Lock Shop Direct measurement guide
How to Measure a Nightlatch or Rim Lock
Use this guide to identify and measure a surface-mounted nightlatch, rim deadlock or rim sashlock before ordering a replacement.
Important: The backset is only one part of the match. Check the lock function, case and keep dimensions, door thickness, rim cylinder connection and handing.
1. Identify the nightlatch or rim-lock type
Nightlatches are surface mounted on the inside face of a door and are normally operated from outside by a rim cylinder. Before measuring, identify the function.
Standard nightlatch
Latch bolt with internal knob or handle. Some include a snib to hold the latch back.
Deadlocking nightlatch
Includes an additional method of deadlocking the latch from inside or outside.
Auto-deadlocking nightlatch
A trigger or auxiliary bolt automatically blocks the latch when the door closes.
Rim deadlock
Key-operated deadbolt only, surface mounted on the inside of the door.
Rim sashlock
Surface-mounted latch and deadbolt, often with a handle or knob.
Escape or specialist function
Designed for a specific egress or controlled-door application. Match the complete specification.
2. Measure the backset
The backset is the distance from the edge of the door to the centre of the rim cylinder or keyhole. Measure horizontally and use the centre line of the cylinder, not the edge of the cylinder pull.
Common-looking nightlatches can have different backsets, which changes the position of the outside cylinder and existing hole.
| What to check |
How to measure or identify it |
Record |
| Backset |
Door edge to centre of rim cylinder or keyhole. |
_____ mm |
| Case width |
Horizontal width of the inside lock case. |
_____ mm |
| Case height |
Top to bottom of the inside lock case. |
_____ mm |
| Case projection |
How far the case projects from the face of the door. |
_____ mm |
| Door thickness |
Measure the full door thickness at the lock. |
_____ mm |
| Fixing centres |
Centre-to-centre spacing of the main case fixing holes. |
_____ mm |
3. Measure the keep and frame arrangement
Measure the keep or staple fitted to the frame, including its height, width, projection and fixing positions. Check whether the door and frame are flush, rebated or have a large stop that affects the keep.
- Record the gap between the door edge and frame.
- Check whether the keep is surface mounted or recessed.
- Note any anti-thrust pin, trigger or auxiliary bolt and where it enters the keep.
- Check the latch bevel and door closing direction.
4. Check the rim cylinder and tailpiece
If the outside rim cylinder is being reused, confirm that its tailpiece, fixing posts and screw centres are compatible with the replacement case. The tailpiece must suit the door thickness and lock case without binding.
If replacing the complete set, record the required finish, cylinder security level and whether keyed alike or master-keyed operation is needed.
5. Handing, operation and safety checks
- State whether the door is hinged on the left or right when viewed from the inside.
- Confirm whether the latch is reversible or the lock must be ordered handed.
- Record the internal operation: knob, lever, paddle, snib, holdback or key.
- Check whether the product is intended to be used with a separate deadlock.
Escape and fire doors: Do not select a nightlatch for a fire exit or communal escape door solely by size. The hardware and complete door arrangement must be suitable for the building’s fire-safety and access requirements.
Still not sure which product you need?
Send our team clear photographs and your measurements before ordering. Please include:
- A photograph of the inside lock case and outside cylinder.
- Backset, case width, case height and door thickness.
- Photographs and dimensions of the keep or staple.
- The door handing and internal operating features.
- Any manufacturer name, model or number.
This guide is intended to help identify and measure replacement hardware. Product designs, terminology and handing conventions can vary by manufacturer. Always compare the complete product specification and fitting instructions. Where a door or fitting forms part of a fire escape, fire door, powered system or other safety-critical installation, obtain advice from a competent person before ordering or altering the hardware.