Measuring door lock size accurately is essential for selecting the correct replacement or ensuring a new lock fits properly. It involves taking several key dimensions of the lock mechanism itself, typically the part installed inside the door (the mortice lock body or latch).
Key Measurements for Door Locks
To measure a door lock size, you need to identify and measure specific points on the lock body. The most common measurements, as highlighted by experts, include:
- The case depth: This measurement runs from the outer edge of the lock body (the part visible when the lock is installed in the door's edge) to the very back of the lock case. It tells you how deep the pocket needs to be routed into the door.
- The backset: This is a crucial measurement taken from the centre of the keyhole (or cylinder opening) to the leading edge of the lock case (the faceplate where the bolt/latch comes out). It determines how far the keyhole will be from the door's edge.
- The case height: This measures the total length of the main lock body case, from the top edge to the bottom edge.
- The centres: This measurement is specifically for locks that combine a handle and a separate locking mechanism (like a sash lock or deadlock with handle). It is taken from the centre of the spindle hole (where the handle fits) to the centre of the keyhole or cylinder opening. This ensures the handle and lock cylinder/keyhole are spaced correctly on the door.
These measurements are typically taken from the existing lock once it has been removed from the door.
Summary Table of Door Lock Measurements
| Measurement | Description | Where to Measure From/To |
|---|---|---|
| Case Depth | Depth of the lock body | From the outer edge of the case to the inner back of the case |
| Backset | Distance of the keyhole/cylinder from the door edge | From the centre of the keyhole to the lock's leading edge |
| Case Height | Total height of the lock body | From the top edge of the case to the bottom edge of the case |
| Centres | Distance between handle spindle and keyhole/cylinder (if applicable) | From the centre of the handle slot to the centre of the keyhole slot |
Understanding these specific dimensions ensures that a new lock will fit the existing cut-out in the door and frame, or helps determine the correct size for a new installation. Always measure carefully, often to the nearest millimetre, especially for case depth and backset, as these are critical for fitting.
