How to Measure a Euro Cylinder Correctly

If you've ever ordered a Euro cylinder and found it sticks out too far, sits too deep, or simply won't fit, you've run into the single most common mistake people make: not measuring properly. Learning how to measure a euro cylinder correctly takes two minutes, and it matters far more than most people realise — because a wrongly sized cylinder is either useless or a genuine security risk.

This guide explains exactly how to measure, why each millimetre matters, and how cylinder size ties directly into lock snapping — the dominant attack on uPVC and composite doors across Manchester.

What a euro cylinder is

The Euro cylinder is the barrel-shaped lock in the centre of most uPVC and composite doors, and many aluminium ones. The key goes in one or both ends, and turning it operates a small cam that drives the door's multipoint mechanism — the row of hooks and bolts that lock the door into the frame.

Because doors vary in thickness and the handle arrangement isn't symmetrical, cylinders come in many lengths and in symmetrical and offset versions. There is no one universal size, which is why you must measure your own.

How to measure a euro cylinder — step by step

You'll need a tape measure or ruler in millimetres. Ideally measure the cylinder in place; you can also remove it first if you're comfortable doing so.

1. Find the centre. Open the door and look at the door edge (the thin face where the lock and hooks are). You'll see a single screw on the underside of the lock face — the cam screw. This screw sits at the centre of the cylinder, where the cam is.
2. Measure the inside half. From the centre of that cam screw, measure to the external end of the cylinder on the inside of the door (the side you turn with the thumbturn or key). Note that figure in millimetres.
3. Measure the outside half. From the same centre point, measure to the end of the cylinder on the outside of the door (the side facing the street). Note that figure too.
4. Write it as inside/outside. For example, 35mm inside and 40mm outside is written 35/40. The overall length is the two figures added together — here, 75mm.

The golden rule: measure each side from the centre separately. Never just measure the total length and split it in half, because most cylinders are offset.

Why offset cylinders exist

An offset cylinder has different measurements on each side of the cam — for instance 40/50 or 35/45. This isn't a quirk; it's the norm. The handle backplate, the door build and the position of the cam mean the two halves are rarely equal.

If you order a symmetrical cylinder (say 40/40) for a door that actually needs 35/45, one side will protrude and the other may sit too deep. That's why measuring both sides independently is non-negotiable.

It also explains a frequent mistake we're called out to fix: someone measures the total length, buys a symmetrical cylinder of the right overall size, and ends up with the external end poking out while the internal thumbturn sits buried. The total was correct; the split was wrong. The door is now both harder to use and easier to attack.

By convention the figure is written inside/outside — the side you operate from within the home first, then the street-facing side. Get into the habit of recording both numbers the moment you measure, so there's no guesswork when you order.

Common UK sizes

Euro cylinders step in 5mm increments from each side of the cam. Common combinations include 35/35, 40/40, 35/40, 40/45, 45/50 and many more, giving overall lengths from roughly 70mm upwards. There is no "standard" size that fits every door — your door tells you what it needs.

If your measurements land between sizes, round so the cylinder sits flush or very slightly inside the escutcheon (the metal plate around the keyhole), never proud of it.

Why a protruding cylinder is a security risk

This is the part that really matters. A cylinder that sticks out more than about 3mm beyond the handle escutcheon gives a burglar something to grip. Lock snapping — breaking the cylinder to expose the mechanism — is the most common forced-entry method on uPVC and composite doors, and a protruding standard cylinder makes it quicker and easier.

A correctly sized cylinder sits flush, leaving little to grab. But sizing alone isn't enough: a flush standard cylinder still snaps in under 30 seconds. The real fix is a flush cylinder that is also 3-star anti-snap (TS007 3-star or Sold Secure Diamond). We explain the security case fully in are anti-snap locks worth it.

So sizing does two jobs: it makes the door work properly, and it removes the protruding "handle" that snapping relies on. Get both right — correct size and anti-snap rating — and you've closed off the main attack.

Why getting the size wrong costs you

Order the wrong size and one of two things happens. Too long, and the cylinder protrudes — a snapping risk and an ugly, vulnerable fit. Too short, and the cylinder sits recessed; the key may not reach the cam properly, the lock can feel stiff, and in the worst case the door won't lock at all. Either way you're looking at a re-order, lost time, and possibly an insecure door in the meantime.

If your uPVC door is already stiff or struggling to lock, the cylinder may not be the only issue — see why your uPVC door won't lock for the other common causes before you replace anything.

Why anti-snap sizing matters specifically

Some anti-snap cylinders have a deliberate sacrificial section on the outer end that snaps away safely under attack, leaving the secure core intact. For that to work as designed, the cylinder must be the right length for your door — sized so the sacrificial part sits where it should and the cylinder is flush. An oversized anti-snap cylinder that protrudes badly undermines its own clever design.

This is one reason we measure carefully on every fit. The numbers matter, and on uPVC doors around Salford and the newer estates we see plenty of poorly sized cylinders left protruding by a previous quick job.

There's a related point worth understanding. The sacrificial section is designed to break away from the outside under a snap attack, leaving the protected core and cam intact inside the door. If the cylinder is too long and sticks out, an attacker has more of the cylinder to grip and can apply more leverage — partly defeating the very feature you paid extra for. If it's too short and recessed, the line where it's meant to break may not sit correctly relative to the door face. Either way, sizing isn't a cosmetic detail on an anti-snap cylinder; it's part of how the security works.

It's also why we don't recommend simply buying the longest anti-snap cylinder you can find "to be safe". Bigger is not better here. The correct size, fitted flush, with the sacrificial section where the maker intended, is what gives you the rated protection.

A quick way to sanity-check your measurement

If you're not fully confident, two simple cross-checks help:

• Look at the fit from outside. With the cylinder in place, does it sit roughly flush with the escutcheon, or does it stick out? A noticeable overhang means it's too long for the door — and a snapping risk regardless of the exact numbers.
• Compare to the old cylinder if you're replacing one. If you've removed the original, measure it the same way (centre cam screw to each end) and match the new one to it — but only if the old one fitted flush in the first place. If the old one protruded, don't copy the mistake; size down.

A tip from experience: if you can comfortably grip the outer end of the cylinder with your fingers and it stands proud of the handle plate, it's sized wrong for security. The aim is a cylinder a burglar can't get purchase on.

What "tolerance" means here

Euro cylinders come in fixed 5mm steps, so you won't always hit your exact door thickness on the nose. That's normal. The rule when you're between sizes is to choose the option that leaves the cylinder flush or marginally recessed on the outside, never proud.

A cylinder a couple of millimetres short inside the door is usually fine and far safer than one a couple of millimetres long outside it. When in doubt, err towards the shorter overall length on the external side — it's the street-facing end that matters most for snapping resistance.

When to call a locksmith

Call a locksmith if you're unsure of your measurements, if your current cylinder protrudes more than a few millimetres, if the lock has become stiff or unreliable, or if you simply want it done right with an anti-snap cylinder fitted flush. We bring a range of sizes, measure on the doorstep and fit in one visit.

We size and fit cylinders correctly every day through our uPVC door repair and lock replacement services across Manchester. Call Manchester Locksmith 24 on 0161 394 1724 with your door type and we'll tell you what you're likely to need.