How to Measure a uPVC Door Lock Before Buying
If your uPVC door lock has failed or you want to upgrade to something more secure, the job is very doable, but only if you order the right part. Knowing how to replace a uPVC door lock starts with measuring what you already have, because these locks come in lots of sizes and the wrong one simply will not fit. Here is a clear, practical walk through the measurements you need.
The two parts you are measuring
A uPVC door lock is really two things working together, and you may be replacing one or both:
- The cylinder, the barrel your key goes into. This is the part most people replace, especially to fit a more secure anti-snap type.
- The multipoint mechanism, the long metal strip down the edge of the door that shoots the bolts and hooks into the frame when you lift the handle and turn the key.
Work out which one you need first. If your key turns but the door will not lock, the multipoint strip may have failed. If the lock feels loose or you just want better security, a new cylinder is usually the answer.
Measuring the cylinder
The cylinder is the easy one, and it is the most common upgrade for security. To measure it properly:
- Open the door and look at the edge where the lock sits. The cylinder runs through the door with a keyhole on each side.
- Undo the single screw on the door edge (usually below the latch) that holds the cylinder in place, so you can slide it out. Keep the key in and turned slightly to help it come free.
- Measure from the central fixing screw hole out to each end separately, in millimetres. A cylinder is sized as two numbers, for example 40/50, measured from the centre to each face.
Getting both halves right matters for security. A cylinder that sticks out too far past the handle is easy to snap, so you want it sitting flush or nearly flush. When you buy, look for an anti-snap, anti-bump cylinder that meets a recognised standard such as TS007 three stars, or a Sold Secure rating, for the best protection.
Measuring the multipoint mechanism
If the whole locking strip has failed, you will need to measure the multipoint mechanism. It is a bit more involved, so take your time and note each figure down:
- Backset: the distance from the front edge of the door to the centre of the keyhole (spindle). Common sizes are 35mm and 45mm.
- PZ measurement: the distance between the centre of the spindle (where the handle goes) and the centre of the keyhole. Usually 92mm on newer doors.
- Faceplate width: the width of the metal strip on the door edge, often 16mm or 20mm.
- Overall length and centre point: the full length of the strip top to bottom, and where the lock case sits along it.
Because there are several types of gearbox and locking point, it is worth photographing the old mechanism and noting the maker's name if it is stamped on the faceplate. That makes matching a replacement far easier.
Tips for getting it right first time
A few small habits save a wasted order and a second trip to the door:
- Measure in millimetres, not inches, as locks are sold in metric sizes.
- Write every figure down as you go rather than trusting your memory.
- Take clear photos of both the cylinder and the faceplate before you remove anything.
- If in doubt on the multipoint strip, take the old one out and match it in person.
If you would rather not wrestle with it, a lock swap is a quick job for a fitter, and getting it right first time protects your home. Our window and door repairs team handles lock replacements and upgrades regularly, so it is an easy call if you get stuck.
A word on security
Since the lock is your door's main defence, it is worth using the chance to upgrade rather than just replace like for like. An anti-snap cylinder is one of the most cost-effective security improvements you can make to a uPVC door, as snapping the cylinder is a common way in. If your door and frame are otherwise sound, a better lock brings it up to standard. If the door itself is tired, our uPVC doors in York range is worth a look.
Rough 2026 costs
Costs vary with the type of lock and whether you fit it yourself, so treat these as broad 2026 guides. A quality anti-snap cylinder is often somewhere around 20 to 60 pounds for the part, while a full multipoint mechanism tends to run from around 60 to 150 pounds, plus fitting if you have someone do it. For a supplied and fitted price, request a free quote.
FAQs
How do I measure a uPVC door cylinder?
Remove the fixing screw on the door edge, slide the cylinder out, then measure from the central screw hole to each end separately in millimetres. The result is written as two numbers, such as 40/50.
What size uPVC door lock do I need?
It depends on your door, so measure the backset, the PZ distance, the faceplate width and the overall length of the multipoint strip. For a cylinder, you only need the two centre-to-end measurements.
Can I replace a uPVC door lock myself?
Often, yes, especially the cylinder, which is a simple swap once you have the right size. The full multipoint mechanism is more fiddly, so many people prefer a fitter for that.
What is the most secure type of uPVC door cylinder?
Look for an anti-snap, anti-bump cylinder that meets a recognised standard such as TS007 three stars or a Sold Secure rating. Snapping the cylinder is a common break-in method, so this upgrade matters.
Why will my uPVC door not lock even though the key turns?
That usually points to a failed multipoint mechanism, where the gearbox or hooks have worn out, rather than the cylinder. The strip down the door edge likely needs replacing.
Thinking of upgrading the whole door? Download our latest brochure to see the range, or request a free quote and we will take a look.