Guide
How to Replace a Door Lock: Measuring, Fitting and When to Stop
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Which Lock Is Actually Being Replaced?
The quickest way to find out which lock you need is to ask the door, not the shop.
Is it a timber door, or uPVC or composite?
This guide concentrates on conventional timber door locks. A uPVC or composite door needs a different approach, since many of them use a euro cylinder within a multipoint locking system.
Where is the keyhole?
If it is cut into the edge of the door, the lock is a mortice lock. If it is through the face of the door near the handle, the lock is a euro cylinder. If the key fits a cylinder on the outside and there is a box fitted on the inside face, the lock is a nightlatch with a rim cylinder.
Does lifting the handle send bolts into the frame above and below?
If so, it is a multipoint mechanism, and the cylinder is only one part of it.
What each lock type looks like
| Lock type | How to recognise it |
|---|---|
| Mortice deadlock | Body recessed in the door edge, key-operated deadbolt, normally without a connected handle |
| Mortice sashlock | Recessed in the edge, latch worked by a handle, deadbolt worked by a key |
| Nightlatch and rim cylinder | Box-shaped case on the inside face, key cylinder reached from outside |
| Euro cylinder and multipoint lock | Usually on uPVC and composite doors, cylinder operating alongside a mechanism that may lock at several points |
Timber Door Locks Fitted Into the Edge
A mortice lock sits in a recess cut into the edge of a timber door, most commonly as a deadlock or a sashlock. The deadlock's deadbolt is thrown by key; the sashlock adds a handle-operated spring latch to a key-operated deadbolt. A number of external timber doors use five-lever mortice locks, and that group includes models designed to meet BS 3621. Even so, the number of levers does not, alone, tell you whether a lock meets that standard or satisfies your insurer. Establish the existing lock’s type—deadlock or sashlock—and record its measurements and security specification before selecting its replacement.
A nightlatch is not recessed into the edge at all; it is fitted to the inside face. An internal knob or lever generally works its latch, and a rim cylinder outside takes the key. Since the nightlatch case and the rim cylinder are separate, a fault in the cylinder alone, or a need to change keys, may not justify replacing the whole nightlatch. Replace the complete unit and you will need to consider its backset, fixing arrangement and compatibility with the existing door preparation, all of which are measured later in this guide.
uPVC or Composite Door Locks Fitted Through the Face
On a euro cylinder door, the cylinder and the multipoint mechanism are different components. Replacing one will not necessarily fix a fault in the other.
Why this comes before shopping
You replace a lock by identifying the existing one, measuring it and fitting a compatible replacement, and a straightforward change can often be done with basic tools when the new lock matches the existing fittings. The challenge is choosing correctly. Look at the door material, the position of the lock and how the handle and key work, because mortice locks, nightlatches and euro cylinders differ in design, measurements and fitting requirements; the wrong type may not fit even if it looks similar to the original. Mortice locks and nightlatches, both commonly fitted to timber doors, use different measurements and fixing arrangements, and mistakes with these can leave you with a lock that will not fit or operate correctly. The rest of this guide covers identifying, measuring, removing and fitting a replacement lock, testing it before closing the door, and the cases where professional help is the better option. If the door turns out to be uPVC, see changing the lock on a uPVC or composite door.
Check Whether the Lock Is the Problem
Swapping the lock is the obvious fix for a door that will not lock easily, but it is only the right fix when the lock is what has failed. A stiff or unreliable lock does not always need replacing; problems with hinges, frame or strike plate can cause similar symptoms, and finding where the problem lies first may save you unnecessary work and expense.
The two-minute test
If the door opens normally, open it fully and operate the lock gently with the door open. Key and bolt moving freely, then becoming difficult once the door is closed, suggests misalignment may be contributing. Even with that helpful indication, wear or damage to the lock's internal parts remains a possibility.
The Door, Not the Lock, Needs the Work: Signs
Smooth operation with the door open and stiffness when it is shut can mean the bolt is not lining up correctly with its keep or strike plate. Check also whether:
- the door catches against the frame, or is becoming harder to close;
- you need to lift, push or pull the door before the key will turn;
- the gap between the door and the frame looks uneven;
- there are scrape marks around the strike plate or where the bolt meets the frame.
Hinges that have worked loose or worn, a frame that has shifted, and timber that has taken on or lost moisture can each pull things out of line, and a lock swap by itself may leave the fault exactly where it was.
The Lock Itself Has Failed: Signs
A lock may need repairing or replacing when its internal components no longer operate reliably:
- a key that has snapped inside may leave a fragment that obstructs normal operation and needs professional removal;
- a lock that jams intermittently may have a worn or damaged mechanism stopping the bolt from moving consistently;
- a key that is difficult to insert or turn may be meeting resistance from wear, damage or a problem inside the lock;
- a key that turns without moving the bolt may point to an internal component that is worn, damaged or disconnected.
A complete lock replacement is not an inevitable requirement for any of these symptoms. Repairing or replacing a single part can address some faults, with the lock's construction and state determining whether this is possible.
Reasons beyond a lock fault
Moving into a new property, losing keys or uncertainty about who retains access can also provide sound security reasons for replacing a functioning lock.
Next, Remove the Old Lock
Stop and check two things before you loosen anything. Is the door open and securely supported? And if the new lock cannot be fitted after all, can you still lock up the house? If either answer is no, sort that out first. Then, having confirmed the replacement matches your existing lock, take the old one out.
For a nightlatch
- First, undo the case on the inside face.
- Second, remove the connecting screws.
- Third, withdraw the cylinder from the outside.
- Fourth, set the case, cylinder and screws aside together.
For a mortice lock
- First, if it is a sashlock, remove the handles and spindle.
- Second, take out the faceplate screws.
- Third, ease the lock body out of the mortice without levering.
- Fourth, if it sticks, check for a second screw under the faceplate rather than pulling harder.
The two types differ, but both come out with the door open. To allow comparison with the replacement components, gather the old screws, handles and fittings together. None of this calls for drilling, levering or force, and if a part will not move the right response is to stop rather than push on. Should a key snap while you are working, stop too: a fragment in the mortice makes the job harder, and what to do if a key snaps off in the lock covers what happens next.
Before You Buy It, Measure the Lock
Write the numbers down before you buy anything. That single habit prevents the most common DIY lock failure, which is a new lock that does not fit the door it was bought for. The replacement has to match the existing recess and holes, so measuring the old lock comes first.
What you will need
| Tool | Purpose |
|---|---|
| Tape measure or ruler | Taking the lock's dimensions and confirming the replacement is the right size |
| Screwdriver | A suitable cross-head or flat-head for removing and refitting existing screws |
| Existing keys | Where available, for checking the old lock and comparing it with the new |
| Manufacturer's fitting instructions | Model-specific fixing arrangements, installation requirements and extra tools |
| Compatible replacement lock | Matching type, measurements, fixing positions and required security specification |
That may sound like a lot, yet for a straightforward replacement you may need only the screwdriver and the tape or ruler. Requirements differ depending on whether you are changing a mortice lock, a nightlatch or just the rim cylinder, and some replacements need an Allen key or another tool named by the manufacturer.
Five Measurements on a Mortice Lock
ERA's mortice lock measuring guide sets out five dimensions:
- Case depth, measured from the front edge of the case to the back; in the UK commonly 64 mm (2½ inches) or 76 mm (3 inches).
- Backset, measured from the front edge of the lock to the keyhole centre; often about 44 mm for the shallower case and 56 mm for the deeper one.
- Centres, which apply to sashlocks only: the vertical gap between keyhole centre and square spindle hole centre.
- Case height, the body from top to bottom.
- Faceplate, its length and width, plus the shape of its ends.
The common sizes are typical rather than certain, so measure the lock you have and take the old one with you.
Measuring a Nightlatch or Rim Cylinder
Its case is mounted on the face of the door, which changes what you record: backset from the edge of the door to the centre of the cylinder hole, the case footprint, standard or deadlocking latch, and the length of the cylinder through the door to where its tailpiece passes.
If the new lock would only go in by drilling fresh holes, cutting the door or substantially altering the recess, check you have the right replacement before you proceed. Keep the door open for the entire job, make sure you could secure the property if the job stopped, and do not take out the existing lock until you have confirmed the new one suits it.
Fit the Replacement
A correctly matched lock goes in without persuasion. If anything needs forcing, it is either the wrong size or the wrong way round, and pushing harder only hides the mistake until it fails later.
Dry fit first.
Slide the new lock into the recess with no screws and check the faceplate sits flush with the door edge.
Check which way it faces.
Make sure the bolt and latch point the right way; a latch fitted backwards will stop the door closing.
Fix it loosely, then test.
Put the faceplate screws in finger tight, work the lock with the key, and only tighten once it moves cleanly.
Refit the spindle and handles
on a sashlock, confirming the spindle is long enough to pass properly through the new case.
Look at the keep.
If the new lock throws its bolt a different distance from the old one, the keep on the frame may need adjusting. This is where the job most often goes wrong, so take your time.
A note on cutting. If the new faceplate is larger than the old recess, the door would need cutting to take it, and that is the point at which many people should stop. A fire door or a certified door is a different matter entirely: leave any alteration to someone qualified to work on it. If you reach that point, consider lock installation done for you.
Test It Before You Shut the Door
Test everything with the door wide open, and do not close it until every test has passed. Few stages of the job matter more than this check: you could be left unable to reopen the door if the latch is fitted incorrectly, the bolt is out of alignment or the handle is connected wrongly. Work through these checks before you rely on the lock.
The key.
Insert it and turn it gently both ways, locking and unlocking. It should run smoothly, without sticking or needing force.
The deadbolt.
On a mortice deadlock or sashlock, work the key and confirm the deadbolt goes fully out and fully back, moving freely without catching inside the case.
The latch and handles.
On a sashlock, check the handles draw the latch back and return to their resting position. On a nightlatch, try the outside key, the inside knob or lever, and any extra locking controls as the manufacturer's instructions describe.
Alignment.
With the door still open, hold it in its closed position against the frame and compare the latch and bolt with their keeps. Check whether the heights visibly differ or the fittings show any indication that they may snag.
The internal release.
Check that unlocking and opening the door from within works as intended, paying particular attention if the door forms part of an escape route. If the replacement has changed a required escape function, do not go any further.
Only when all five pass should you close the door. For that first closed-door test, have another responsible person with you or another safe way back in. Check that the latch engages, that the key works normally and that the door reopens easily, then try the remaining keys and every function from both sides. Where a lock has extra testing requirements, its manufacturer's fitting instructions take priority; Yale's official lock fitting instructions are one source of model-specific guidance. If the lock works open but stiffens or will not engage when shut, alignment may be playing a part, so do not force the key or handle.
Once it works
Check every supplied key and decide who needs a copy, keeping a spare somewhere secure rather than hidden outside. After a move, the old keys should no longer open this lock, which helps when you cannot account for copies held by previous occupants or contractors. For a missing or stolen key, assess whether someone could associate it with your address and whether it also operates other locks. Want one key for several doors? Some compatible locks can be supplied keyed alike or rekeyed to match, depending on their design, so think about it before you buy, as not every lock offers the same options. Keep any useful old parts securely until you no longer need them. The goal is not just a new lock but access controlled by the keys you mean to keep in use.
Stop and Call a Locksmith: Knowing When
Knowing when to stop is part of doing this well. Most replacements are within reach: if the new lock matches the fittings already there and the door is in good condition, the job is often manageable. It changes when the door needs altering, components are damaged, or the locking problem is one you do not recognise. Then a professional may be the safer option.
When it will not fit. Case, faceplate or fixing holes that do not line up are a signal to stop. Forcing the lock in, or cutting into the door, risks further damage.
When the old lock will not come out. Seized screws, damaged fittings and jammed mechanisms deserve specialist assessment, not more force.
When there is no key. Without a key, nothing can be removed conventionally.
When the door is out of alignment. A lock that works open but binds closed suggests the hinges, frame or strike plate need attention, not the lock.
When the hardware is specialist. Multipoint systems, including one whose mechanism has failed, unfamiliar mechanisms and locks tied to particular insurance requirements may need a more detailed assessment. If your insurer specifies a standard, check your policy rather than assuming.
When the door is fire-rated or an escape route. Hardware here must keep the door's required fire protection and safe escape functions; unauthorised alterations could compromise them. A flat entrance door onto a shared corridor is one such door, where the lock may need to open from inside without a key.
When testing goes badly. If the new lock does not operate smoothly, or you are uncertain the door can be secured and reopened reliably, stop.
An apparent fit alone does not establish suitability for the door. A locksmith can assess whether you need a compatible replacement, an adjustment or more extensive work, and the cost depends on the lock, its condition and any extra fitting or repair required. A fitted lock change tends to cost less than a lock change and a repair to a door you have cut wrongly.
Deal With Your Old Keys
What did you really want when you decided to change the lock? Usually it was certainty about who can get in. The lock itself delivers only part of that; the rest comes from how you handle the keys.
The checklist
| Do this | Because |
|---|---|
| Count the new keys and note who holds each | You cannot control keys you have not tracked |
| Assume hidden copies after a move | Previous tenants, tradespeople and letting agents may still hold keys |
| Change the lock for a lost key, not just a stolen one | A found key opens the door for whoever found it |
| Ask for keyed alike when buying, if you want one key for several doors | Arranging it later can mean changing or rekeying locks again, and not all locks allow it |
| Keep a spare somewhere other than the property | A spare inside the house is no use when you are locked out of it |
Do this only for a property you own or rent; if you are a tenant, check your tenancy agreement and speak to your landlord or letting agent before changing anything.
If you would rather a locksmith handled the next step, bear in mind that the right solution depends on the door, the existing hardware and the reason for the change, and that repairing or adjusting current fittings is sometimes more suitable than replacing everything. Have your postcode, door type and a brief description of the problem ready, and say whether you still have a working key and whether the door can be opened normally.
Above all, do not hide a key outside. That is not a spare; it is an unlocked door with a few extra steps.
Have a Lock Changed in Farnborough Instead
Handing the job over is a perfectly sensible choice. Wilds Locksmiths Farnborough fits replacement locks throughout Farnborough, including on doors where this guide recommends stopping. Before you ring, have three things ready:
- the door material, such as timber, uPVC or composite;
- where the keyhole is, whether in the edge, the face or a cylinder with a box inside;
- whether you still have a key that works.
From that, we give you a price on the call. If something found on site would change it, you are told before any work begins, and the call-out itself is free whatever the time.
Or get in touch online. Emergency calls are taken day and night, all year round, bank holidays included.
Door Lock Replacement Questions
Is it difficult to replace a door lock yourself?
A suitable replacement often makes this a straightforward DIY task. Many conventional internal locks can be changed with basic tools, including uncomplicated mortice lock and nightlatch replacements. A lock you do not recognise, mismatched fittings or door faults can add time. Consider a locksmith where components are damaged, alignment is poor or the door needs alteration. Take particular care around fire-rated doors, specialist hardware and requirements for escape.
What tools do I need to change a door lock?
For a simple lock replacement, have a tape measure, the correct screwdriver and a new lock compatible with the existing fitting. Additional tools may be necessary for some models. Review the manufacturer's fitting guidance beforehand so the equipment matches the requirements of the lock being installed.
How long does it take to replace a door lock?
Matching the existing lock and door fittings usually makes replacement manageable, particularly for conventional mortice locks or nightlatches. Budget extra time for an unfamiliar design, incompatible parts or problems with the door. A locksmith may be required for damaged hardware, alignment faults or door alterations. A suitable screwdriver, tape measure and compatible lock are normally the basics. Follow the manufacturer's fitting guidance, checking before starting whether further tools are specified.
Can I replace two door locks with one key?
Sharing one key is feasible when the locks permit keyed-alike operation. Suitable locks may arrive matched or sometimes be rekeyed; confirm compatibility before purchasing. A locksmith can explain rekeying options for your type. An existing key needs a compatible replacement keyed to match, otherwise different keys normally follow. Many compatible conventional internal locks require basic tools, with extra care for fire-rated doors, specialist fittings and escape requirements. The wrong size may obstruct recess fit or keyhole, handle or strike-plate alignment. Do not force the lock or alter the door before establishing that it is suitable.
Can I change the locks on a rented property?
A rented property's lock change needs consideration of both the tenancy terms and the situation. Check the agreement and contact the landlord or letting agent before any alteration. If the reason is a faulty lock posing an urgent safety concern, report that problem straight away.
Can I replace a door lock and keep the same key?
It is possible in some cases when the replacement can be keyed for your current key. Different keys are the usual outcome otherwise. Compatible locks that support keyed-alike operation may be supplied together or sometimes rekeyed to share a key. Check suitability before purchasing and seek a locksmith's advice on rekeying your type. For many conventional internal locks, a compatible fitting makes basic tools sufficient for replacement. Fire-rated doors, specialist hardware and specific escape requirements call for extra care.
Can I replace an internal door lock myself?
For many ordinary internal locks, yes, provided the new fitting is compatible and basic tools are available. A matching replacement is usually manageable for conventional mortice locks or nightlatches. Damage, poor alignment and door alterations may require locksmith help. Specialist hardware, fire-rated doors and specific escape requirements need additional attention. Check compatibility before buying and discuss rekeying your lock type with a locksmith. Suitable locks can be supplied or sometimes rekeyed as keyed alike. Your existing key may work with a compatible replacement matched to it; otherwise, expect different keys.
Do I need to replace the door handle when changing a lock?
You can keep a serviceable handle when it is compatible with the new lock. Reuse should follow checks of the spindle, the fixing locations and the way the handle functions. The lock change alone does not establish a need to replace a suitable handle.
What happens if the replacement door lock is the wrong size?
The lock may not fit the door's recess, or the keyhole, handle and strike plate may fail to align with it. Pause before forcing the fitting or altering the door, and confirm suitability. Establish compatibility before purchase. A keyed-alike arrangement allows suitable locks to share a key; compatible models can be supplied that way or sometimes rekeyed.
Why is my new door lock stiff after fitting?
Stiffness can result from poor alignment, an installation error or pressure exerted by the frame. Begin by trying the lock while the door is open. If resistance continues, check that the replacement is compatible and get professional help instead of forcing it. Identifying the lock type and measuring carefully help establish whether the fitting is suitable.