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.
Is the Lock Actually the Problem? Check
Replacing a lock that was never broken is one of the commonest wasted jobs on a front door. A lock that feels stiff or unreliable is not always worn out; trouble with the hinges, the frame or the strike plate can mimic a lock fault closely. Checking which you have, before you buy, may save you unnecessary work and expense.
Signs the Work Is Needed on the Door, Not the Lock
A lock that is smooth with the door open and stiff with it shut suggests the bolt may not be meeting its keep or strike plate in line. Other signs of possible misalignment:
- marks scraped into the strike plate, or into the frame where the bolt strikes;
- an uneven gap between door and frame;
- a door that catches on the frame or takes more effort to close;
- needing to push, pull or lift the door before the key will turn.
Alignment can be thrown out by loose or worn hinges, by movement in the frame and by changes in the timber caused by moisture. These issues may persist after replacing only the lock.
Signs of the Lock Itself Failing
Repair or replacement may be needed if the parts inside a lock stop working dependably. Typical signs:
| Symptom | What it may mean |
|---|---|
| Key turns, bolt stays put | An internal part may be worn, damaged or disconnected |
| Key hard to insert or turn | Wear, damage or a problem inside may be causing resistance |
| Lock jams intermittently | A worn or damaged mechanism may stop the bolt moving consistently |
| Key snapped in the lock | The fragment may obstruct operation and need professional removal |
These symptoms do not automatically mean the complete lock has to be replaced. Depending on the lock's design and condition, repairing or replacing an individual component may be enough.
You might also decide to change a lock that works perfectly well for security reasons: you have moved in, you have lost keys, or you cannot be sure who still has access.
The check takes seconds. With the door opening normally, swing it fully open and operate the lock gently. Does the key turn and the bolt travel freely now, then struggle once the door is closed? Then misalignment may be contributing. Bear in mind that this is a clue, not proof: wear or damage inside the lock is still possible.
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?
Now Fit the Replacement
If the measurements were right, the new lock will go into the old recess without any effort. Treat resistance as information: a lock that has to be forced is either the wrong size or the wrong way round.
First, a dry run. Without screws, slot the lock into the recess and check that the faceplate finishes flush with the door edge.
Second, the direction. The latch and bolt need to face the way the door closes. A latch fitted backwards will not close.
Third, loose screws. Fit the faceplate screws only finger tight, then work the lock with the key before tightening them down.
Fourth, the handles. On a sashlock, refit the spindle and handles, and check the spindle is long enough to engage the new case properly.
Fifth, the keep. If the new lock's bolt projects differently, the keep may need moving or adjusting. More lock changes go wrong at this step than at any other.
There is one further note. When the new faceplate is bigger than the old recess, the door needs cutting, and that is where many people should stop and think again. Fire doors and certified doors are different: alterations to them should be left to someone qualified to work on them.
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.
Dealing With the Old Keys
The tools can go away now. What is left is accounting for keys, because a lock change only does its job when the keys you no longer trust stop working and the ones you do trust are all accounted for.
Key control in five steps
Keyed alike. Decide now, not later, whether one key should open more than one door. Ask for keyed-alike locks when you buy; afterwards it may take changing or rekeying the locks again, and not every lock can be rekeyed.
Spare. Keep one, but keep it away from the property, with someone you trust or somewhere secure.
Unknown copies. In a property you have just moved into, expect that keys exist which you know nothing about, held by former tenants, tradespeople or letting agents.
Lost keys. Even if a key was only lost, the change is justified, because a found key works for anyone who finds it.
Records. Note how many keys came with the new lock and who is holding each of them.
Only change the locks on a property you own or rent, and tenants should check their agreement with the landlord or letting agent first.
Handing it over
If you would rather a locksmith took this on, the best solution depends on the door, the existing hardware and the reason you are replacing the lock; adjusting or repairing what is fitted can be more suitable than replacing everything. Have your postcode, door type and a brief description ready, and mention whether you still have a working key and whether the door opens normally.
Hiding a key outside does not give you a spare. It gives you an unlocked door with extra steps.
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.
Instead, Get a Lock Changed in Bournemouth
If you would rather not do it yourself, HD Locksmiths Bournemouth can supply and fit the replacement lock anywhere in Bournemouth, and that includes fire doors, multipoint systems and the other jobs flagged above as ones to leave alone. You will hear the price on the call, based on what you tell us; should anything on site alter it, we explain before starting. Coming out to you is never charged, whatever the time of day.
Three details help us price it: the door material, the position of the keyhole, and whether a key still works.
Emergency calls are answered round the clock, every day of the year. Not urgent? Use the contact form.
When to Stop and Then Call a Locksmith
Plenty of lock changes go exactly to plan, especially when the replacement matches the existing fittings and the door is in good shape. A few do not, and recognising them early is the skill. If you find yourself needing to alter the door, deal with damaged components or solve a locking problem you have not met before, a professional may be the safer option.
Do not continue with an ill-fitting lock
If the case, faceplate or fixing holes are misaligned, further damage could result from forcing the lock into position or cutting into the door.
Stop if the door is out of line
When the replacement works with the door open but binds when it is shut, the hinges, frame or strike plate may need attention.
Stop if the door is fire-rated or an escape route
Its hardware must preserve the door's required fire protection and safe escape functions, and unauthorised alterations could compromise them. Flat entrance doors onto shared corridors belong here, because the lock may need to open from inside without a key.
Stop if the old lock will not come out, or there is no key
Seized screws, damaged fittings or a jammed mechanism may need specialist assessment rather than extra force, and with no key nothing can be removed conventionally.
Stop if the hardware is specialist or insured
Multipoint systems, including a failed multipoint mechanism, and unfamiliar mechanisms may need a more detailed assessment. If your insurer specifies a standard and you are unsure the replacement meets it, check your policy.
Stop if testing leaves doubt
A lock that does not operate smoothly during testing, or that leaves you uncertain whether the door can be secured and opened reliably, is a reason to stop.
Bear in mind that an apparent fit does not establish that a lock is suitable for the door. A locksmith can assess whether a compatible replacement, an adjustment or more extensive work is needed, with the cost depending on the lock, its condition and any additional fitting or repair. A fitted change typically costs less than a change plus fixing a door that was cut wrongly; see having the lock replaced for you.
Door Lock Replacement Questions
Is it difficult to replace a door lock yourself?
Usually, it is manageable when the replacement suits the existing lock and door fittings. A conventional mortice lock or nightlatch can be straightforward, and many internal locks need only basic tools. Unfamiliar mechanisms, incompatible parts or door problems can extend the work. Damage, poor alignment or necessary door alterations may call for a locksmith. Fire-rated doors, specialist fittings and doors with specific escape requirements deserve extra care.
What tools do I need to change a door lock?
The usual essentials are a replacement that fits, a measuring tape and a screwdriver appropriate to the job. Confirm the equipment requirements in the manufacturer's instructions before starting. A straightforward change may need only those basics, while certain models call for further tools during fitting.
How long does it take to replace a door lock?
Conventional mortice locks and nightlatches can be straightforward if the replacement matches the existing lock and door fittings. Most such jobs use an appropriate screwdriver, measuring tape and compatible new lock. Check the maker's installation guidance in advance for model-specific extra tools. Unfamiliar locks, incompatible fittings and door faults can cause delays; damage, misalignment or door alterations may make professional locksmith help necessary.
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?
That depends on the tenancy terms and the circumstances involved. Read the agreement and speak to the landlord or letting agent before replacing a lock. If a defective lock presents an urgent safety problem, report the fault immediately rather than treating it as a routine change.
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?
A compatible conventional internal lock can often be changed yourself using basic tools. Ordinary mortice locks and nightlatches are usually manageable when the replacement matches existing fittings. A locksmith may be needed for damage, alignment problems or door modifications. Take extra care with fire-rated doors, specialist fittings and particular escape needs. Confirm compatibility before purchase. Keyed-alike locks can share one key through compatible supply or sometimes rekeying; ask a locksmith about your type. Retaining your current key needs a replacement matched to it. Without that option, new locks generally include 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?
An installation problem, frame pressure or poor alignment could explain the stiffness. Operate the lock with the door open as the initial test. Where resistance persists, confirm that the lock is compatible and arrange professional help rather than forcing movement. Check its type and measurements when deciding whether the replacement is suitable.