Smart Locks

The smart lock purchase gets made on app screenshots and goes wrong on a tape measure. The features people compare - remote unlock, guest codes, auto-lock, fingerprint - are broadly similar across the category. The things that decide whether the lock ends up on your door or back in its box are three measurements and one fork in the road, and none of them are in the marketing copy.

The three measurements that decide fit

Backset

The distance from the edge of the door to the center of the large hole. US residential doors are almost always 2-3/8 in or 2-3/4 in, and most replacement deadbolts ship with an adjustable latch that covers both. Older doors, commercial slabs and some imported units are neither, and an adjustable latch will not stretch to meet them.

Cross-bore and edge-bore

The cross-bore is the big face hole, normally 2-1/8 in. The edge-bore is the smaller hole in the door edge that the bolt passes through, normally 1 in. Locks with a large interior escutcheon or an in-door mechanism can need a bore you do not have, and enlarging one is a jig-and-hole-saw job, not a screwdriver job.

Door thickness

Standard is 1-3/8 in to 1-3/4 in. Thicker doors need a spindle or screw extension kit, which is usually sold separately and occasionally not sold at all. Measure the slab, not the frame.

Two more physical checks save most of the returns in this category: does the interior side of the door have a raised panel, glass insert or applied molding within a few inches of the bore that a wide interior plate would foul, and does the existing deadbolt throw smoothly when the door is closed? Both are worked through in will a smart lock fit my door.

Retrofit or replacement

This is the fork that determines almost everything else. A retrofit lock replaces only the interior thumbturn and drives your existing deadbolt. The exterior of the door is untouched, your keys keep working, nothing gets drilled, and it goes on and comes off in about ten minutes - which is why it is the default answer for renters. The trade-off is real: there is no exterior keypad unless you add a separate one, the interior unit is visibly bulky, and the whole thing depends on your existing deadbolt being smooth and well aligned, because the motor has less leverage than your wrist did.

A replacement deadbolt swaps the entire lock. You get the exterior keypad or fingerprint reader, a cleaner installation, and often a higher published grade. You also inherit a new keyway, which means re-keying or carrying a different key, and some models are fully keyless - no cylinder to pick or bump, but also no mechanical fallback, so the emergency power path matters more. Retrofit locks that need no drilling covers the first route; the main smart lock rankings cover both.

What a published BHMA grade measures

BHMA, working with ANSI, grades locks 1 to 3 under the A156 standards. The grade is earned on a bench: cycle count, bolt projection, torque and impact resistance, finish durability. Grade 1 is the highest. For electronic locks the relevant documents add tests for the keypad, the motor and the battery-fail behavior.

What the grade does not cover is everything past the metal. It says nothing about the firmware, the account recovery flow, whether the Bluetooth pairing is authenticated, or how long the manufacturer will keep the servers running. A Grade 1 deadbolt with a weak app is still a Grade 1 deadbolt, and a Grade 2 lock on a properly reinforced jamb is a better assembly than a Grade 1 lock held by half-inch screws into soft pine. In a residential door the weak component is usually the strike and the frame, not the bolt - which is why the cheapest upgrade in this entire hub is a longer set of strike screws that reach the stud.

The radio only carries remote access

The common worry here is what happens when the internet drops. For most locks the answer is: nothing much. The keypad, the fingerprint reader, the Bluetooth handshake with your phone at the door and the physical key all work locally. Wi-Fi, or a hub bridge, carries remote lock and unlock, away-from-home notifications and remote guest code management. Lose the connection and you lose those three things, not the lock. The exception is worth knowing before you buy, and do smart locks need wifi goes protocol by protocol.

Battery life follows directly from the radio. Four AA cells is the category standard. A lock with an always-listening Wi-Fi radio drains them fastest; Bluetooth, Thread and Z-Wave locks sip by comparison because the radio is asleep between events. Whichever you choose, find the emergency power path before you need it - a 9V contact, a USB-C port, or a key - and check you can physically reach it from outside.

What most buyers get wrong

Enabling auto-lock on a door that does not close squarely. The bolt meets the strike plate edge, the motor stalls, and the lock reports locked while the bolt sits in fresh air. Fix the alignment first. This is also the single best argument for pairing a lock with a contact sensor on the same door: the lock knows the bolt moved, only the sensor knows the door is actually shut.

Treating fingerprint as a security feature. It is a convenience feature, and a good one. Capacitive readers get less reliable with cold, wet or dry hands, so every fingerprint lock needs a code or key you have actually memorized or carried.

Buying a protocol without the thing that speaks it. A Z-Wave deadbolt with no Z-Wave controller is a keypad deadbolt. A Thread-only lock with no border router is a Bluetooth deadbolt. Both are fine purchases if that is what you meant to buy.