PRACTICAL GUIDE • MEASURING UP
How to measure a euro cylinder
Measure from the centre of the fixing screw hole to each end of the cylinder separately, and quote the outside measurement first. A cylinder is described as two numbers, such as 40/50, because the halves are usually different lengths. Getting it wrong by 10mm either leaves the door insecure or stops it closing.
What you need
A steel rule or tape measuring in millimetres, and two minutes. A ruler marked only in inches will not do: cylinders are sold in 5mm steps and converting from fractions is how people end up 5mm out.
You do not need to remove the cylinder to measure it, though it is easier if you do.
The measurement, step by step
- Open the door and look at its edge. Along the metal faceplate, below the latch, is a single screw holding the cylinder in place. That screw hole is the centre point for both measurements.
- Measure from the centre of that screw hole to the outside face of the cylinder. The outside is the side you put your key in from the street. Measure to where the cylinder ends, not to the outside of the handle plate.
- Measure from the centre of that screw hole to the inside face. Same method, other side.
- Write the outside measurement first, so a cylinder that is 40mm outside and 50mm inside is a 40/50. Some suppliers list it the other way round, so say which is which when ordering.
- Add the two together and check. A 40/50 is a 90mm cylinder overall. The total should be a multiple of 5, and if it is not, measure again.
- Note the type while you are there. Single, double, or thumbturn on the inside, and whether the key enters from both sides.
Why the two halves are usually different
The screw hole sits where the cylinder passes through the lock case, and that is rarely in the middle of the door. Door thickness, handle backplates and the position of the mechanism all push it off centre. A 35/45 and a 45/35 are physically different cylinders and are not interchangeable: fitted the wrong way round, the long half sits outside, projecting, which is the exact arrangement lock snapping relies on.
The mistake that matters
A cylinder should sit as close to flush with the outside handle plate as possible, and certainly no more than about 3mm proud. People commonly buy a cylinder that is too long, because it fits, the key turns and the door works.
What it also does is leave a length of brass sticking out of the door for someone to grip. That is the single most common avoidable weakness on a uPVC front door, and it is created at the moment of ordering rather than by any fault in the lock.
Too short is a different problem and immediately obvious: the cam will not reach the mechanism, or the fixing screw will not bite.
What to buy, once you have the number
With the two measurements you have what you need. Worth deciding at the same time:
- Anti-snap rating. TS007 three star or Sold Secure Diamond, for a front door.
- Thumbturn or key inside. A thumbturn means no key needed to get out, which matters for escape; a key both sides means a glazed door cannot be unlocked by reaching through. The right answer depends on the door and who uses it.
- Keyed alike. Several doors on one key, if you have more than one.
- Number of keys. Three to five is typical.
Or let someone else measure it
If you would rather not, a cylinder replacement is listed from £120 with the cylinder measured, supplied and fitted and new keys included, with call-out and labour at the published rates. It is also the better route if the existing cylinder is stiff, if the key has to be jiggled, or if the door has to be lifted to lock — because in that case the cylinder is probably not the only thing that needs attention.
Common questions
Can I measure the cylinder without taking it out?
Yes. Open the door, find the fixing screw on the faceplate, and measure from the centre of that screw hole to each end. Removing the cylinder makes it easier but is not necessary, and removing it means the door cannot be locked until the new one arrives.
What does 40/50 mean on a euro cylinder?
40mm from the centre screw to the outside face, 50mm to the inside face, 90mm overall. The outside measurement is conventionally written first, but not every supplier follows that, so state which half is which when ordering.
What happens if the cylinder is too long?
It projects beyond the handle, giving an attacker something to grip, which is what snapping relies on. The door will work normally, which is why the mistake goes unnoticed. Anything more than about 3mm proud should be changed.
Are all euro cylinders the same shape?
The profile is standardised, which is why a replacement fits the hole. The length, the cam position and the security rating are not standardised, and those are what you are choosing between.
Can I get all my doors on one key?
Yes, cylinders can be supplied keyed alike. One key covers front, back and garage, which is convenient — with the trade-off that one lost key affects every door rather than one.
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