There are a few measurements you’ll need to take to assess this. Firstly, measure the thickness of your door. Next, measure the thickness of the backplate of your handle on the interior side of the door. Take the same measurement on the exterior side of the door. For example, if your door width is 70mm and your backplate is 5mm thick on both the interior and exterior, then the total width will be 70 + 5 + 5 = 80mm. This value is then split (normally in half), to give two values of 40mm, which relates to a cylinder size of 40/40. If you’re still unsure about which size cylinder you need, please feel free to contact us!