with your key inventory. Next time you need a lock, the numbers will be ready.

Questions people ask