How to Measure a Euro Cylinder

A euro cylinder is the oval-shaped barrel that sits inside the lock on most uPVC and composite front doors, as well as many back doors and French doors. It is the part your key actually turns. Unlike a traditional 5-lever mortice deadlock — the type buried into the edge of a timber door and often required by home insurers under BS3621 — a euro cylinder is held in place by a single central screw and can, in theory, be removed in seconds with the right tool. That convenience cuts both ways: it makes legitimate replacement straightforward, but it also means a poorly sized cylinder is one of the most common points of attack on UK doors.

If the cylinder protrudes more than three millimetres beyond the face of the door furniture (the external handle or escutcheon plate), a burglar can grip it with a cheap pair of pliers or mole grips, apply rotational force, and snap the cylinder in under a minute. This technique — known as lock snapping — is well documented across London and requires no specialist skill. A correctly sized, flush or near-flush anti-snap cylinder rated to TS007 3-star removes that vulnerability almost entirely. Getting the measurement right before you buy a replacement is therefore not a minor admin task; it is the most important step in the whole job.

Remove the door furniture if possible — but if you cannot, you can still measure accurately with the hardware in place. Open your door and look at the cylinder face-on. You will see it is divided into two halves by the central fixing hole where the retaining screw sits. The side facing outside your home is the external half; the side facing into your hallway is the internal half. These two halves are almost always different lengths, and that asymmetry matters. Use a metal tape measure or a rigid steel rule — a cloth tape can stretch and give you a reading that is a millimetre or two out, which is enough to cause problems.

Measure from the centre of the fixing hole to the outer edge of the cylinder on the external side. Write that number down. Then measure from the centre of the fixing hole to the outer edge on the internal side. Write that number down too. A euro cylinder is sold using a format that expresses these two measurements together: external measurement first, then internal. So if your external side is 40 mm and your internal side is 45 mm, you need an 40/45 cylinder. Common sizes in UK residential doors include 35/35, 40/40, 35/45, 40/45, and 45/55, though doors vary — especially older timber-framed doors in Tooting's Victorian terraces, which sometimes have non-standard depths. Always measure your specific door rather than relying on a neighbour's size or a sticker left by the previous owner.

Once you have your two figures, check the cylinder currently in the door. If the external face sits flush with or slightly recessed into the door furniture, that is correct. If it sticks out noticeably — particularly on the outside — that overhang is a snapping target. When you buy a replacement, choose a size that keeps the external face flush. If the door furniture itself is very thin, you may need to also check whether a thumb-turn cylinder (which replaces the internal keyway with a rotating knob, handy for those with mobility needs or who want to avoid double-locking themselves in) is a suitable option for your door type.

Once you have the correct measurements, the next decision is which cylinder to fit. This is where many homeowners unknowingly cut corners. A basic replacement cylinder from a builders' merchant might be the right physical size, but it may offer no meaningful resistance to snapping, drilling, or picking. For doors in and around London, the benchmark worth aiming for is a cylinder that carries a TS007 3-star rating — the highest level under that British Standard, combining snap resistance, pick resistance, and drill resistance in a single product. Some insurers in the UK now specifically ask for TS007-rated hardware, so it is worth checking your policy wording before you buy.

Anti-snap cylinders are designed with a deliberate break point. If a burglar attempts to snap the external section off, the cylinder sacrifices that outer portion — but crucially, the internal mechanism remains intact and the door stays locked. Without that feature, a successful snap attack takes the whole barrel with it and leaves the door bolt free to retract. When our team carries out a lock change London homeowners frequently ask about, we always assess whether the existing cylinder has any snap resistance at all, and in many cases the answer — particularly in older uPVC doors — is no. Upgrading at the same time as replacement is straightforward and costs no more in labour.

The most frequent error is measuring the cylinder itself rather than measuring from the fixing-hole centre. If you measure the full length of the cylinder and then split it in half, you will get the wrong figure for any asymmetric barrel, which is most of them. Always measure each side independently from the centre of the fixing hole. The second most common mistake is ignoring the door furniture thickness. The escutcheon plate or handle surround adds its own depth on the external face. Measure to the face of the door furniture, not to the cylinder edge poking through it — that way your chosen replacement will sit correctly within the hardware rather than proud of it.

A cylinder that is too short brings its own problems: it may not engage the cam (the internal lever that throws the bolt) properly, leaving you with a lock that feels loose, fails to lock fully, or requires the key to be held at an awkward angle. If you have ever struggled with a key that only works when you lift the handle simultaneously, a slightly undersized or misaligned cylinder is often the cause. If you are unsure after measuring, or if your door is a composite door with a multipoint lock — which engages several bolt points up and down the door frame simultaneously — it is well worth calling a professional locksmith before ordering parts. Getting the wrong size means a wasted journey to the trade counter and potentially a night with the door not fully secured. If you would like advice specific to your door, call us on 020 3675 7793 — we answer around the clock.

Measuring and replacing a euro cylinder is a reasonable DIY job on a straightforward uPVC door, provided you are comfortable removing the door handle assembly and you have the correct tools. But there are situations where calling an experienced locksmith is the right move from the outset. If you are locked out of your home in London — whether in Tooting, Streatham, Balham, or anywhere else across the capital — do not attempt to force entry. Check first whether any other door or window has been left unlocked, whether a trusted neighbour holds a spare key, or whether your letting agent or housing association has emergency access arrangements. If none of those apply, our team can reach most addresses in Tooting within around 30 minutes of your call, and we will confirm an exact up-front price before any work begins — the final quote depends on factors such as the lock type, the time of day, travel distance, and whether parts are needed.

Beyond lockouts, there are other scenarios where professional help is the sensible option. If the cylinder has already been snapped or drilled — perhaps discovered on returning home — the door lock replacement London residents need in that situation involves assessing whether the door frame and multipoint lock mechanism have also been damaged before fitting a new cylinder. A skilled locksmith can spot secondary damage that is easy to miss. Similarly, if you are a landlord managing a property in Tooting and a tenancy has ended, changing the cylinder promptly and fitting one of an appropriate security rating protects both the property and your legal position. Our team carries out upvc door lock repair London wide, as well as mortice lock work, nightlatch replacement, and full door lock replacement — all as a 24 hour locksmith London service with no appointment needed. If security feels uncertain or you simply want a professional eye on your door, call 020 3675 7793 at any hour and we will send someone straight out.

Our mobile team covers Tooting and surrounding areas as a fully insured emergency locksmith London service. Here is a specific breakdown of what we do: euro cylinder measurement and replacement; anti-snap TS007 cylinder upgrades; uPVC multipoint lock repair and replacement; Yale nightlatch replacement and upgrade; 5-lever mortice deadlock installation to BS3621; door lock replacement for timber, uPVC, and composite doors; emergency lockout assistance for residential properties; non-destructive entry where possible to avoid door damage; destructive entry when no other option is safe; lock change after a break-in; lock change following end of tenancy; communal entrance lock repair for blocks of flats; door alignment adjustment when a warped frame causes locking difficulty; keep and striker plate repair and reinforcement; door frame repair following forced entry; letterbox restrictor fitting; door chain and door limiter fitting; thumb-turn cylinder installation for accessibility needs; internal door lock fitting; bathroom lock replacement; wooden door lock repair; uPVC door handle and mechanism repair; French door lock repair; back door lock replacement; emergency boarding-up service after break-in or damage.