American residential door hardware is unusually standardized, which is why this question has a reassuring answer most of the time. The vast majority of exterior doors in the US were prepped at the factory to the same dimensions, and every major lock brand builds to them. The locks that do not fit are not fitting because of one of five specific things — so measure those five and you will know before you spend anything.
Measurement 1: the backset
The backset is the distance from the edge of the door to the center of the round hole the lock sits in. In the US, it is almost always either 2-3/8 inches or 2-3/4 inches, and most replacement deadbolts ship with an adjustable latch that handles both — so this is usually a non-issue that only becomes a problem on an unusual custom or imported door. Measure it with a tape from the door edge to the middle of the cylinder. If the number is not close to one of those two, you are in custom territory and a retrofit lock is the safe route, because it does not care about the backset at all.
Measurement 2: the door thickness
Measure the edge of the door. Standard US exterior doors are 1-3/4 inches thick; interior and some older doors are 1-3/8 inches. Replacement locks typically state a supported range covering both, sometimes with an included spacer or a longer screw set for the thicker case. Where thickness genuinely bites is at the extremes: a very thick custom door, a solid-core slab with applied panels, or a door with heavy interior molding. The Level Lock+ is the interesting case here because its mechanism lives inside the bore rather than in an exterior escutcheon, which changes what "fits" means — read its published range before you assume either way.
Measurement 3: the bore holes
Standard prep is a 2-1/8 inch circular hole through the face of the door and a 1 inch hole drilled into the edge for the bolt. If your door already has a deadbolt, these holes already exist and already match — that is the whole point of standardization, and it is why a deadbolt swap is a screwdriver job rather than a drilling job.
You only need to think about this if your door has no deadbolt and you are adding one. That means drilling, which means a hole saw, a template and the risk of doing it slightly wrong in a door you cannot un-drill. If the door has no deadbolt and you are not confident, that is the one scenario on this page where paying a locksmith for an hour is genuinely good value.
Measurement 4: interior clearance — the one retrofit buyers miss
If you are buying a retrofit lock — one that mounts over your existing deadbolt and turns the thumb-turn with a motor — the backset and bore do not matter at all, because you are not touching them. What matters instead is physical room on the inside of the door. Retrofit bodies are noticeably larger than a thumb-turn, and three things take the space away: deep interior trim or molding close to the lock, a thumb-turn set unusually near the door edge, and anything that swings across the door such as a storm door, a closet door or a coat hook.
The test takes ten seconds. Hold your phone flat against the door where the lock body would sit, and open the door fully. If the phone clears everything, a retrofit will clear it too. This is the single most common reason a retrofit lock goes back, and it is entirely avoidable.
Measurement 5: does your current deadbolt actually work properly?
Not a measurement, and the most important item on the list. Go and throw your deadbolt now. If you have to lift the door, lean on it, pull the handle toward you or jiggle the key, your bolt is binding against a misaligned strike plate — and a smart lock will make that worse, not better. A human adjusts unconsciously. A small motor cannot: it will strain, drain the batteries, log jam errors and eventually fail to lock when you need it to.
The fix is usually adjusting or repositioning the strike plate so the bolt enters cleanly, which is a twenty-minute job with a chisel and a screwdriver and should be done before the lock arrives. This applies to retrofit locks most of all, because a retrofit is doing nothing but turning your existing, possibly misaligned, bolt. Our install guide covers the alignment check in the right order.
The doors where the answer is genuinely "no"
Five cases, and it is better to know now.
Sliding patio doors. A deadbolt smart lock does not fit a slider, full stop. Different hardware, different problem.
Mortise locks. Common in older apartment buildings and pre-war homes: the lock body sits in a deep rectangular pocket cut into the door edge rather than in round bores. Most consumer smart locks will not fit a mortise prep, and the ones that do are specialist products. A retrofit may still work if there is a separate conventional deadbolt with a thumb-turn.
Multi-point locks. Doors — often uPVC or fiberglass — where turning the handle throws several bolts into the frame at once. The smart-lock ecosystem for these is thin.
Glass and full-glass doors. No stile to mount into.
Metal doors with no existing deadbolt. Technically drillable, practically a job for someone with the right bits and nerve.
If you are in one of those categories and still want keyless access to something, the answer is often a different device class — a smart padlock for a gate, shed or storage, or a no-drill video doorbell for the access problem a lock was going to solve.
Retrofit versus replacement, decided by what you measured
Here is the decision tree in one paragraph. If your bore and backset are standard, your door is a standard thickness and your bolt throws smoothly, a replacement lock gives you a keypad on the outside and a cleaner look — buy Schlage Encode Plus or Level Lock+. If anything is non-standard, if you rent, or if you simply do not want to remove the existing hardware, buy a retrofit — it sidesteps the backset, the bore and the thickness entirely and only asks for interior clearance. If your bolt binds, fix that first regardless of which you buy. For the rental version of this decision in full, see best smart locks for renters, and for the whole field ranked, best smart locks and how to choose a smart lock.
The four locks below
Two retrofits and two replacements, chosen for the fit cases this page is about rather than ranked on features. If your door is standard and you have decided on a replacement, the direct comparisons worth reading are Level Lock+ vs Schlage Encode Plus and Level Lock+ vs August.
Take four measurements and a photo before you order
Write these down: door thickness at the edge, backset from edge to cylinder center, the distance from the thumb-turn to the nearest obstruction on the inside, and the distance from the door edge to the nearest obstruction on the outside (trim, sidelight glass, a storm door frame). Then photograph both faces of the door around the lock. Those four numbers and two photos answer every fit question on this page, and they let you check a lock's published dimensions against your actual door rather than against an assumption.
Fix the strike plate first, always
If the bolt does not glide into the strike with the door at rest, stop and fix that before the lock arrives. Loosen the strike plate screws and reposition it, or file the opening slightly, so the bolt enters with no resistance. A motor has no judgment and no leverage; a bolt that a human has been muscling closed for years will defeat it. Doing this first turns most smart lock installations from frustrating into boring, which is what you want from a lock.
Check what the lock needs behind it, not just in front
Beyond physical fit, three practical requirements catch people out. The lock needs 2.4GHz Wi-Fi within reach of the door if it has built-in Wi-Fi — a front door at the far end of a house is a common dead spot. It needs a battery compartment you can actually open with the door in place. And if it needs a hub, that hub needs an outlet somewhere sensible, which is covered in do smart locks need a hub.
Keep the old lock and the old keys
Whichever lock you fit, box the original hardware with its screws, its strike plate and its keys, and label it. If you rent, that box is how you hand the apartment back cleanly. If you own, it is how you get back into your house on the day something unexpected happens to the new lock — and the honest discussion of that scenario is in do smart locks work without power and can you use a key with a smart lock.
