How to Measure a Euro Cylinder Lock (UK Guide, With the Mistakes to Avoid)

Max the Locksmith · July 2026

The short answer: a euro cylinder is measured as two figures from the centre of the cam fixing hole — outside and inside (say, 45/50), never one overall length. Ten millimetres wrong in either direction and the lock won’t fit — or worse, fits and sticks out of the door as a pre-made snapping handle for burglars. This guide covers both measuring methods, the mistakes that catch people, and what to choose while you’re at it. Prefer it done for you? Supply-and-fit runs the flat £85 + parts (+ VAT), sized by eye, guaranteed right — price guide.

Method 1: cylinder out (accurate — 5 minutes)

Open the door so you can’t lock yourself out. On the door edge, level with the cylinder, find the single long retaining screw among the faceplate screws — undo it. Insert the key and turn it slightly (roughly the 20-past position) so the cam tucks flush into the body, then slide the cylinder out toward you.

Now measure from the centre of the cam fixing hole to each end. That hole is the dividing line: the two figures — external first, internal second — are your size. A cylinder measuring 45mm and 50mm is a 45/50 (95mm overall). Sizes run in 5mm steps, most UK doors land between 40/40 and 50/50, and offset sizes are completely normal — composite doors with thick external furniture often need 5–10mm more on the outside. If yours has a thumb-turn, note which side it’s on: thumb-turn cylinders are sized and handed the same way, turn-side stated.

Method 2: cylinder in place (quick check — 1 minute)

Measure from the centre of the retaining screw on the door edge out to each face of the door, to where the cylinder ends beyond the handle plate. It’s a couple of millimetres less precise — plenty to identify your size class, and exactly the numbers to WhatsApp over so the right cylinder arrives on the first visit.

The mistakes that catch people (from jobs that started as DIY)

Measuring overall length only. A “95mm cylinder” might be 45/50 or 50/45 — fitted the wrong way round, it protrudes 5–10mm on the outside. Always two figures, always know which is external.

Buying to the old cylinder when the old cylinder was wrong. If the current lock sticks out past the handle, matching it repeats the fault. Measure the door, not the mistake.

Ignoring the flush rule. A correctly sized cylinder finishes flush to 3mm proud of the handle furniture. Anything more is leverage — protruding cylinders are precisely what lock snapping attacks, and “it only stuck out a centimetre” is a sentence that comes up at after-burglary visits more than it should.

Forcing a cam that doesn’t sit. If the new cylinder goes in but the key won’t turn smoothly, the cam is clashing with the gearbox follower — wrong cam profile or a misaligned fit. Forcing it damages the gearbox: back out and re-check rather than lean on the key.

Oiling a new stiff lock. Household oil gums cylinders up and drags dust in. Graphite or PTFE only — the same rule as for stiff, sticking locks generally.

While you’re choosing: get the right cylinder, not just the right size

Format: double (key/key), thumb-turn (key outside, knob inside — quick exits, good for families and anyone who fumbles keys), or half (one-sided, garage doors). Security: this is the moment to fit TS007 3-star or BSI Kitemark anti-snap — the price gap over budget brass is small and the difference is the entire point of 1-star vs 3-star. Convenience: several doors can be supplied keyed-alike so one key runs front, back and garage — worth deciding before buying, not after. And if you’ve just moved in, the size question is moot until you’ve changed the locks anyway — you don’t know who kept keys.

Sizing done for you, and the stock question

Max carries cylinders across the full size range — doubles, thumb-turns, halves, anti-snap as standard, keyed-alike sets — so the “measure, order, wait, hope” loop compresses into one visit with the size read by eye and the fit tested through the whole mechanism. Odd sizes (very long, very offset, restricted profiles) occasionally need ordering: when that’s yours, a correctly-functioning temporary cylinder goes in the same day so the door stays fully lockable, and the final cylinder swaps in when it lands. Nobody waits overnight on an unlockable door.

What it costs (on average)

Supply-and-fit averages £85 labour + £25–£120 parts + VAT — the range covering budget-decent through 3-star anti-snap and thumb-turns. High-security restricted or specialist cylinders can run beyond that (occasionally £150–£250+), always quoted first. Several doors in one visit share the single labour charge with small per-door top-ups — the keyed-alike whole-house swap is the classic good-value order. Price guide.

Quick answers

What are standard euro cylinder sizes in the UK?

From about 70mm overall (35/35) up to 120mm+ on thick composite and timber doors, in 5mm steps per side. Most Nottingham doors land between 40/40 and 50/50 — but measure; “most” isn’t “yours”.

Does it matter which way round an offset cylinder goes?

Yes. A 40/50 fitted backwards protrudes 10mm outside — a snapping handle. External figure faces the street, and the finished lock should sit flush to 3mm proud.

My new cylinder is stiff and the key catches — wrong size?

Possibly wrong cam profile or a fit that’s clashing with the gearbox follower. Don’t force it — that’s how a cheap cylinder swap becomes a gearbox job.

Can I upgrade to anti-snap in the same size?

Almost always — 3-star cylinders come in the full size range, including thumb-turns. Same slot, same five minutes, much better lock.

Do you cut extra keys for the new cylinder?

Key cutting isn’t something Max offers — but new cylinders come with their keys, extra keys can be supplied with the cylinder on request, and keyed-alike sets mean fewer keys to manage in the first place.

Is a thumb-turn less secure than key both sides?

Beside glazing or a letterbox within reach, a standard thumb-turn can be turned by an intruder — there are guarded and double-check turn designs for exactly that layout. On solid doors away from glass, thumb-turns are a safety win: fast exits, no hunting for keys.

Sorted? What most readers check next

• 1-star vs 3-star euro cylinders
• Are anti-snap locks worth it?
• uPVC & Composite Door Lock Types
• Lock replacement & installation service
• Moving house? Change the locks first

Rather not do maths on your front door? Call 07552 421433 — sized by eye, fitted flush, £85 + parts (+ VAT), no call-out fee.