The back set dimension on a window locking mechanism is referred to as the space from the front of the faceplate, to the centre of the spindle, this should be measured in mm.

The detailed image below illustrates where you should be measuring to and from to obtain the correct measurement.

Window Backset Measure

Now you have obtained the correct backset measurements you can purchase a suitable replacement.

View our range of UPVC Window Locks for a suitable replacement.