You measure sash locks primarily by two dimensions: the centres measurement and the lockcase depth.
Here's a breakdown of how to measure each dimension:
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Centres: This measurement is specific to sash locks. It's the vertical distance from the centre of the follower (the part where the door handle spindle goes through) to the centre of the keyway (where the key is inserted).
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Lockcase Depth: Also known as case width. This is the horizontal distance from the front face of the forend (the plate you see on the edge of the door) to the back edge of the lockcase (the main body of the lock).
In summary, to accurately measure a sash lock, you need these two dimensions: centres and lockcase depth. Measuring both ensures you select a compatible replacement or new installation.
