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
Ask one question before shopping: is it the lock, or the door around it? A replacement solves a faulty lock and no other problem, while a stiff or unreliable lock does not always need replacing at all. Problems with the door's hinges, frame or strike plate can produce symptoms almost identical to a lock fault, and sorting out which is which first may save you unnecessary work and expense.
The Lock Itself Has Failed: Signs
Locks can need repair or replacement when their internal components stop operating reliably. Four symptoms suggest it:
- The key turns but does not move the bolt, which may mean an internal component is worn, damaged or disconnected.
- Resistance when inserting or turning the key may result from wear, damage or an internal lock fault.
- Occasional jamming may occur when wear or damage to the mechanism interferes with consistent bolt movement.
- A fragment left by a key that has broken off inside may interfere with normal operation and need professional removal.
A full lock replacement is not necessarily needed when these symptoms appear. The lock's construction and state determine whether a fault can be put right through repair or replacement of a single part. Security concerns also lead some people to replace a functioning lock after moving into a new property, losing keys, or becoming uncertain about who retains access.
The Door, Not the Lock, Needs the Work: Signs
Stiffness that appears only when the door is shut, while the lock works smoothly with it open, may mean the bolt is out of alignment with its keep or strike plate. Other signs of possible misalignment:
- the gap between door and frame looks uneven;
- the door catches against the frame or has become harder to close;
- scrape marks appear around the strike plate or where the bolt meets the frame;
- the door needs lifting, pushing or pulling before the key will turn.
Loose or worn hinges, movement in the frame and moisture-related changes in the timber can all affect alignment, and replacing the lock alone may not solve them.
The door-open test. If the door opens normally, open it wide and work the lock gently without closing it. Difficulty operating the key and bolt after closing the door, despite both moving freely in that position, may indicate that misalignment is part of the problem. Wear or damage to the lock's internal parts remains possible, even with that helpful indication.
The Old Lock: Remove It
Here is the safety line, before any instruction: do this with the door open and securely supported, and with a means of securing the property ready in case the replacement cannot be completed. Once you are sure the new lock matches the old one, the old one can come out.
Nightlatch removal
- From the inside, unscrew the nightlatch case.
- Remove the case from the door face.
- Take out the connecting screws.
- Pull the cylinder free from the outside.
Mortice lock removal
- Remove the handles and spindle if the lock is a sashlock.
- Unscrew the faceplate from the door edge.
- Slide the lock body out of the mortice, easing it rather than levering.
- If it holds fast, check under the faceplate for a second screw instead of pulling harder.
Although the two lock types come out differently, the door stays open for both. A comparison between the new components and the original screws, handles and fittings may be needed, so put the originals to one side together.
Drilling, levering or forcing a lock is never part of this job. If the lock will not come out, stop. If a key snaps while you are removing it, stop then too, as a fragment in the mortice makes the remaining work harder.
Measure the Lock Before Buying It
Buying the wrong size is where most DIY lock changes go wrong, and it happens before a single screw is touched. The rule is simple. You measure the lock already in the door, then buy one that matches it, because the replacement must suit the existing hole rather than the other way round.
The kit
Often a screwdriver plus a tape measure or ruler is enough for a straightforward change. The full list depends on what you are replacing, whether a mortice lock, a nightlatch or just a rim cylinder.
| Item | Why you need it |
|---|---|
| Screwdriver | Cross-head or flat-head, to remove and refit the existing screws |
| Tape measure or ruler | To take the dimensions and confirm the new lock's size |
| Compatible replacement lock | Must match the existing type, measurements, fixing positions and required security specification |
| Existing keys | Where available, for checking the old lock and comparing it with the new one |
| Manufacturer's fitting instructions | To identify model-specific fixings, installation requirements and any extra tools |
Some locks also need an Allen key or another tool the manufacturer specifies.
The Five Measurements of a Mortice Lock
Five measurements describe the lock, and the definitions below follow ERA's guide to measuring a mortice lock:
- Backset, from the lock's front edge to the keyhole centre.
- Case depth, the whole depth of the case from front to back.
- Centres, for sashlocks only, between the keyhole centre and the centre of the square spindle hole.
- Case height, from the top of the body to the bottom.
- Faceplate, its length and width and whether the ends are square or rounded.
For mortice locks commonly used in the UK, a 64 mm (2½ inches) case depth often corresponds to a backset of about 44 mm, while a 76 mm (3 inches) case depth often corresponds to about 56 mm. Those are typical figures only. Write down your own five and take the old lock to the shop.
Measuring Rim Cylinders or Nightlatches
Here the case sits on the inner face. Note the backset from the door edge to the centre of the cylinder hole; the case footprint on the door; whether the latch is standard or deadlocking; and the cylinder length from the outer face to where the tailpiece comes through.
Should the new lock need fresh holes drilled, the door cut or the recess substantially altered, double-check that you have the right replacement before you continue. The door remains open from start to finish. Be sure you can still secure the property if the work stalls, and do not remove the old lock until the replacement has been checked as suitable.
Before You Shut the Door, Test It
A lock you have just fitted can lock you out of your own home, which is why nothing in this guide matters more than this: test it with the door open, and do not close the door until every test passes. A poorly fitted latch, a misaligned bolt or an incorrectly connected handle are all easy to correct with the door open and very hard to correct from the wrong side of it.
First close
Close the door only once those checks have passed, and for the first closed-door test make sure another responsible person is present or you have another safe means of access. Check the latch engages, the key works as normal and the door reopens easily, then test the remaining keys and functions from both sides. The manufacturer's fitting instructions take priority where a lock has additional testing requirements, and Yale's official lock fitting instructions are one place to find model-specific guidance. Fine when open but stiff or not engaging when closed? Alignment may be contributing. Do not force the key or handle.
Afterwards
Check every supplied key works and decide who needs a copy; keep a spare secure rather than hidden outside. A lock changed after moving home means old keys should no longer work through it, useful when copies held by previous occupants or contractors cannot be accounted for. Consider whether a missing or stolen key also operates other locks and whether it could be traced to your address. Some compatible locks can be supplied keyed alike or rekeyed so one key serves several doors, depending on design, though not every lock supports it, so weigh that up before buying. Keep useful old components secure until they are no longer needed. You are aiming for access controlled by the keys you intend to keep in use, not merely a new lock in the door.
Alignment
With the door still open, hold it in its closed position and look at where the latch and bolt meet their keeps on the frame. Note any obvious difference in height, or anything suggesting the hardware will catch when the door closes.
Internal release
Check the door can be unlocked and opened from inside as intended. This matters most where it forms part of an escape route. Should the replacement have changed a required escape function, do not proceed.
Key
Use a gentle turning motion with the new key to check both locking and unlocking. Movement should be smooth, with no sticking and no excessive force.
Deadbolt
On a mortice deadlock or sashlock, operate the key and confirm the deadbolt extends and retracts fully, moving freely without catching inside the case.
Latch and handles
With a sashlock, the handles should retract the latch and return to their normal position. For nightlatch checks, follow the manufacturer's instructions as you try the key from outside, the knob or lever from inside, and any extra locking controls.
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.
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.
Get a Lock Changed in Boston Instead
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Have ready when you call: what the door is made of, where the keyhole sits, and whether you still have a working key.
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Knowing When to Stop and Call a Locksmith
This guide has given you the whole job, so it is only fair to say where it ends. Many replacements are perfectly manageable: the new lock matches the old fittings, the door is in good condition, and the work goes as described. Altering the door, coping with damaged parts or working through an unfamiliar locking problem is different, and there professional help may be the safer option.
A failed multipoint mechanism or other specialist hardware.
Multipoint systems, mechanisms you do not recognise and locks with specific insurance requirements may need a more detailed assessment.
No key.
With nothing to operate the lock, it cannot be removed conventionally.
A fire door or escape route.
Replacement hardware must preserve required fire protection and safe escape functions, and unauthorised changes could compromise them; a flat entrance door onto a shared corridor, which may need to open from inside without a key, is a typical case.
A lock that will not fit.
If case, faceplate or fixing holes do not align, forcing or cutting could cause further damage.
An old lock that will not come out.
Screws that have seized, fittings that are damaged or a mechanism that has jammed are reasons to have it assessed, never to push harder.
A door that binds.
Fine open, tight closed, means the hinges, frame or strike plate may need attention.
An insurance standard you cannot confirm.
Check your policy; do not assume.
Stopping points
Also stop if the replacement does not operate smoothly in testing or leaves you unsure the door can be secured and opened reliably. Fitting the hole is not the same as being suitable for the door. The need for a compatible replacement, an adjustment or more extensive work is something a locksmith can evaluate. The cost depends on the lock, its condition and any additional fitting or repair, yet a fitted lock change normally costs less than a lock change plus a repair to a door cut in the wrong place.
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?
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?
Start by checking the manufacturer's fitting instructions: some models need additional tools beyond the usual suitable screwdriver, tape measure and compatible lock. Replacing a conventional mortice lock or nightlatch is often uncomplicated when it matches the current arrangement. Unfamiliar hardware, unsuitable fittings or door trouble can add time. A locksmith may be needed where components are damaged, alignment is wrong or the door requires modification.
Can I replace two door locks with one key?
Yes, provided both locks are compatible with keyed-alike operation. Suitable locks can be supplied together or sometimes rekeyed for one key. Check compatibility before buying and ask a locksmith about rekeying your lock type. Keeping an existing key requires a replacement that accepts that key; otherwise, different keys normally accompany it. Many conventional internal locks need only basic tools if compatible. Fire-rated doors, specialist hardware and escape requirements need extra care. A mismatch may affect the recess, keyhole, handle or strike alignment. Never force the lock or alter the door until suitability is confirmed.
Can I change the locks on a rented property?
Check the tenancy agreement first, because the answer depends on its terms and your circumstances. Contact the landlord or letting agent before carrying out a replacement. If a lock fault creates an urgent concern about safety, raise it immediately rather than delaying the report.
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?
Yes, basic tools often suffice for replacing a compatible conventional internal door lock. Replacement is usually manageable when existing fittings match, including ordinary mortice locks and nightlatches. Door alterations, damage or alignment faults may need a locksmith. Give fire-rated doors, specialist hardware and particular escape provisions extra care. Check compatibility before ordering; ask a locksmith whether your lock type permits rekeying. Locks supporting keyed-alike arrangements may be supplied or sometimes rekeyed for one key. A compatible replacement may match your current key, but otherwise different keys normally accompany the new lock.
Do I need to replace the door handle when changing a lock?
A new handle is not automatically necessary. Retain the existing one if its condition is good and it works with the new lock. Check where the fixings sit, examine the spindle and assess the handle's operation before choosing to use it again.
What happens if the replacement door lock is the wrong size?
Poor fit may prevent the new lock entering the recess or aligning with the keyhole, strike plate or handle. Avoid forcing it or modifying the door without first establishing suitability. Check compatibility before ordering, including whether keyed-alike operation is supported if a shared key is wanted. Suitable locks may be supplied for one key or sometimes rekeyed to achieve that.
Why is my new door lock stiff after fitting?
Frame pressure, fitting mistakes or an alignment fault may make a replacement lock stiff. Try operating it with the door open first. If stiffness remains, establish compatibility and seek help from a qualified locksmith rather than applying force. Assess the lock type and measurements as part of checking that a suitable replacement has been fitted.