When you need to replace door locks on a uPVC or composite door, getting the exact size of the euro cylinder is the most important step. A cylinder that is the wrong size can stick out and be easier to attack, and many people want to know how sizes are written before they order or ask a locksmith for help. The size is two lengths taken from the edge of the door, not one total length. Understanding the measurements can help you avoid ordering the wrong part.
What a euro cylinder is
A euro cylinder is the barrel the key goes into on a uPVC or composite door. It sits inside the multipoint locking mechanism, which is the strip of hooks and bolts worked by lifting the handle. The cylinder passes through the door and the lock case to operate the gearbox, which is the central part of that mechanism. When people talk about how to replace door locks on modern doors, they usually mean changing this cylinder rather than the whole strip.
Euro cylinders are made in different lengths and to different security standards, so the size and the standard both matter.
Why the size matters
The length of a euro cylinder dictates how securely it sits in your door. The MLA says the more a cylinder sticks out, the more prone it is to lock snapping, because it gives an attacker something to grip.
If you simply copy an old cylinder without checking how it fits, you might repeat a previous mistake. An old cylinder that protruded more than 3mm was the wrong size in the first place. You must measure the door itself to ensure the new lock sits flush and keeps your home secure.
How cylinder sizes are written
When you buy a replacement, you will see the size written as two numbers in millimetres. These numbers represent the internal and external lengths of the barrel. For example, a 90mm euro lock might have a 40mm internal and 50mm external measurement. This requires a 40 by 50mm euro cylinder.
The order of the numbers varies between sellers, so you must always check which side the first number refers to. Some retailers write it as internal by external, while others reverse the order or use a slash between the numbers. Because many cylinders are a different length on each side, you cannot just measure the whole length and halve it. A cylinder with the wrong lengths on either side will not fit the door properly.
How to take the two measurements
The reference point is the fixing screw on the edge of the door, and the lengths are taken from its centre. You need one reading to each end of the cylinder.
You need to take two separate readings to find the internal and external lengths. Follow these steps to record the exact numbers.
- Measure from the centre of the fixing screw to the outside end of the cylinder.
- Measure from the centre of the fixing screw to the inside end of the cylinder.
- Write both numbers down and note clearly which one is the external measurement.
The cylinder should sit flush with the handle or stick out no more than about 3mm. If your old cylinder protrudes more than that, measure the door instead of copying it.
Inside and outside and which number is which
Getting the internal and external measurements mixed up is a common mistake when people learn how to change door locks. If your door needs a cylinder that is 40mm on the inside and 50mm on the outside, ordering it the other way round gives the wrong length on each side.
When you write down your measurements, label them clearly. When you shop for the new lock, read the product description carefully to see which number the manufacturer lists first. Some cylinders have a thumb turn on the inside instead of a keyhole, which means they can only be fitted one way round. A thumb turn is often needed on main exit doors so you can get out quickly in a fire without searching for a key.
Why it should not stick out
The Master Locksmiths Association states that the outside cylinder shall not protrude more than 3mm from the face of the door or the lock furniture. Ideally, it should be completely flush with the handle. The more exposed a lock cylinder is, the more prone it is to lock snapping.
If your old cylinder protrudes more than 3mm, it was the wrong size. You must measure the door rather than copying the old lock, or you will simply repeat the mistake and leave your home exposed.
Choosing the right standard
Finding the right size is only half the job when you replace front door locks. The new cylinder must also meet modern security standards to protect your home. There is no government licensing of locksmiths in the UK and anyone can trade as one, which makes choosing vetted products and professionals vital.
The top level of protection is Sold Secure SS312 Diamond. The MLA calls this the highest level of approval for a euro cylinder and recommends it as a cylinder change only solution. The other main route is the TS007 standard, which uses a star rating system to grade security. The MLA says either route must total three stars to be effective against attack.
You can achieve this by fitting a TS007 three-star rated cylinder. Alternatively, you can fit a TS007 one-star cylinder combined with a two-star rated handle. The Master Locksmiths Association warns that a one-star lock is only anti-snap if fitted with two-star door furniture. Look for third party approval such as the Kitemark rather than relying on the maker's claim, and check your home insurance policy wording to see what standards are required.
When to ask a locksmith to measure
Removing and fitting a lock affects the security of the door, and doing it incorrectly can leave your home vulnerable. The Master Locksmiths Association advises that changing locks is not a do-it-yourself job. A vetted locksmith can select the correct size lock and ensure proper installation. If you inspect your current lock from the outside and find it protrudes beyond the frame handle, Secured by Design suggests considering a member of the Master Locksmiths Association to do the work.
If you are struggling to get an accurate measurement, or if your door handles are loose and worn, it is safer to call a professional. A locksmith can measure the door and select the correct size, and can also check the gearbox and the multipoint locking mechanism at the same time. That is useful because uPVC door lock problems are not always caused by the cylinder.
If you decide to hire a professional, watch out for common industry scams. Avoid any trader who shows these warning signs when you contact them or when they arrive.
- National call centres pretending to be local and passing the job to whoever is nearest.
- Quoting a very low price on the phone that multiplies as soon as they arrive on the doorstep.
- Refusing to give a fixed price before starting the work.
- Drilling a lock that could have been opened with non-destructive entry methods like picking or bypassing.
- Replacing a whole multipoint mechanism when only the cylinder or gearbox had failed.
- Fitting cheap cylinders at premium prices.
- Demanding cash only and providing no written invoice.
- Arriving with no ID in an unmarked vehicle.
What a locksmith charges to fit one
Hiring a locksmith to supply and fit a new euro cylinder is usually a quick and straightforward job. Most single lock changes and lockouts take well under an hour, with a standard cylinder replacement typically taking about 20 minutes. Jobs are usually priced as labour plus parts. Many locksmiths charge no separate call-out fee but some do, and VAT may or may not be included, so you must always ask.
The table below shows the MLA 2026 guide prices to fit a cylinder on a uPVC door.
|
Type of euro cylinder |
Typical fitting time |
Fitted cost including VAT |
|---|---|---|
|
Standard euro cylinder on a uPVC door |
About 20 minutes |
From £105 |
|
Anti-snap euro cylinder on a uPVC door |
About 20 minutes |
From £137 |
These are from prices in the MLA 2026 guide and local quotes will differ. Price is driven by the time of day, your location, and the security grade of the lock. If you live in London or the South East, or if you need a locksmith in the evening, at night, at the weekend or on a bank holiday, the cost will be higher.
To keep costs down, wait until normal daytime working hours if the door is secure and the replacement is not an emergency. Under the Consumer Rights Act 2015 a service must be carried out with reasonable care and skill, and at a reasonable price if none was agreed beforehand. Before work starts, tell the locksmith you want a total price that includes the parts, the labour, the VAT and any out-of-hours charges.
Under the Consumer Contracts Regulations 2013 there is normally a 14-day cancellation period for work arranged at home or by phone. This does not apply where you specifically asked for a visit for urgent repairs. However, any extra work sold during that visit is not covered by the exception.
Common questions
How to replace door locks on a wooden door?
Timber doors usually have different locks to uPVC doors. They typically use a mortice deadlock or a sashlock fitted inside the wood, or a night latch fitted on the surface. A night latch is the surface latch often called a Yale lock.
Can I replace locks on doors without changing the keys?
If you are changing the cylinder because it is broken or sticks out too far, you will get a new set of keys with the new lock. However, if you have multiple uPVC doors, a locksmith can supply replacement cylinders that are keyed alike. This means the internal pins are matched so that one single key operates all the doors in your house.
Do I need to replace the whole multipoint mechanism?
No, you do not usually need to replace the entire strip of hooks and bolts just to change the key. The euro cylinder is a separate component that slots through the middle of the lock case. A common scam is a rogue locksmith insisting on replacing a whole multipoint mechanism when only the cylinder or the gearbox had failed. Replacing just the cylinder is much cheaper and faster.
How do I know if I have a three-star lock?
Look for the standard mark on the cylinder, or ask your locksmith to confirm the standard. A one-star cylinder only counts if it is fitted with two-star door furniture such as the handle. You can read more in our guide on anti-snap locks and how they work.
Can a tenant or landlord replace door locks?
For rented homes, a landlord who changes the locks to keep a tenant out without a court order commits illegal eviction, which is a criminal offence. Tenants should always check their tenancy agreement and get the landlord's agreement before changing locks themselves. If you are unsure about your rights regarding door locks, contact Citizens Advice or Shelter for guidance on your specific situation.
Next steps
Before you order a replacement cylinder or hire a professional, take a few minutes to check your current door setup. Gathering the right information will help you avoid buying the wrong size or falling for a scam.
- The two lengths measured from the central fixing screw to each end.
- Which side is the outside measurement.
- Whether you need a thumb turn or a key on the inside if the door is needed for escape.
- The standard mark on the cylinder you plan to buy.
- How far the new cylinder will sit from the handle once fitted.
If you are unsure about taking the measurements, or if you want the peace of mind that comes with professional fitting, it is best to call a local expert to replace the lock for you. When you contact a locksmith, ask them to confirm they will fit an anti-snap cylinder and ask for a total fixed price before they arrive. Check they have a real local address and read their reviews over time.
You can use our directory to find a locksmith near you who can measure the door and fit the right cylinder.
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