Measure both internal and external dimensions of your cylinder
Before you start replacing your cylinder, it is worth taking a look at the required dimensions. To select the right cylinder size, you need the external size and the internal size of your cylinder.
Measurements are always taken from the center of the cylinder. The center of the cylinder is where the retaining screw is located.
It is best to measure your cylinder while it is still installed. Measure from the outside edge of the door (including door fitting) to the center of your door (A). This will ensure that your new cylinder is not too short. Next, measure from the inside edge of your door (including door fitting) to the center of your door to find the internal dimension of your cylinder (B).
Any commercially available folding rule or angle ruler is suitable for measuring a cylinder.