Guide
How to Replace a Door Lock: Measuring, Fitting and When to Stop
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Which Lock Are You Replacing?
Stand in front of the door and answer three questions. Between them they tell you which lock is fitted, and that, rather than anything on a shop shelf, decides what you will be replacing.
Which material is the door?
A timber door points towards a mortice lock, a nightlatch, or both. If the door is uPVC or composite, that suggests a multipoint locking system incorporating a euro cylinder.
Where does the key go in?
- A keyhole in the door's edge means a mortice lock.
- A keyhole in the door's face, near the handle, means a euro cylinder.
- A cylinder on the outside face with a box-shaped case on the inside means a nightlatch with a rim cylinder.
What does the handle do?
If lifting it throws bolts up and down the frame, you have a multipoint mechanism, with the cylinder as only one part of it.
Reading the answers
| Lock type | Recognising it |
|---|---|
| Mortice deadlock | Lock body inside the door edge, key-operated deadbolt, usually no handle connected |
| Mortice sashlock | Fitted in the edge like a deadlock, with a handle-operated latch and a key-operated deadbolt |
| Nightlatch and rim cylinder | Box-shaped case on the inside face, key-operated cylinder accessible from outside |
| Euro cylinder and multipoint lock | Often on uPVC and composite doors, a cylinder working with a mechanism that may secure the door at several points |
Mortice locks in more detail
These sit in a recess cut into the edge of a timber door, and the common types are deadlocks and sashlocks. With a deadlock, the key operates a deadbolt. With a sashlock, a key-operated deadbolt is combined with a spring latch that the handle draws back. You may find a five-lever mortice lock on an external timber door, and some of these are models designed to meet BS 3621. Even so, neither compliance with that standard nor fulfilment of your insurance requirements can be established from the lever count. For your replacement choice, establish the lock’s measurements and security specification and whether its type is deadlock or sashlock.
Nightlatches in more detail
A nightlatch is mounted on the inside face of a door, not recessed into its edge. The usual arrangement combines an outside rim cylinder operated by a key with a latch worked from inside by a knob or lever. The case and the cylinder are separate components, which means that if only the cylinder is faulty, or the keys need changing, you may not need a whole nightlatch. If the complete nightlatch must be replaced, its backset, fixing arrangement and compatibility with the existing door preparation come into play, and those measurements are dealt with later in this guide.
Euro cylinders in brief
A fault in the multipoint mechanism will not necessarily be remedied by changing the cylinder, or vice versa, since the two are separate components.
Identify the lock before buying a replacement. Mortice locks, nightlatches and euro cylinders vary in design, measurements and fitting requirements, and choosing the wrong type can mean the new lock will not fit even if it resembles the original. The common timber door locks, mortice locks and nightlatches, use different measurements and fixing arrangements, and getting these details wrong can leave a lock that does not fit or work. In short, you identify the existing lock, measure its dimensions and fit a compatible replacement, and with matching fittings a straightforward change can often be completed with basic tools. Choosing the replacement is the challenge. This guide covers identifying, measuring, removing and fitting it, including testing before the door is closed, concentrates on conventional timber door locks, explains where uPVC and composite doors need a different approach and highlights when professional help is the better option.
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.
Measure the Lock Before Buying It
Here is the step that decides whether your lock change works. The replacement has to slot into the recess and fixings the current lock already uses, so you are matching a lock, not picking one, and you match it by measuring first. If that sounds like more than you want to take on, consider having a new lock fitted properly.
The Five Measurements of a Mortice Lock
ERA defines five dimensions for a mortice lock in its mortice lock measuring guide. Three of them, case depth, backset and centres, decide most fits.
| Measurement | How to take it | Common UK size |
|---|---|---|
| Faceplate | Length and width of the plate on the door edge; note square or rounded ends | No single size |
| Centres | Sashlock only: keyhole centre to centre of the square spindle hole | No single size |
| Case height | Lock body, top to bottom | No single size |
| Backset | Lock's front edge to keyhole centre | Approximately 44 mm or 56 mm, matching the case |
| Case depth | Case front edge to back | 64 mm (2½ in) or 76 mm (3 in) |
Each common size is typical, so record your own figures and bring the old lock when you buy.
Measuring Rim Cylinders or Nightlatches
With a nightlatch, the case is fixed to the inside face of the door, so you note the backset from the door edge to the middle of the cylinder hole, the case footprint, whether it deadlocks or is a standard latch, and the cylinder length through the door to the tailpiece. If only the cylinder is worn, it can often be replaced without the case.
Gather the tools
For a straightforward change you may need just a screwdriver and a measuring tape or ruler, though it depends whether you are changing a mortice lock, a nightlatch or only the rim cylinder. Before starting, have ready a suitable cross-head or flat-head screwdriver for the existing screws; a tape measure or ruler for dimensions and size checks; a compatible replacement matching the existing type, measurements, fixing positions and the security specification you need; your existing keys, where available, to check the old lock and compare it with the replacement; and the manufacturer's fitting instructions for model-specific fixings, installation requirements and extra tools. An Allen key or another manufacturer-specified tool is required by some replacements.
Before anything comes out
If you would have to drill fresh holes, cut into the door or substantially alter the recess to fit the new lock, stop and confirm you have the correct replacement. Keep the door open throughout. Keep the old lock fitted while you confirm the replacement's suitability, and ensure that the property can be secured if you cannot finish the work.
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 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.
The Replacement: Fit It
A matched lock slides home; a mismatched one fights you. Keep that in mind throughout, because anything that needs forcing is the wrong size or is being fitted the wrong way round, and the faults it causes tend to appear months later.
How to fit it
- Orientation check. Confirm the bolt and latch face the correct way before you go further. A latch installed backwards will not close the door.
- Dry fit. Put the lock in its recess without screws. Its faceplate ought to lie level with the edge, with no lip standing proud.
- Screws, loosely. Drive the faceplate screws finger tight, operate the lock, and tighten them once everything moves freely.
- Handles back on. For a sashlock, refit the spindle and handles, having checked the spindle is long enough for the new case.
- Keep alignment. If the new bolt projects a different distance, the keep may need adjusting. Fitting goes wrong here more often than anywhere else.
The honest caveat
A new faceplate that is larger than the existing recess means cutting the door, and that is a reasonable point at which many people stop. A fire door or a certified door should not be cut or altered by anyone except someone qualified to work on it.
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.
Dealing With the Old Keys
Most locks are changed because of a key, not because of a lock. That makes this the step that actually finishes the job: deciding which keys still matter, and making sure the rest no longer do.
Your key checklist
- Know your set. Note how many keys arrived with the new lock and write down who has each one.
- Plan for the keys you cannot see. In a home you have just moved into, expect that previous tenants, tradespeople or a letting agent may still hold copies.
- Treat a lost key like a stolen one. Whoever picks up a found key can use it, so a lost key is still a reason to change the lock.
- Ask about keyed alike at the counter. One key for several doors is far simpler to arrange when you buy; doing it afterwards may involve changing or rekeying locks again, and some locks cannot be rekeyed at all.
- Store the spare off the premises. A trusted friend or relative is a better home for it than a drawer in the same house.
This advice assumes the property is yours or you rent it; tenants should check their agreement before changing any lock.
If you would rather a locksmith dealt with it
The right solution depends on the door, the existing hardware and the reason for replacement, and in some cases adjusting or repairing the current fittings is more suitable than replacing them all. When you call, have your postcode, the door type and a brief description of the problem to hand, and mention whether you still have a working key and whether the door opens normally.
Finally, a key under the mat or in a plant pot is not a spare; it is an open door that takes a few seconds longer.
Instead, Get a Lock Changed in Hastings
Want the lock changed without doing it yourself? B.T Locksmiths Hastings supplies and fits replacement locks in Hastings, and that covers the doors this guide advises against tackling alone.
To price it accurately, we need to know the door material, where the keyhole is, and whether you still have a key. With those answers you get a price on the call, and if the door turns out to need something different, that is explained before any work starts.
You are never charged a call-out fee, day or night.
Emergency calls are taken at any hour throughout the year, weekends and bank holidays included. Contact us here if you would rather not phone.
Door Lock Replacement Questions
Is it difficult to replace a door lock yourself?
Matching the replacement to the existing lock and fittings usually keeps the job manageable. Conventional mortice locks, nightlatches and many internal door locks can be straightforward to change with basic tools. Extra time may be needed for an unfamiliar lock, unsuitable fittings or door trouble. A locksmith may be necessary for damage, alignment faults or door alterations. Fire-rated doors, specialist hardware and escape requirements warrant closer attention.
What tools do I need to change a door lock?
Most uncomplicated replacements need three basics: a suitable new lock, an appropriate screwdriver and a tape measure. Check the maker's fitting directions before work begins. The model determines whether other tools are necessary, so treat that basic list as a starting point rather than a complete list for every lock.
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?
It is possible if the two locks allow keyed-alike matching. Ask a locksmith whether your lock type can be rekeyed, and verify compatibility before buying. Compatible locks may be supplied or sometimes rekeyed for shared operation. Retaining your present key needs a replacement matched to it; a new lock otherwise normally brings different keys. Many ordinary internal locks are basic-tool replacements when compatible. Specialist hardware, fire-rated doors and particular escape needs require care. Unsuitable sizing may mismatch the recess, keyhole, handle or strike plate. Do not force installation or modify the door without confirming suitability.
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?
Sometimes a compatible new lock can be set up around the key you already use. If not, it will normally come with different keys. Before committing to a purchase, check compatibility and ask a locksmith about rekeying that type of lock. Suitable locks may be supplied or sometimes rekeyed as keyed alike, allowing shared key use. Basic-tool replacement is often possible for conventional internal locks with matching fittings. Additional care applies to fire-rated doors, specialist equipment and doors with defined escape requirements.
Can I replace an internal door lock myself?
A straightforward internal lock change is often achievable with basic tools and a compatible replacement. Conventional mortice locks and nightlatches are usually manageable when the current fittings match. Damaged hardware, misalignment or alterations may need locksmith assistance. Take additional care with fire-rated doors, specialist equipment and particular escape requirements. Confirm compatibility before purchasing. For shared keys, suitable locks supporting keyed-alike arrangements may be supplied or sometimes rekeyed. Ask a locksmith whether your lock type allows rekeying. Retaining your current key depends on a compatible replacement being matched to it; otherwise, new keys are usual.
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?
Misalignment is one possible explanation, alongside incorrect installation and pressure from the frame. First assess how the lock works while the door remains open. If it still resists, check compatibility and consult a qualified locksmith without forcing the mechanism. Establishing the lock type and taking careful measurements support that suitability check.