Replacing a mortise lock can be surprisingly tricky if you’re not familiar with the two key measurements that determine whether a lock will actually fit your door: the backset and the PC size (also called the centre distance).
Both measurements tell you where the spindle and cylinder sit inside the door. Get them right, and your new lock will fit perfectly. Get them wrong, and nothing lines up.
This guide explains what these measurements mean, why they matter, and how to measure them accurately.
What is the backset?
The backset is the distance between:
-
the front edge of the lock case (the faceplate), and
-
the centre of the spindle hole (square hole) or, on some locks, the cylinder hole.
For most modern European mortise locks, common backsets are 50 mm and 55 mm, though older doors often use 60 mm or other non-standard sizes.
Why backset matters
If the backset doesn’t match the existing cut-out in your door:
-
the handle sits too close to the door frame,
-
the faceplate won’t align properly, or
-
the lock simply won’t fit into the recess.
Even a few millimetres can make the difference.
Always measure—never guess.
How to measure the backset
-
Hold the tape measure at the front edge of the door (or the faceplate if the lock is still installed).
-
Measure to the centre of the spindle hole or cylinder hole.
-
Record the distance in millimetres.
You can measure with the lock still in the door or with it removed.
What is the PC size (centre distance)?
The PC size is the centre-to-centre distance between:
-
the spindle hole (square), and
-
the cylinder hole (round).
This distance determines where your handle and cylinder will sit in relation to each other.
Most common PC sizes
-
PC 55 mm — widely used in interior doors
-
PC 72 mm — common in modern heavier doors
-
PC 92 mm — typically found in new build properties and security doors
How to measure the PC size
-
Measure from the centre of the spindle hole.
-
To the centre of the cylinder hole.
-
Do not round the measurement — it must be exact.
Only buy a new lock once both the backset and PC size match your existing door.
Measuring the backset on a rim lock
Rim locks work differently from mortise locks. For these:
-
Measure from the inside edge of the rim lock
-
To the centre of the cylinder on the outside of the door
If the new lock has a different backset, you’ll need to drill a new cylinder hole through the door.
Some rim locks have different inside and outside backsets, especially on thicker doors.
An anti-burglar strip can make measuring harder — removing the lock temporarily usually helps.
Backset vs PC size — quick summary
| Measurement | What it tells you | Why it matters |
|---|---|---|
| Backset | Distance from door edge to spindle/cylinder centre | Determines whether the lock fits the cut-out |
| PC size | Distance between spindle and cylinder centres | Ensures correct alignment of handle and cylinder |
Both must be correct — otherwise the handle or cylinder will sit skewed or won’t function properly.
When to double-check your measurements
-
Older doors often use unusual backsets (45 mm, 60 mm, etc.)
-
Replacing rim locks — cylinder alignment is critical
-
Security doors almost always use PC 72 or PC 92
-
If in doubt: take a photo with a measuring tape and send it to a specialist
FAQ
1. What happens if I buy a lock with the wrong backset?
It won’t sit correctly in the door, and the handle will be misaligned.
2. Can I adjust a backset?
No — backsets are fixed. The lock must match the door.
3. Can I change the PC size?
Only by drilling new holes, which is specialist work.
4. My lock says PC55, but I measure 56 mm — why?
This almost always comes from measuring slightly off-centre. PC sizes are fixed standards.
Measure precisely through the exact centres of both holes.