How do I know what size cylinder my door has?

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It is measured from the centre of the screw that holds the cylinder, on the edge of the door, to each of its ends, the outside one and the inside one. That gives two measurements, one for each side.

Open the door and look on the edge, next to the lock, for the long screw that goes through the faceplate. That screw marks the centre of the cylinder. Measure with a ruler from there to the outer end and then to the inner end, in millimetres.

If the two measurements match, the cylinder is symmetrical, for example 30 by 30. If not, it is offset, such as 30 by 40, and you have to keep the right side facing outwards.

A cylinder that sticks out too far is a weak point, so it should be matched to the door. There is an illustrated guide in how to measure the cylinder. If you get in a muddle, send us a photo on WhatsApp.

The service that solves it

Cylinder size calculator

Measure both sides of the cylinder and get its approximate size, with a warning if it would stick out of the door.

Related terms

Other questions on this topic

What is the difference between a lock cylinder and a lock?

The cylinder is the part the key goes into; the lock is the mechanism inside the door that moves the bolts. Often changing the cylinder is enough. Read the full answer

What is an anti-bump cylinder?

It is a cylinder built to resist bumping, a technique that opens basic cylinders by tapping a doctored key, leaving hardly any marks. Read the full answer

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Lock cylinders

Everything people usually ask about the lock cylinder: when to change it, how to measure it, what protection it has and how to look after it on the coast.

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