How to Measure a Euro Cylinder: Sizes & Fitting Guide
A euro cylinder is the barrel-shaped lock you find in most uPVC and composite doors, and plenty of aluminium and timber ones too. When a key stops turning smoothly, or you simply want to upgrade your security, replacing the cylinder is one of the easiest jobs on the door. The only part people tend to get wrong is the measuring, so let's walk through it properly.
What a euro cylinder actually is
The euro cylinder is a standardised lock body that slots into the door and drives the multipoint mechanism when you turn the key or thumbturn. Because the shape is standardised across Europe, a huge range of cylinders will physically fit your door. The catch is that they come in dozens of lengths, so getting the measurement right is what separates a five-minute swap from an ill-fitting, insecure result.
Measure the cylinder, not the door
The single most important rule: measure your existing cylinder, not the thickness of the door. Door thickness varies with weatherseals, glazing and handles, so it tells you very little about the barrel inside.
If you can, remove the old cylinder first. Undo the single fixing screw on the edge of the door (it sits below the latch, on the faceplate), put your key in and gently turn it a few degrees until the cam lines up, then slide the cylinder out. With it in your hand you can measure it accurately on a flat surface.
How the sizing works
Euro cylinders are measured from the central fixing screw hole to each end, and quoted as two numbers, for example 35/45mm. That central point is where the retaining screw passes through, and it splits the cylinder into two sides.
- The first number is usually the outside (external) measurement, from the centre to the end that faces the world.
- The second number is the inside (internal) measurement, from the centre to the end that faces into your home.
Add the two figures together and you have the overall length. A 35/45 cylinder is 80mm long overall. Sizes typically step in 5mm increments, so measure carefully with a steel rule or callipers and round to the nearest 5mm. If your cylinder has a thumbturn on the inside, measure to the base of the turn rather than the tip.
Offset, half and single cylinders
Most doors use a double cylinder (key both sides) or a key-and-thumbturn cylinder. Note whether yours is symmetrical (e.g. 40/40) or offset (e.g. 35/45), because ordering the halves the wrong way round will leave one side proud and the other recessed.
Why protrusion matters
Fit and security go hand in hand here. A cylinder that sticks out too far past the handle or escutcheon is a genuine weak point, because it gives leverage to anyone trying to snap it. As a rule of thumb, aim for the cylinder to finish flush with the handle, and never let it protrude more than around 3mm on the outside.
If your existing lock stands proud, that is a reason to size down or upgrade rather than replicate the old measurement. For high-security doors, look for anti-snap cylinders that meet a recognised British Standard, and remember that a well-fitted, flush cylinder is far harder to attack than a longer one left exposed.
Quick fitting checklist
- Remove the old cylinder and measure centre-to-end on each side.
- Note which side is external and which is internal.
- Round each figure to the nearest 5mm and add them for the overall length.
- Check the new cylinder will sit flush, protruding no more than about 3mm.
- Slide the new one in, line up the cam with your key, and refit the single fixing screw.
Get the measurement right and the rest is genuinely straightforward. When you're ready to buy, browse our full range of euro cylinders and locks, add a matching pair of escutcheons to finish the look, and take a look at our wider door security options if you're upgrading for peace of mind. We offer next-day delivery on in-stock orders and free UK delivery over £100.
