How to measure a euro cylinder lock

Two numbers, one screw, five minutes. Here's how to get the size right first time, and the one rule that tells you if your current cylinder is wrong.

1. Find the fixing screw

Open the door and look at the thin edge. You'll see a long metal strip (the faceplate) with several screws. The one you want is the screw directly in line with the keyhole, usually a little below the handle spindle. That screw goes into the cylinder and holds it in the door.

2. Measure to each face

Hold a ruler or tape across the door edge. Measure from the centre of the screw to the outside face of the door, then from the centre of the screw to the inside face. Measure to the door surface, not to the handle plate; handles vary in thickness and the cylinder size doesn't care about them.

3. Round to 5mm

Euro cylinders come in 5mm steps from 30mm upwards. 38mm becomes 40, 52mm becomes 50. Write the result as outside/inside: 40/50 means 40mm to the outside face and 50mm to the inside face. The overall length is the two added together; the screw sits at the join.

4. The protrusion rule

A correctly sized cylinder sits flush with the handle plate or sticks out no more than about 3mm. Anything more gives a burglar something to grip and snap. If yours sticks out 5mm or more on the outside, that's the number one reason to replace it, and the number one reason to go for a 3-star anti-snap cylinder when you do.

Key both sides, or thumbturn?

If the inside has a knob you turn instead of a key, that's a thumbturn cylinder. Measure it the same way; the size is the same, you just order the "key & turn" version with the turn on the inside dimension.

Rather not measure?

Take the cylinder out (it's the same one screw, then turn the key slightly and pull), lay it on white paper with a 10p resting on it and take a photo. Our photo lock finder measures it and shows you the exact cylinder to order.