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What Are The Key Sizes Needed To Replace My Door Lock?

Sunday, 23 March 2014  |  Israr Sarwar

ALTReplacing a door lock can be a straight forward DIY task, but sometimes choosing the right lock can prove difficult.

When replacing your lock there are a few key dimensions that you need to take down from your old lock to help you select the correct replacement, so we thought it would be a good idea to share these with you. Below we have tried to demystify the 4 key dimensions of a door lock to make that swap over quick and simple:

So our 4 key dimensions when replacing your door lock are...

1) Lock Centres

The majority of door locks are sashlocks and these provide 2 key functions within one lock. Firstly they allow you to open a door with a door handle and secondly they allow you to lock the door either with a key or a bathroom turn & release.

You will notice that on a sashlock you will have a square hole (located in the follower) for the door handle spindle and then underneath it you will see a keyhole. The lock centre is the distance between the centre of the square spindle hole and the centre of the circular part of the keyhole. If you have a bathroom lock then rather than a keyhole you will have another spindle hole.

This should be clearer on the illustration below:

Lock Centres Diagram

2) Backset

The Backset is simply the distance between the edge of the door where the lock is installed, to the centre of the square spindle hole (located in the follower).

This dimension is essential when replacing a lock as you already have holes drilled through the door to match the old backset and changing the backset can be awkward.

Lock Backset Diagram

3) Lock Case Size

The Lock Case Size is the depth of the lock case and this generally ties into the Backset dimension (i.e. a larger lock backset will generally mean a larger lock case size).

Matching the existing case size is important as this dictates the existing cut-out that has already been made in the door. The existing cut-out in the door can be modified if required but this will lengthen the replacement time.

Lock Case Size Diagram

4) Spindle Hole Size

The spindle hole is the square hole in the lock that is located within the lock follower (Latch Spindle). The spindle hole size dictates the size of spindle that can be used on the door handles and matching the existing size will ensure that you do not need to replace your door handles. Generally within the UK the spindle hole on door handles is 8mm.

Word of caution though, on bathroom locks you will have 2 spindle holes with the upper spindle hole (Latch Spindle) used for the door handle and the lower spindle (Turn Spindle) used to lock the door. Bathroom locks generally still come with an 8mm Latch Spindle to operate the door handle however the Turn Spindle can vary between 5mm or 8mm. Please ensure you check which spindle size you need for the Turn Spindle, spindle size converters are also available.

Spindle Hole Size Diagram

We hope that this has been useful and be sure to check out our full range of door locks.

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