A euro cylinder can look like a straightforward replacement until the new lock arrives and projects beyond the handle, sits too far back in the door, or simply does not fit. Getting euro cylinder sizes right is the difference between a clean, secure installation and a lock that needs returning. The good news is that the measurement is simple once you know where to start.
How euro cylinder sizes are shown
Euro cylinders are measured in two halves from the centre fixing screw. You will usually see a size written as two numbers, such as 30/30, 35/45 or 40/50. The first number is the measurement from the centre of the cylinder to one end; the second is the measurement from the centre to the other end.
A 35/45 cylinder is therefore 80mm long overall. It is not a 35mm cylinder with an extra 45mm added for the handle. Both figures describe the lock body itself, measured from the centre line of the screw hole.
This matters because most external doors are not perfectly symmetrical. A composite door, uPVC door or timber door may have thicker external furniture, a security escutcheon, internal handles or a thumbturn. That can mean the outside and inside measurements are different.
When ordering, think of the size as internal/external rather than left/right. Once fitted, the longer side must face the side of the door that needs it. If you reverse an offset cylinder, it may protrude on one side and sit recessed on the other.
Measure from the centre screw, not the edge
The cylinder is held in place by a long fixing screw positioned on the door edge, level with the keyway. This screw is the reference point for every euro cylinder measurement.
For the most accurate result, remove the existing cylinder first. Open the door, remove the centre fixing screw, insert the key and turn it slightly while gently pulling the cylinder out. The small cam in the middle must be aligned with the cylinder body before it will slide free. On a thumbturn cylinder, turn the thumbturn slightly to align the cam instead.
Place the cylinder on a flat surface and measure from the centre of the fixing-screw hole to each end. Record both measurements in millimetres. For example, if one side measures 40mm and the other measures 50mm, you need a 40/50 cylinder.
If you cannot remove the lock, measure from the centre of the fixing screw on the door edge to the outer face of each handle or escutcheon. This can be effective, but it is easier to make a small error because door furniture can hide the true position of the cylinder. Removing the cylinder remains the better option where possible.
Include handles, escutcheons and door thickness
Do not measure the door slab alone. A 44mm timber door may need a 35/35 cylinder, but this can change once handles, backplates and protective escutcheons are included.
Measure to the point where the cylinder will finish, normally flush with the outer face of the handle or security escutcheon. A very slight projection can be acceptable depending on the cylinder and furniture, but the cylinder should not stand noticeably proud. An exposed cylinder can be easier to grip or attack, particularly on an external door.
Equally, avoid choosing a cylinder that is too short. If it is recessed too deeply, the key may be awkward to use and the handle furniture may obstruct access. The aim is a neat, usable fit with minimal projection on both sides.
Common euro cylinder sizes in the UK
There is no single standard size for every door, but several sizes appear regularly in UK homes and commercial properties. A 30/30 cylinder is common on slimmer doors with fairly even furniture, while 35/35, 40/40 and 45/45 cylinders suit thicker or more heavily fitted doors.
Offset sizes are just as common. A 35/45 or 40/50 cylinder may be needed where an external security escutcheon adds depth, or where an internal thumbturn and handle arrangement requires more length. Flats with fire doors, communal entrance doors and commercial aluminium doors can have particularly varied requirements.
The existing size is the best starting point, but do not assume a like-for-like number is always correct. If you are changing handles, adding a security escutcheon or replacing a basic cylinder with one that has a different profile, measure again before placing the order.
Choose the right cylinder type as well as the size
Correct sizing ensures the cylinder fits the door. It does not, by itself, determine how secure or convenient the lock will be. Before buying, check the function you need.
A double cylinder requires a key on both sides. It can suit doors where key control is preferred, but consider emergency exit needs carefully. If a key must be available to leave the property, keep it accessible without leaving it in the lock where that could create a security risk.
A thumbturn cylinder has a key outside and a turn knob inside. It is popular for domestic front doors because it allows quick exit without a key. However, it should be selected thoughtfully where letterboxes, nearby glazing or other openings could allow someone to reach the thumbturn.
Keyed alike cylinders are useful when you want one key to operate several doors. This is often practical for landlords, small businesses and households with a front door, back door and garage access. The cylinders must be compatible within the same product range and key system, so plan the set before ordering.
For external doors, consider a security-rated cylinder from a recognised manufacturer. Features can include anti-snap, anti-drill, anti-pick and anti-bump protection. The best choice depends on the door, its handles and the overall security requirement. A high-security cylinder is most effective when paired with suitably strong door furniture and a correctly installed multipoint lock.
Avoid the mistakes that cause fitting problems
The most frequent error is measuring the whole cylinder and ordering that as one figure. A cylinder described as 70mm overall may be 35/35, 30/40 or another split entirely. The total length alone does not tell you which side is longer.
Another common issue is measuring from the end of the cylinder cam rather than from the centre of the fixing screw. The cam is not the sizing reference and can lead to a wrong result.
It is also worth checking whether you have a standard euro profile cylinder. Oval cylinders, rim cylinders and certain specialist locks use different formats and cannot be substituted simply because the overall length looks similar. If your lock has an unusual shape, take clear measurements and identify the lock type before ordering.
Finally, do not force a new cylinder into place. If the fixing screw will not line up or the cylinder will not sit level, remove it and recheck the cam position and measurements. Forcing parts can damage the lock case or multipoint mechanism.
When a locksmith is the sensible option
Measuring and replacing a cylinder is manageable for many property owners, especially when the door is open and the existing lock can be removed. It is less straightforward if the key is lost, the cylinder is broken, the multipoint lock is stiff, or the door is misaligned.
A locksmith can identify whether the issue is the cylinder, handles, gearbox or door alignment, then fit the correct replacement. This is particularly valuable for landlords and facilities managers dealing with several doors, restricted key systems or occupied properties. Secure Choice can also help customers arrange professional fitting through the Lockfit locksmith network.
Before you order, write down both half measurements, the cylinder type, whether you need keys or a thumbturn inside, and which side is external. That small check takes minutes and gives you the best chance of fitting a secure cylinder that looks right and works properly from day one.