How to Measure Euro Cylinder Lock? | Center-To-End Method

Learn how to measure a euro cylinder lock before you order, and the whole job takes two numbers: the distance from the center of the cam hole to each end, recorded as internal/external in millimeters.

The two halves don’t have to match, and together they add up to the full cylinder length. A cylinder recorded as 30/40, for example, is 70 mm end to end.

Measuring a Euro Cylinder Lock: Two Numbers That Matter

A euro cylinder lock’s size is a pair of center-to-end distances, written as internal/external and expressed in millimeters. The euro cylinder is the rounded lock body that slots through the door and links the handles and key to the locking mechanism. The first number is the inside half in most guides, though some sellers list external/internal — what matters is that you know which side faces into the house.

Both measurements start from the same reference point: the dead center of the cam or retaining screw hole at the back of the cylinder. The cam is the rotating piece that drives the locking bolt, which is why sizing guides use its center as the reference. The most common mistakes are measuring the full visible length of the barrel and measuring to the black cam ring instead of the true center.

Once you have the two figures, the math is simple — the halves add up to the whole cylinder length:

Measuring the Cylinder After You Remove It

Removed cylinders are the easiest to size: pull the lock out, then read from the center of the fixing hole out to each end. You don’t have to remove the cylinder to get its size — the in-place method below works too — but this way gives you the cleanest numbers.

1. Remove the retaining screw — it sits in the faceplate on the door edge and holds the cylinder in place.
2. Insert the key and turn it slightly so the cam retracts from the locking mechanism.
3. Pull the cylinder out from the inside of the door.
4. Measure from the center of the fixing hole to each end of the cylinder.
5. Record the pair inside-first — 40/50 means 40 mm on the inside half and 50 mm on the outside half.

If the two numbers add up to the total length of the barrel, you measured it correctly. The sequence matches the one in Lathams Steel Doors’ measuring guide, if you want the procedure confirmed step by step.

How Do You Measure a Cylinder That’s Still in the Door?

In place, measure from the center of the retaining screw to the outside face and to the inside face — and include the thickness of the door furniture so the new lock sits flush. A cylinder gauge or a simple ruler works for this, and it’s the way to go when you’d rather not disturb a working lock.

Ignoring the handle backplates or escutcheons is a common cause of an undersized replacement. Several guides recommend measuring to the face of the furniture, not to the bare door surface, when the lock is still installed.

Fit matters as much as the numbers. The cylinder should sit flush or nearly flush with the door furniture; one that protrudes too far is easier to grip and snap, which multiple sources warn leaves the lock vulnerable, and one that’s too short may not engage properly.

If you’re replacing the lock rather than just sizing it, comparing the best cylinder locks on the market before you order saves a second trip to the hardware store.

The routine, end to end: measure from the center of the cam or fixing hole to each end, record the pair inside-first in millimeters, round up to the nearest 5 mm, and confirm the installed cylinder sits flush with the door furniture.