How to Measure a Euro Cylinder

A euro cylinder is the barrel-shaped lock mechanism fitted into the majority of uPVC, composite, and aluminium doors in the UK. It is the component your key actually turns. It sits inside a housing within the door, held in place by a single central screw, and it is flanked by door furniture — the handle plate or escutcheon — on the face of the door. The cylinder has two halves: the external side (outside the property) and the internal side (inside). Those two measurements, combined, give you the total length, and that total determines whether a replacement will fit flush, sit recessed, or — the dangerous scenario — stick out past the door furniture.

Why does sticking out matter so much? Because a euro cylinder that protrudes more than roughly 3 mm beyond the escutcheon plate can be gripped with ordinary pliers and snapped in seconds. Cylinder snapping is not a niche concern — it is one of the most common methods used to defeat uPVC and composite doors across London and the wider UK. A snapped cylinder defeats the lock entirely, leaving the door openable from the outside without any key at all. That is why the measurement is not just about getting the right part; it is about understanding whether your current lock is already a vulnerability.

You need a metal ruler or a steel tape measure — fabric tape measures flex and give inaccurate readings. First, open the door so you can work comfortably on both faces. Look at the face of the cylinder: you will see it divided by the central cam (the small metal tongue in the middle that retracts the bolt). That centre point is your reference. Measure from the centre of that cam to the outer edge of the external (outside-facing) half, in millimetres. Write that number down. Then measure from the centre of the cam to the outer edge of the internal (inside-facing) half. Write that down too. Your cylinder size is expressed as external x internal — for example, 35 x 45 mm, or 40 x 40 mm for a perfectly symmetrical fitting. The total length in that first example would be 80 mm.

A few things to double-check while you have the ruler out. First, confirm the cylinder is a standard euro profile — it has a figure-of-eight or oval cross-section when viewed from the end. If your lock looks significantly different, it may be a different standard (oval or British Standard cylinders, for instance). Second, look at how much of the cylinder sits proud of the escutcheon plate on the external face. If you can see more than about 3 mm of cylinder exposed beyond the plate, that is a snapping risk and worth flagging immediately. Third, note whether your door has a thumbturn on the internal side rather than a keyhole — this affects which replacement cylinder you order. If in any doubt, our team covering Streatham and surrounding areas of south London can measure and advise in person; call 020 4530 9860 and we will send someone out, usually arriving within around 30 minutes.

Once you have your measurement, you are in a position to do more than simply replace like for like. The upgrade worth considering is a TS007 3-star anti-snap cylinder. These are designed with a deliberate break point: if someone attempts to snap the cylinder, the outer portion shears away cleanly but leaves the internal locking mechanism completely intact and functional. The door remains secure. Many home insurers now either recommend or require anti-snap cylinders, particularly on uPVC and composite doors, so fitting one can also have implications for your policy — worth checking with your insurer before or after a lock change.

When ordering an anti-snap cylinder, your measurement is used in exactly the same way — external x internal in millimetres — but you should be aware that some anti-snap cylinders are slightly longer than their standard equivalents due to the additional internal mechanism. Always check the finished external protrusion against your door furniture before the central fixing screw is tightened. A trained locksmith handling a door lock replacement London-wide will check this as a matter of course and trim or adjust the cylinder seating if the door design allows. If you are sourcing the cylinder yourself and fitting it, measure twice before you buy and confirm the escutcheon plate will cover the cylinder flush or leave no more than 2–3 mm proud on the outside face.

Whether you are thinking about how to change a lock on a composite front door or simply replacing a worn cylinder on a first-floor flat door, the measurement process is the same. The variables are the door thickness, the handle plate design, and whether a thumbturn or key-operated internal is needed. Getting those details right before purchase saves a wasted trip and an unnecessary second call-out.

The most frequent error is measuring the total visible length of the cylinder without accounting for the cam in the centre — this gives you an overall figure but not the split between external and internal halves. A cylinder ordered on total length alone may fit the door but leave the wrong amount exposed on each side. For example, if your door is fitted asymmetrically and you replace a 35 x 45 mm cylinder with a 40 x 40 mm one, the external side will now protrude further than it should, creating the snapping risk described above.

The second common mistake is measuring with the door closed and the cylinder still in situ, trying to eyeball the figures through the letterbox gap or from the outside. This almost always produces inaccurate results because you cannot see the cam centre clearly and the door furniture partially obscures the cylinder ends. Always open the door fully. A third mistake, particularly in older properties, is assuming all euro cylinders are interchangeable. Some doors — particularly older uPVC frames common in Streatham's stock of 1980s and 1990s housing — were fitted with non-standard cylinder lengths or unusual escutcheon geometries. If you are dealing with a lock that looks unusual or if the existing cylinder has no visible markings, it is worth calling a locksmith London homeowners trust to assess in person rather than ordering blind.

If you have ever been locked out and found yourself wondering how to pick a door knob lock or whether there is a quick bypass, the honest answer is that for a modern euro cylinder lock the legitimate options are: checking for a spare key, accessing the property through a secured but unlocked window or back door, or calling a professional. Legitimate locksmiths use specialist tools and training to open locks non-destructively — not methods you would want to attempt without that background. Calling us on 020 4530 9860 means a qualified engineer arrives promptly, verifies ownership, and opens the door without unnecessary damage.

Measuring your cylinder is not always a prelude to a DIY job. Sometimes it is the first step in deciding whether to call a professional for a full door lock replacement London residents can trust to be done correctly. There are several situations where professional installation is strongly advisable rather than optional: if the door has been forced or the cylinder has already been partially snapped; if you have just moved into a property in Streatham or elsewhere in SW16 and want to know who might still hold a key to the previous cylinder; if the door is a modern composite with a multipoint mechanism where the cylinder replacement requires careful realignment of the locking points; or if the existing cylinder is so worn that the cam or tailpiece has deformed and the measurement itself is unreliable.

When our team carries out a lock change London-wide, the final price depends on a number of factors: the type of door and frame, the cylinder specification required (standard, anti-snap, thumbturn or double-keyed), the time of day — including evenings, weekends and bank holidays — and travel distance to your postcode. We confirm an exact, up-front quote on the phone before anyone travels to you, so there are no surprises. For anyone who needs an emergency locksmith London can rely on at any hour, or who simply wants the job done right the first time, we are on 020 4530 9860 around the clock. We cover Streatham, Streatham Common, Streatham Hill, Streatham Vale, and surrounding areas of south London.

It is worth noting that on uPVC and composite doors, the euro cylinder is one part of a larger multipoint locking system. If your door is difficult to lock, if the key feels stiff, or if the handle does not fully lift the locking points into their keeps, the problem may not be the cylinder at all — it could be the gearbox within the door, the alignment of the door in the frame (a common issue in older uPVC frames as the hinges wear), or a damaged locking point. A uPVC door lock repair London call-out will typically involve the engineer assessing the whole system rather than just the cylinder, because replacing a cylinder in a misaligned door will not solve a handle or gearbox fault.

Similarly, if someone has previously fitted a cylinder that is too long and it has been protruding for some time, the cam and the gearbox actuator may have worn unevenly. In that case the measurement exercise confirms the problem but the solution involves more than simply swapping the cylinder. A thorough assessment is the right starting point — and that is exactly what a trained locksmith Streatham Common residents can call on 020 4530 9860 will carry out, day or night, as part of a 24 hour locksmith London service that does not stop for evenings, weekends or bank holidays.