Lock Hardware Basics
Before you buy a lock, you need five numbers and one letter: door thickness, backset, bore diameter, bolt faceplate size, strike position, and the handing of the door. On a standard New York apartment or brownstone door that usually means a 1-3/4 inch thick door with either a 2-3/8 or 2-3/4 inch backset. Measure those first and most of the wrong purchases disappear.
Almost every lock returned to a hardware store goes back for the same reason. The lock was fine. It simply did not match the holes already in the door, the thickness of that door, or the direction it swings. New York makes this worse than average, because the housing stock is a mix of hollow metal apartment doors, prewar wood doors carrying mortise locks, brownstone entry doors with rim hardware bolted to the inside face, and newer units with multi-point patio doors. One template does not cover them.
This guide walks the measurements in the order a locksmith takes them, with the New York specifics called out. If you already know what hardware you have, our guide to identifying your door lock type is the companion piece; this one is about the dimensions.
Measure the edge of the open door with a tape, not the frame. Two numbers cover most doors:
Anything thicker than 1-3/4 inch is a real possibility on prewar buildings and custom entry doors, and it matters most for two things. First, a cylindrical lock needs a tailpiece long enough to reach. Second, a rim lock or mortise lock needs a cylinder of the right length, and mortise cylinders are sold in 1/8 inch steps. Measure to the nearest 1/16 inch and write it down rather than rounding in your head.
Backset is the distance from the edge of the door to the center of the large bore hole where the knob or deadbolt sits. Measure horizontally across the face of the door, from the edge to the middle of the hole, not to the edge of the hole.
Many retail deadbolts ship with an adjustable latch that covers both 2-3/8 and 2-3/4 inch. Many do not, and the ones that do not are often the better locks. Check the box before you get to the door.
A door prepared for a modern cylindrical lock has two holes. The crossbore is the round hole through the face of the door, almost always 2-1/8 inch in diameter on residential prep. The edge bore is the smaller hole drilled into the edge of the door for the latch or bolt, usually 1 inch.
Measure both. If the crossbore in your door is smaller than 2-1/8 inch, you are probably looking at an older or non-standard prep, and the new lock will need the hole enlarged. If there is no crossbore at all because the existing lock is a rim or mortise type, do not assume you can simply drill one. Keep reading to the New York section below.
On the edge of the door, the metal plate around the latch or bolt is the faceplate. Note its shape and size. A rectangular faceplate roughly 1 inch by 2-1/4 inch is the common residential deadbolt size. A radius corner plate and a square corner plate are not interchangeable without touching up the mortise in the door edge, which is a visible repair if you get it wrong.
Note the bolt throw as well, meaning how far the bolt extends when you turn the key. A full inch of throw is what you want on an entrance deadbolt, and it is one of the differences between hardware grades. Our guide to ANSI lock grades explains what those numbers actually test.
Handing describes the hinge side and swing direction together. Knobs and most residential deadbolts do not care. Mortise locks, lever sets with a fixed return, multi-point gearboxes, panic hardware and many electronic locks care very much.
The convention locksmiths and manufacturers use is to stand on the outside of the door, meaning the secure side, the side you need a key for:
Apartment entrance doors in New York are frequently a reverse hand, because the door opens inward into the apartment while the hallway is the keyed side. If you are ordering a lever or an electronic lock online and the listing asks for handing, this is the question it is asking, and getting it wrong usually means a return rather than a field fix.
Measurements only mean something inside a format. Four cover nearly all New York doors.
Knob, lever or deadbolt mounted through a round crossbore. This is the format most retail hardware is built for, and the one where backset and bore size are the deciding numbers. See whether you need a locksmith to install a deadbolt.
A rectangular steel case buried in a pocket inside the door edge, common on prewar apartment and building doors. Here you are measuring the pocket, the backset, the spacing between the knob spindle and the cylinder, and the cylinder length and cam. These are not retail purchases in practice. Our guide to mortise lock repair in NYC covers what can be rebuilt and what has to be replaced.
The lock body bolts to the inside face of the door and the cylinder passes through to the hallway. The jimmy proof or so called police lock on thousands of New York apartment doors is this family. The critical dimension is cylinder length relative to door thickness, plus the distance from the door edge to the cylinder. See jimmy proof and police locks.
Tall patio, terrace and French doors that throw bolts at several heights from one handle. These are measured by gearbox model, spindle size, backset and the spacing of the lock points, and parts are brand specific. See multi-point lock repair.
Three local realities change the answer even when your tape measure is accurate.
The lock is half the assembly. Measure the strike opening in the jamb and note whether it is a flat plate, a box strike with a metal cup behind it, or a T shaped strike. Note the screw spacing, and whether the screws reach past the jamb into the framing behind it. On a wood frame, long screws into the stud do more for security than the next grade of deadbolt, which is why strike plate and frame reinforcement is usually the better dollar.
Write these down before you shop, and photograph the door edge, the face around the lock, and the strike in the jamb:
That list is also exactly what a locksmith wants on the phone. It is the difference between a van that arrives with the right cylinder and a van that has to come back. If you are choosing hardware rather than matching it, the best lock type for a New York apartment is the next read.
Measure first, then decide honestly. Hand this one off when:
For everything else, accurate numbers make this a short job. We do lock change and rekey work across the five boroughs, and our residential locksmith page covers the rest of what goes on a home door.
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Need a licensed NYC locksmith? Emergency Locksmith NY is licensed and insured, NYC DCA License #1362785-DCA, serving all five boroughs 24/7. Call (718) 412-1332.
Residential doors use either a 2-3/8 inch or a 2-3/4 inch backset, measured from the edge of the door to the center of the large bore hole. Mortise locks commonly use 2-1/2 inch. Measure to the middle of the hole rather than its edge, and check whether the lock you are buying ships with an adjustable latch that covers both residential sizes.
Stand on the outside of the door, meaning the side you need a key for. If the hinges are on your left and the door swings away from you it is left hand, and if it swings toward you it is left hand reverse. The same applies on the right. New York apartment entrance doors are often a reverse hand because the keyed side is the hallway and the door opens inward.
Usually not. A standard residential deadbolt and most knobs are not handed. Handing matters for mortise locks, lever sets with a fixed return, multi-point gearboxes, panic hardware and many electronic locks. If a product listing asks you to choose a hand, assume it genuinely matters and measure before ordering.
Ask building management before you drill. Many New York apartment entrance doors are steel and part of a labeled fire rated assembly, and cutting new holes can affect that rating. Steel doors also have reinforcement only in specific places, so a new hole may land where there is nothing to hold the lock. Existing factory prep is almost always the better path.
Most apartment entrance doors and exterior doors are 1-3/4 inch thick, and most interior passage doors are 1-3/8 inch. Prewar and custom entry doors can be thicker. Thickness decides tailpiece length on a cylindrical lock and cylinder length on a rim or mortise lock, so measure the edge of the open door rather than assuming.
Door thickness, backset, crossbore and edge bore diameters, faceplate size and corner shape, the handing, the existing lock format, any brand stamped on the faceplate or cylinder, and the strike type in the jamb. Photographs of the door edge, the face around the lock and the strike are worth more than a description. That is enough to arrive with the right part the first time.