The thickness of the door is not the size of its cylinder. For a euro-profile lock, record two distances from the centre of the cylinder’s fixing screw: one to the outside end and one to the inside end. A 35/40 cylinder has 35 mm on one side and 40 mm on the other. Its 75 mm total length does not tell you which side needs the extra 5 mm.
Keep the door open and your access secure while checking. If it is shut or the key resists, do not dismantle anything for the sake of a measurement. This guide helps you describe the cylinder already in place, then separate that observation from the decision about a suitable replacement.
MEASURE, DO NOT GUESS
See both measurements together
Record the dimensions of an existing euro-profile cylinder. This aid cannot confirm which replacement will fit.
Do not buy on these numbers alone: check the profile, cam, thumbturn, security furniture and external projection with the manufacturer or a professional.
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Start at the fixing screw, not at the edge of the door
Open the door and look at its narrow edge. In a typical euro-profile installation, the cylinder fixing screw runs through the lockcase at roughly the height of the cylinder. The centre of that screw is the origin for both dimensions. The edge of the door leaf, the centre of the keyhole and the handle spindle are different references; using one of them can shift both readings.
Measure from the screw towards the exterior end of the cylinder, then from the same point towards its interior end. Write “outside” and “inside” next to the numbers. This matters when the interior has a thumbturn or when the two sides have different security furniture. Product listings may order their numbers differently, so confirm the manufacturer’s orientation rather than assuming a printed 35/40 always reads the way you do.
Example: 35 mm outside plus 40 mm inside equals 75 mm overall. A 40/35 cylinder is also 75 mm long, but places the fixing screw at a different point relative to each face.
Measuring an installed cylinder with the door open
Stabilise the open door and identify the screw associated with the cylinder. A straight ruler or square lets you read from its centre to the visible end of the existing cylinder on each face. View the scale head-on to reduce parallax, and write down what you actually read rather than rounding immediately to a catalogue size. If a security escutcheon hides the screw or cylinder end, record the uncertainty instead of inventing precision.
You do not need to put a tool into the keyway or remove the entire lock to take this observation. You certainly do not need to close the door to test it. A photograph of the open door edge and one of each side of the furniture can help a locksmith understand why a measurement is unclear. Keep a full key profile, security card numbers and identifiable address details out of any public image.
The size of the existing cylinder is only the start. ABUS explains that a suitable length depends on both the door and its furniture. If the old cylinder already protrudes too far, buying the same size may reproduce the exposure. The next check is how a proposed replacement sits within the actual protective escutcheon, not just whether its numbers match the removed part.
When the cylinder has already been removed
If the part is out during an authorised repair, measure from the centre of its fixing-screw hole to each end of the body. Mark which end faced outside before that context is lost. The loose screw is not itself a dimension; the centre of the hole in the cylinder defines the split.
Do not remove an otherwise working cylinder merely to complete this tool if you cannot preserve safe access. A thumbturn, an unusual cam or an integrated protective fitting can complicate removal and replacement. With a closed door, the priority is addressing the access problem, not obtaining a number by force.
Why door thickness alone gives the wrong answer
Furniture and protective plates add thickness on one or both faces. The lockcase, and therefore the fixing screw, need not sit exactly at the midpoint of the door leaf. Two equally thick doors can take different cylinder lengths. Equally, two cylinders with a 75 mm total can have a different outside/inside split.
ABUS notes that catalogue lengths often progress in 5 mm steps per side. That helps explain sizes such as 30/35, 35/35 and 35/40; it is not a reason to round an uncertain reading. If you read 37 mm, check where you placed the zero and whether the cylinder disappears behind the escutcheon. A listing does not make an inaccurate measurement true.
External projection matters as much as the total
A cylinder that sticks out unnecessarily beyond the outside furniture is more exposed. In its replacement guidance, ABUS advises no more than 3 mm of projection in the situations it discusses and recommends appropriate protective furniture. This is manufacturer guidance, not a universal promise about every door or insurance policy. The specific cylinder and escutcheon must be considered together.
If the existing part visibly projects, do not assume its size should be copied. If it sits deep behind a cover, do not automatically order a longer one either: a different design may interfere with operation or protection. The final installation must be checked for smooth use with the door open before anyone relies on it as the main lock.
What the two dimensions cannot confirm
Profile and cam
This tool is only for a euro-profile cylinder. Oval cylinders, lever locks and some electronic systems need different parts. The cam transfers rotation to the lockcase and must match the installed mechanism. A similar length does not establish that the profile or cam will fit.
Double key or inside thumbturn
A double-key cylinder and a thumbturn model can share dimensions while working differently for the people inside. Identify the inside face and the intended means of exit. The configuration and any model-specific instructions matter as much as the measurement stamped on a package.
Security furniture and key control
Dimensions address longitudinal fit, not the security of key duplication or resistance to particular attacks. If you are replacing a cylinder after losing keys or to improve security, verify documented protection and compatibility with the escutcheon. Our lock replacement service in Lleida considers the whole door rather than a single number on a product label.
Five common measuring mistakes
- Recording only the total. A 75 mm length does not reveal its outside/inside split.
- Reversing the sides. Label the exterior and interior before comparing product references.
- Measuring only the door leaf. Furniture and the fixing-screw position also affect the fit.
- Copying a poorly fitted old cylinder. A new part may repeat its excessive projection.
- Ignoring profile, cam or thumbturn. Matching millimetres do not prove mechanical compatibility.
It helps to ask three separate questions: “How long is the existing part?”, “How should the replacement sit inside the furniture?” and “Which model works with this lock?”. The ruler primarily addresses the first. Treating it as an answer to all three is where many avoidable errors begin.
What to tell a locksmith before choosing a part
Explain whether the door is open, whether the lock works normally and why you are considering a change. Give both dimensions with the outside and inside clearly identified, and say whether you measured from the screw centre. Mention a security escutcheon, thumbturn or hidden cylinder end. “I cannot see the outside end under the cover” is more useful than a confident but guessed dimension.
A general picture of the furniture may fill in the gaps without exposing a full key or your address. If you need help locally, a locksmith in Lleida can check the profile, dimensions and protective fit before recommending material. The aim is to remove uncertainty, not to turn a partial measurement into a rushed purchase.
Questions people ask at this point
What does 30/40 mean on a cylinder?
It denotes the two distances from the fixing-screw centre to the ends, in millimetres. Check which number the maker assigns to the outside. Adding them gives 70 mm overall, but the total does not replace the two separate readings.
Can I measure without taking the cylinder out?
Often yes, when the door is open and the screw and cylinder ends are visible. If security furniture obscures the reference, use a reliable measuring method or ask a professional; do not remove protection just to guess a dimension.
Does a 70 mm thick door need a 70 mm cylinder?
Not necessarily. Furniture adds depth, and the fixing screw may not be central within the leaf. Measure the exterior and interior from the screw rather than relying on the door thickness alone.
Does this apply to an armoured door?
Only after confirming that it uses an accessible euro-profile cylinder and the protective fitting allows a trustworthy measurement. Some security doors have specific configurations; equal numbers do not automatically mean an interchangeable part.
What if the door is shut and the key will not turn?
Stop measuring. Do not loosen hardware or force the key to reach the screw. Explain the access problem first; diagnosis and replacement sizing are distinct decisions.
Bring the two measurements, with their sides clearly marked, but leave room for a final check of the furniture and cam. In Lleida we can look at the complete door and explain what still needs confirming before a replacement is chosen.
Reviewed content
Professional experience behind this guide
Serralleria Catalana reviews this content against real work with openings, locks, cylinders, doors and safes in Lleida. When price depends on the door, time, hardware or diagnosis, we explain that before giving a fixed figure.
- Published
- 21/10/2026
- Updated
- 21/10/2026
- Reviewed by
- Serralleria Catalana
- Standard
- Local experience, proportionate security and clear pricing.
Professional locksmith in Lleida with over 25 years of experience. I write to help you protect your home wisely — no jargon, no fear.
