Guide
How to Replace a Door Lock: Measuring, Fitting and When to Stop
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Which Lock Are You Replacing?
That answer matters more than any other, because the door determines what you replace. The whole job reduces to three moves: identify the existing lock, measure it, and fit a replacement that is compatible with it.
Question one: timber, or uPVC and composite? Look at the door material first. Timber doors usually carry a mortice lock, a nightlatch or both. For many uPVC and composite doors, the arrangement instead consists of a multipoint locking system incorporating a euro cylinder.
Question two: where does the key go? If the keyhole is in the door edge, think mortice lock. If it is in the face, close to the handle, think euro cylinder. If a box sits on the inside face and the key enters a round cylinder from outside, think nightlatch with a rim cylinder.
Question three: what happens when you lift the handle? If bolts shoot up and down into the frame, the door has a multipoint mechanism, and the cylinder you can see is just one piece of it.
Also check how the handle and key behave. A key that throws a bolt with no handle involved suggests a mortice deadlock. A handle that draws back a latch while the key works a separate bolt, all from a case set into the edge, suggests a mortice sashlock.
Locks Fitted in the Edge of a Timber Door
Mortice locks fit into a recess cut into the edge of a timber door, and they come in two common forms. A deadlock moves a deadbolt with a key. A sashlock adds a spring latch, worked by a door handle, to that key-operated deadbolt. On some external timber doors you will find a five-lever mortice lock, and some of those models are designed to meet BS 3621. The number of levers on its own does not prove a lock meets that standard, nor that it satisfies your insurance terms. So before you shop, record three things: deadlock or sashlock, its dimensions, and its security specification.
If There Is a Box on the Inside
That is a nightlatch, which mounts on the inside face of the door instead of being let into the edge. It generally works a latch from an inside knob or lever, with a separate key-operated rim cylinder on the outside. Because the two are separate parts, a faulty cylinder or a need for new keys may not require a whole new nightlatch. If the complete latch must go, its backset, fixing arrangement and fit with the existing holes in the door all start to matter; this guide covers those measurements further on.
If the Keyhole Is Beside the Handle
On a euro cylinder door the cylinder and the multipoint mechanism are separate components, so replacing one will not necessarily fix a fault in the other.
Why go to this trouble? Mortice locks, nightlatches and euro cylinders differ in design, measurements and fitting requirements, and a lock of the wrong type will not fit, however similar it looks. Mortice locks and nightlatches, the two found most often on timber doors, also measure and fix differently, and errors there leave a lock that will not fit or operate correctly. A straightforward change can usually be managed with basic tools as long as the new lock matches the existing fittings; choosing it is the real challenge. From here, this guide covers identifying, measuring, removing, fitting and testing a replacement before you close the door. Alongside its main coverage of conventional timber door locks, it outlines the different approach required for uPVC and composite doors and the situations better suited to professional help.
Check the Lock Is Actually the Problem
A new lock cures a faulty lock and nothing else, so spend two minutes finding out whether the lock really is at fault. A stiff or unreliable lock does not always need replacing: worn hinges, a shifted frame or a badly placed strike plate can produce much the same symptoms. Open the door fully, provided it opens normally, and gently work the lock with the door standing open. If the key and bolt move freely like that but turn stiff once the door is shut, misalignment may be part of the problem. Treat that as a strong clue rather than a verdict, because it does not rule out wear or damage inside the lock. Settling the question first can spare you unnecessary work and expense.
Signs the Lock Itself Has Failed
A lock may need repair or replacement once its internal parts stop working reliably. Symptoms that point that way include:
- The key turns but the bolt does not move. Something inside may be worn, damaged or disconnected.
- The key is hard to insert or turn. Wear, damage or a fault within the lock could be creating the resistance.
- The lock jams now and then. A worn or damaged mechanism can stop the bolt moving consistently.
- A key has snapped inside. The broken piece may block normal operation and need professional removal.
None of these automatically means the whole lock must go, since some faults can be fixed by repairing or replacing one component, depending on the design and condition. There are also good reasons to change a lock that works perfectly: moving into a new home, losing keys, or not knowing who still has a copy.
Signs the Door, Not the Lock, Needs the Work
When the lock runs smoothly with the door open but stiffens once it is shut, the bolt may not be meeting its keep or strike plate squarely. Other signs include:
- you have to lift, push or pull the door before the key will turn;
- scrape marks show around the strike plate or where the bolt meets the frame;
- the gap between door and frame looks uneven;
- the door catches on the frame or is getting harder to close.
Loose or worn hinges, movement in the frame and moisture changing the timber can all upset alignment, and a new lock on its own may not cure any of them. For the wider decision, see how to decide between repairing and replacing.
Before Buying, Measure the Lock
Think of this step as matching, not shopping. The new lock has to fit the recess and holes the old one occupies, so you measure what is there and buy to those numbers. It is the step most guides rush, and it is the one that decides whether the job takes one trip or three.
Mortice Lock: The Five Measurements
Taking the dimensions as ERA's mortice lock measuring guide defines them:
| Dimension | Definition | Typical UK figure |
|---|---|---|
| Case depth | Depth of the lock case, measured front edge to back | 64 mm (2½ in) or 76 mm (3 in) |
| Backset | From the lock's front edge to the keyhole centre | About 44 mm with a 64 mm case; about 56 mm with a 76 mm case |
| Centres | Sashlocks only: keyhole centre to square spindle hole centre | Varies, so measure |
| Case height | Height of the body, top to bottom | Varies, so measure |
| Faceplate | Length and width of the visible plate, square or rounded ends | Varies, so measure |
These are common sizes, not universal ones; record your own and carry the old lock with you when you buy.
Measuring Up a Rim Cylinder or Nightlatch
The case is fixed to the face of the door rather than buried in it, so the list changes: backset from door edge to the centre of the cylinder hole, the footprint of the case, whether it is a standard nightlatch or a deadlocking one, and the cylinder length, taken through the door to the point where the tailpiece passes. Cylinders and cases are frequently sold separately.
Tools and preparation
A straightforward change may need only a screwdriver and a measuring tape or ruler, though that varies between a mortice lock, a nightlatch and a rim cylinder on its own. Have ready:
- the manufacturer's fitting instructions, which can show model-specific fixing arrangements, installation requirements and any additional tools;
- your existing keys, if available, to check the old lock's operation and compare it with the replacement;
- a compatible replacement that matches the existing type, measurements, fixing positions and the security specification you require;
- a tape measure or ruler for the dimensions and for confirming size;
- a suitable cross-head or flat-head screwdriver for the existing screws.
Some replacements also need an Allen key or another manufacturer-specified tool.
If drilling new holes, cutting into the door or reshaping the recess substantially looks necessary, check that the replacement is correct before you proceed. Keep the door open the whole time, have a way to secure the property if you cannot finish, and do not take the existing lock out until you have confirmed the new one is suitable. Prefer to hand it over?
Fit the Replacement Next
Fit, test, then tighten: that is the pattern to follow, and the new lock should cooperate at every stage. If you find yourself pushing, tapping or squeezing a part into place, stop, because it is the wrong size or fitted the wrong way round.
Sequence
- Spindle check (sashlocks). Before refitting handles, confirm the spindle is long enough for the new case.
- Dry fit. Sit the lock in the recess with no screws and check the faceplate is flush.
- Orientation. Make sure the latch and bolt face the right way; a backwards latch will not close the door.
- Fix and test. Faceplate screws go in finger tight, the lock is worked, and only then are the screws tightened, before the handles go back on.
- Keep. If the bolt projection differs from the old lock, the keep may need adjusting. This is the most common point of failure.
Where to stop
If the new faceplate is larger than the old recess, the door would have to be cut, and many people will want to stop at that point rather than reshape it. Fire doors and certified doors should only ever be altered by someone qualified to work on them. If you are at that stage, look at.
Remove the Old Lock
With the replacement checked against the old lock, removal is next, and it starts with a precaution most instructions skip. Open the door, wedge or support it securely, and be sure you can secure the property if the replacement cannot be completed.
Mortice locks and nightlatches come out in different ways.
| Step | Mortice lock | Nightlatch |
|---|---|---|
| 1 | Sashlock only: remove the handles and spindle | Undo the case fixings on the inside face |
| 2 | Remove the faceplate screws | Take the case off |
| 3 | Ease the lock body out of the mortice | Undo the connecting screws |
| 4 | If it sticks, check for a second screw under the faceplate | Withdraw the cylinder from outside |
Gather the original screws, handles and other fittings and keep them together. They are your reference when you compare the new components.
What you should not do is drill, lever or force any part of the lock. If it will not come out with the steps above, stop. Likewise, if a key snaps during removal, stop: a piece left in the mortice makes the rest of the job harder. For that situation, read what to do if a key snaps off in the lock.
Test It Before the Door Is Shut
Keep the door wide open for every test, and close it only when all of them pass. Why so strict? Because a latch that has been fitted poorly, a bolt that sits out of line or a handle connected incorrectly could leave you unable to open the door once it shuts. Checking the lock with the door fully open is one of the most important steps in the job.
Closing the door for the first time
Only close the door once these checks have passed, and for that first closed-door test make sure another responsible person is with you or you have another safe means of access. The latch should engage, the key should work normally and the door should reopen without difficulty. Move on to checking the other keys and trying each available function from either side of the door. Where a lock has additional testing requirements, the manufacturer's fitting instructions take priority, and Yale's official lock fitting instructions are available for model-specific guidance. Alignment may be a factor when the new lock works with the door open but becomes stiff or will not engage after closing; never force the key or handle.
Keys, copies and spares
Every supplied key should operate the lock; check them and decide who needs one. Choose a secure place for a spare key; never conceal it outside the property. Changing a lock after moving home means old keys should no longer work in it, which matters when copies held by previous occupants or contractors are unaccounted for. Assess whether a missing or stolen key could be associated with your address, and check whether it operates any other locks. One key for several doors is possible with some compatible locks, supplied keyed alike or rekeyed depending on their design, though not every lock supports it, so decide before buying. Store useful old components securely until they are no longer needed. The real aim is that access to your property is controlled by the keys you intend to keep in use.
Start with the key. Put the new key in and turn it gently in both directions. It should move smoothly, never sticking or needing excessive force.
Then the deadbolt. On a mortice deadlock or sashlock, use the key and check the deadbolt extends and retracts fully, moving freely without catching inside the lock case.
Then the latch and handles. Once the sashlock is fitted, operate the handles to confirm that they draw back the latch and settle into their usual position. Use the manufacturer's instructions to check the nightlatch's outside key, inside knob or lever, and any further locking controls.
Then the inside release. Check that the door's intended unlocking and opening operation works from within, taking particular care if it is part of an escape route. If fitting the replacement has changed a required escape function, do not proceed.
Finally, alignment. With the door still open, hold it in its closed position against the frame and compare the latch and bolt with the keeps. Look for differences in height or any sign the hardware will catch.
Deal With the Old Keys
If you changed the lock because of a key, and most people do, this is the step that matters most. A new lock is only worth fitting if the old keys stop counting.
Lost key? Change it anyway.
It does not need to have been stolen; a key that turns up in someone else's hand works just as well for them.
New home? Assume there are copies.
Previous tenants, tradespeople and letting agents may all still have keys you have never seen.
Keep a written tally.
Note how many keys came with the new lock and who has each one.
Want one key for several doors? Ask first.
Keyed-alike locks are best ordered when you buy; getting there later can mean changing or rekeying locks a second time, and some locks will not allow it.
Keep the spare elsewhere.
It belongs with a trusted person or somewhere secure away from the property, not inside it.
All of this applies to property you own or rent. If you rent, check what your tenancy agreement says and contact your landlord or letting agent before making changes.
Some readers will prefer to pass this on. The right solution depends on the door, the hardware already there and the reason for the change, and occasionally a repair or adjustment beats a full replacement. Have your postcode, the door type and a short description of the problem ready, and say whether a working key still exists and whether the door opens normally. As for hiding a key outside: that is not a spare, it is an unlocked door with extra steps. If the only key has gone, see what to do when the only key is lost.
When to Stop and Call a Locksmith
You can do most of this job yourself. The exceptions are specific, and each comes with a reason, so here they are.
Testing does not go smoothly.
If the new lock does not operate cleanly during the checks, or leaves you uncertain whether the door can be secured and opened reliably, stop.
Your insurer names a standard.
If you are not certain the replacement meets it, check your policy before fitting it.
It is a fire door, or people escape through it.
Replacement hardware must preserve the door's required fire protection and safe escape functions, and unauthorised alterations could compromise its performance. That includes a flat entrance door onto a shared corridor, where the lock may need to open from inside without a key.
The hardware is specialist.
A multipoint locking system, particularly one with a failed mechanism, or any unfamiliar mechanism may need a more detailed assessment.
The door or frame is misaligned.
Working open but binding closed points to hinges, frame or strike plate.
There is no key.
Conventional removal is not possible without one.
Removal stalls.
Seized screws, damaged fittings or a jammed mechanism may need specialist assessment rather than more force.
The replacement is the wrong shape for the recess.
If the case, faceplate or fixing holes do not align, forcing it or cutting the door could cause further damage.
With the door in good condition and a new lock that matches its existing fittings, a simple lock replacement is often a manageable task. If the job needs the door altered, involves damaged components or turns into a locking problem you are not familiar with, professional assistance may be safer.
Stop the replacement when:
An apparent fit does not, by itself, make a lock suitable. By assessing the problem, a locksmith can establish whether a compatible replacement will suffice, an adjustment is needed or more extensive work is required. What it costs depends on the lock, its condition and any additional fitting or repair, and a properly fitted change usually costs less than a change plus repairing a wrongly cut door. For that route, see having the lock replaced for you.
Have a Lock Changed in Runcorn Instead
Handing the job over is a perfectly sensible choice. Eds Locksmiths Runcorn fits replacement locks throughout Runcorn, 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?
For a compatible replacement, changing the lock is not usually especially difficult. Conventional internal locks often require only basic tools, while standard mortice locks and nightlatches can be uncomplicated. Fire-rated doors, specialist fittings and doors with particular escape needs require greater care. Unfamiliar hardware, incompatible fittings or door problems add work. Damaged components, poor alignment or changes to the door may make a locksmith necessary.
What tools do I need to change a door lock?
Begin with a screwdriver suited to the lock, a tape measure and a compatible replacement. That is the typical equipment for an uncomplicated change. Read through the maker's fitting instructions before proceeding, as individual models can require tools beyond this basic selection.
How long does it take to replace a door lock?
Usually, replacement is manageable when the lock and existing fittings match; conventional mortice locks and nightlatches can be uncomplicated. A tape measure, appropriate screwdriver and compatible replacement are generally needed. Before beginning, review the manufacturer's instructions for any extra tools. Unfamiliar mechanisms, mismatched fittings or door problems can lengthen the job. Damaged parts, misalignment or changes to the door may warrant a locksmith.
Can I replace two door locks with one key?
One key can serve two locks when they support a keyed-alike arrangement. Before purchase, establish compatibility; suitable locks may be supplied or sometimes rekeyed to share a key. A locksmith can advise whether your type permits rekeying. An existing key works only with a replacement keyed to match, otherwise expect new keys. Basic tools often suffice for compatible conventional internal locks. Take extra care with fire-rated doors, specialist fittings and escape requirements. Wrong sizing can prevent recess fit or alignment with the keyhole, handle or strike plate. Confirm suitability before forcing anything or changing the door.
Can I change the locks on a rented property?
There is no single answer without considering your tenancy agreement and circumstances. Read the terms governing the tenancy and speak with the landlord or letting agent before changing a lock. Report a faulty lock immediately when it gives rise to an urgent safety concern.
Can I replace a door lock and keep the same key?
You may retain the key if a suitable replacement can be keyed to match it. Check compatibility before ordering, and ask a locksmith whether rekeying is possible for the lock type involved. Without matching, different keys normally accompany the new lock. Keyed-alike arrangements also let compatible locks share a key, through supply or sometimes rekeying. Many conventional internal locks are manageable with basic tools if the replacement fits. Extra care is necessary with specialist hardware, fire-rated doors and doors governed by specific escape requirements.
Can I replace an internal door lock myself?
You can often replace a conventional internal lock yourself when the new fitting is compatible; basic tools may be sufficient. Matching existing fittings usually keeps mortice lock and nightlatch changes manageable. Damage, alignment issues or door modifications may require a locksmith. Specialist hardware, fire-rated doors and escape requirements need extra attention. Before purchase, check compatibility and ask a locksmith about your type's rekeying options. Compatible keyed-alike locks can be supplied or sometimes rekeyed for shared use. An existing key needs a replacement matched to it; otherwise, expect different keys.
Do I need to replace the door handle when changing a lock?
Not necessarily: a sound handle that matches the replacement lock can remain. Examine the fixing positions, assess the spindle and check the handle's movement before using it again. The decision depends on the existing handle's compatibility and condition rather than the lock change alone.
What happens if the replacement door lock is the wrong size?
An incorrectly sized lock may fail to fit the recess or line up with the keyhole, handle or strike plate. Establish suitability before forcing it into position or changing the door. Check compatibility before purchase, including any keyed-alike requirement. Locks supporting that arrangement can share a key, with compatible models supplied or sometimes rekeyed for that purpose.
Why is my new door lock stiff after fitting?
The cause may be incorrect fitting, misalignment or pressure where the door meets its frame. Check operation with the door open before testing further. Persistent resistance calls for a compatibility check and professional assistance, without forcing the mechanism. Confirm the lock type and its measurements when assessing whether the replacement is appropriate.