Guide

How to Replace a Door Lock: Measuring, Fitting and When to Stop

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Which Lock Is It That You Are Actually Replacing?

A replacement lock has to suit the door it goes into, so the door, rather than preference, decides what you buy. Here are three questions to answer while you look at it, each with an "if this, then that" answer.

Material. If the door is timber, you are in the right guide. If it is uPVC or composite, a different approach is needed: many such doors use a euro cylinder within a multipoint locking system.

Keyhole. If it is in the edge, then it is a mortice lock. If it is in the face near the handle, then it is a euro cylinder. If there is a box on the inside face and a cylinder outside, then it is a nightlatch with a rim cylinder.

Handle. If lifting it moves bolts up and down the frame, then a multipoint mechanism is fitted, and the cylinder is only one part of it.

While you are there, watch how the handle and key operate, since that separates the two mortice types.

Deadlock or sashlock?

Mortice locks are fitted into a recess cut into the edge of a timber door, and the common pair are deadlocks and sashlocks.

A mortice deadlock has its body inside the door edge and uses a key to operate a deadbolt, usually with no handle linked to it.
A mortice sashlock is fitted into the edge in the same way but combines a key-operated deadbolt with a spring latch that a handle operates.

Five-lever mortice locks are fitted to some external timber doors, including certain models designed to meet BS 3621. Nevertheless, the number of levers is insufficient to establish either compliance with that standard or fulfilment of your insurance requirements. Check which you have, together with its dimensions and security specification, before choosing a replacement.

Nightlatch and rim cylinder

Mounted on the door’s inside face, a nightlatch has a box-shaped latch case instead of one recessed into the edge; its cylinder is operated by a key from outside. In the usual arrangement, that outside rim cylinder is combined with a latch operated from inside using a knob or lever. The two are separate components. A complete nightlatch replacement may be unnecessary when the fault is confined to the cylinder or you simply want different keys. If the complete nightlatch is being replaced, its backset, fixing arrangement and compatibility with the existing door preparation matter, and later sections of this guide show how to measure them.

Euro cylinder and multipoint lock

This arrangement, common on uPVC and composite doors, combines a cylinder and a mechanism through which the door may be secured at several points. As each is a separate component, a fault in either will not necessarily be resolved by replacing the other.

The point of all this

Mortice locks, nightlatches and euro cylinders have different designs, measurements and fitting requirements, and the wrong type can fail to fit even when it looks similar to the original. Mortice locks and nightlatches, the locks commonly found on timber doors, use different measurements and fixing arrangements as well, and getting those wrong can leave you with a lock that will not fit or operate properly. Replacing a door lock means identifying what is fitted, measuring its dimensions and fitting a compatible replacement; with matching fittings, basic tools often suffice, and choosing is the hard part. This guide goes on to measuring, removing and fitting the replacement and testing it before the door closes. Conventional timber locks form its main subject, with explanations of the differences affecting uPVC and composite doors and guidance on when to favour professional help. For the complete taxonomy, see the full range of UK door lock types.

Check Whether the Lock Is the Problem

Swapping the lock is the obvious fix for a door that will not lock easily, but it is only the right fix when the lock is what has failed. A stiff or unreliable lock does not always need replacing; problems with hinges, frame or strike plate can cause similar symptoms, and finding where the problem lies first may save you unnecessary work and expense.

The two-minute test

If the door opens normally, open it fully and operate the lock gently with the door open. Key and bolt moving freely, then becoming difficult once the door is closed, suggests misalignment may be contributing. Even with that helpful indication, wear or damage to the lock's internal parts remains a possibility.

The Door, Not the Lock, Needs the Work: Signs

Smooth operation with the door open and stiffness when it is shut can mean the bolt is not lining up correctly with its keep or strike plate. Check also whether:

the door catches against the frame, or is becoming harder to close;
you need to lift, push or pull the door before the key will turn;
the gap between the door and the frame looks uneven;
there are scrape marks around the strike plate or where the bolt meets the frame.

Hinges that have worked loose or worn, a frame that has shifted, and timber that has taken on or lost moisture can each pull things out of line, and a lock swap by itself may leave the fault exactly where it was.

The Lock Itself Has Failed: Signs

A lock may need repairing or replacing when its internal components no longer operate reliably:

a key that has snapped inside may leave a fragment that obstructs normal operation and needs professional removal;
a lock that jams intermittently may have a worn or damaged mechanism stopping the bolt from moving consistently;
a key that is difficult to insert or turn may be meeting resistance from wear, damage or a problem inside the lock;
a key that turns without moving the bolt may point to an internal component that is worn, damaged or disconnected.

A complete lock replacement is not an inevitable requirement for any of these symptoms. Repairing or replacing a single part can address some faults, with the lock's construction and state determining whether this is possible.

Reasons beyond a lock fault

Moving into a new property, losing keys or uncertainty about who retains access can also provide sound security reasons for replacing a functioning lock.

Next, Remove the Old Lock

Before the first screw, make the door safe to work on and the house safe to leave: open the door, support it securely, and keep something to hand that will secure the property should the job stop halfway. This assumes you have already matched the replacement to the existing lock.

If it is a mortice lock

Handles and spindle come off first, but only on a sashlock. Then unscrew the faceplate from the door edge. Ease the lock body out of the mortice gently; there is no need to lever it. If it will not slide out, check for a second screw beneath the faceplate rather than pulling harder.

If it is a nightlatch

Work from the inside out. Undo the case on the inside face and lift it off, remove the connecting screws, then withdraw the cylinder from the outside of the door.

Whichever it is

Keep the original screws, handles and fittings together for comparison with the new components.
Never drill, lever or force any part of a lock; if it will not come out, stop.
If a key snaps during removal, stop, because a fragment left in the mortice makes the job harder.

The process differs between the two lock types, yet both are removed with the door open and supported. If you have been left with a snapped key, see what to do if a key snaps off in the lock.

Measure the Lock Before You Buy It

Most failed lock changes fail at the counter. The replacement must fit the space the old lock already uses, so the existing lock is measured before anything is bought, and the aim is a match rather than a choice.

The Five Measurements on a Mortice Lock

Faceplate. Measure the length and width of the plate on the door edge and note whether its ends are square or rounded.
Case height. Measure the lock body from top to bottom.
Case depth. Measure the case from its front edge to its back. Common UK depths are 64 mm (2½ inches) and 76 mm (3 inches).
Backset. Take the measurement between the lock's front edge and the keyhole's centre. It is often around 44 mm on the 64 mm case and around 56 mm on the 76 mm case.
Centres. On a sashlock only, measure vertically from the keyhole centre to the centre of the square spindle hole.

These definitions and typical sizes come from ERA's measuring guide for mortice locks. Typical is not certain, so measure yours, note all five and bring the old lock along.

Measuring a Rim Cylinder or Nightlatch

With the case on the inside face, you record different things: the backset from door edge to cylinder hole centre, the size of the case where it sits on the door, whether the latch is standard or deadlocking, and the cylinder length from the outside face through to where the tailpiece passes. A worn cylinder can often be changed without touching the case.

Before going further, if the new lock would need fresh holes, cuts in the door or a substantially altered recess, confirm you have the correct replacement. The door stays open throughout. Make sure the property can be secured if you cannot complete the job, and keep the old lock in the door until you have checked the replacement is suitable.

Before you measure, gather the tools

A screwdriver and a measuring tape or ruler may be all a straightforward replacement needs. Changing a mortice lock, a nightlatch or only the rim cylinder each asks for slightly different things, so have these to hand:

Existing keys: if you have them, they let you check how the old lock works and compare it with the new one.
Screwdriver: cross-head or flat-head, matched to the screws you will take out and refit.
Manufacturer's fitting instructions: useful for model-specific fixings, installation requirements and any additional tools.
Tape measure or ruler: to measure the lock and confirm the replacement is the correct size.
Compatible replacement lock: matched to the existing type, measurements, fixing positions and required security specification.

Depending on the model, the maker may also specify an Allen key or a different tool as necessary.

Fit the Replacement Next

The principle is simple: a lock that matches the door goes in easily. Needing force means the size or the orientation is wrong, and small errors at this stage produce a lock that works on day one and struggles six months on.

Step 1 — Dry fit. Hold the lock in the recess without screws. The faceplate should sit flush with the door edge.

Step 2 — Orientation. Check that the bolt and latch face the correct way. Fitted backwards, a latch will not close.

Step 3 — Faceplate screws. Run them in finger tight, test the lock with the key, and then tighten.

Step 4 — Spindle and handles. For a sashlock, refit both, making sure the spindle is long enough for the new case.

Step 5 — The keep. Where the new bolt projects further or less than the old one, the keep may need adjusting. More fitting jobs go wrong here than anywhere else.

One honest note before you go further. A new faceplate that is larger than the old recess means cutting the door, and many people will reasonably decide to stop at that point. If the door is a fire door or a certified door, do not alter it yourself; that work belongs with someone qualified to carry it out. If you would rather not cut, look at lock installation done for you.

Deal With Your Old Keys

Fitting the lock was the mechanics. What you actually wanted, in most cases, was control over who can get in, and that depends on the keys.

Who has a key? Start by listing every key supplied with the new lock and the person holding it. A written list beats memory.

Who might have one you don't know about? After moving in, assume extra copies exist. Former tenants, contractors and letting agents are the usual holders.

Was a key lost, or taken? It makes less difference than people think. Anyone who finds a lost key can use it, so the lock is still worth changing.

Do you want one key for two doors? Say so when you buy. Keyed-alike locks are easiest to get at the outset, while sorting it out later can mean changing or rekeying locks again, and that is not possible with every lock.

Where is the spare? Somewhere other than the property, with someone or somewhere you trust.

One boundary applies throughout: change locks only on a home you own or rent, and if you rent, read your tenancy agreement and speak to your landlord or letting agent first.

Prefer to get help? The correct fix depends on the door, its current hardware and why the change is needed, and repairing or adjusting existing fittings is sometimes more suitable than a full replacement. Have your postcode, your door type and a brief account of the problem ready, along with whether you still have a working key and whether the door opens normally. And remember: a key hidden outside the house is an unlocked door with a few extra steps.

Test It Before You Shut the Door

Before the door closes even once, the new lock has to pass every check with the door fully open. State it plainly and follow it to the letter: a poorly fitted latch, a misaligned bolt or a handle that is not connected properly could leave you unable to open the door again, and the only safe place to find that out is with the door open.

Five checks, door open

Release from inside. Confirm the door unlocks and opens from the inside exactly as intended, and pay particular attention if it forms part of an escape route. Where the replacement has changed a required escape function, go no further.
Key. Put the new key into the lock, then gently turn it through the locking and unlocking actions. Nothing should stick or require excessive force.
Latch and handles. For a sashlock, the handles should pull the latch back and then return to their normal position. Check the nightlatch by following the manufacturer's instructions for its key on the outside, knob or lever on the inside, and any extra locking controls.
Deadbolt. On a mortice deadlock or sashlock, use the key to extend and retract the deadbolt fully; it should travel freely without catching in the case.
Alignment. Hold the open door in its closed position against the frame and compare latch and bolt positions with their keeps, looking for differences in height or signs the hardware may catch.

Then, and only then, close it

Once every check passes, close the door, but for that first closed-door test have another responsible person present or another safe way of getting in. Verify that the door opens again easily, that the latch engages and that the key works as usual. Try each of the other keys, then check every function on both sides of the door. If the lock has additional testing requirements, follow the manufacturer's fitting instructions over anything here; Yale's official lock fitting instructions provide model-specific guidance. A lock that works open but turns stiff or fails to engage closed may have an alignment problem. Avoid forcing the key or handle.

Make the keys count

With the lock working, confirm every supplied key operates it and decide who should hold a copy. Store a spare key in a secure place instead of concealing it outdoors. If you changed the lock after moving in, the old keys should no longer give access through it, which is helpful when you cannot account for copies held by previous occupants or contractors. Consider which other locks, if any, a missing or stolen key opens and whether someone could connect it with your address. Some compatible locks can be supplied keyed alike or rekeyed so one key works several doors, depending on their design, and since not every lock supports this, consider it before purchasing. Keep useful old components securely until you are sure they are no longer needed. In the end, the aim is for access to your property to be controlled by the keys you mean to keep in use.

Get a Lock Changed in Chester Instead

Some lock changes are best left to someone else, and Safe Locksmiths Chester supplies and fits replacement locks across Chester for exactly those cases, including every door type this guide has marked as one to stop on.

Before calling, check: the door's material, the keyhole's position and whether a working key remains.

We price each job on the call from your description. Anything on site that alters the figure is explained before a screw is turned, and no charge is made for a technician arriving. Evenings, weekends and bank holidays are charged at a higher rate, and you still hear the price first.

Get a Price for a Lock Change

Emergency lines are open around the clock, every day. You can also message us through the contact page.

When to Stop and Then Call a Locksmith

Nothing here is meant to put you off. Swapping a lock is often a manageable job when the replacement matches the existing fittings and the door is in good order. Some situations, though, involve altering the door, handling damaged components or untangling an unfamiliar problem, and those are where professional assistance may be the safer choice. Here are the stopping points, each with its reason, so you can judge for yourself.

Stop if...	Because...
The new lock does not fit the recess	Misaligned case, faceplate or fixing holes mean forcing it or cutting the door could cause more damage
The old lock will not come out normally	Seized screws, damaged fittings or a jammed mechanism need assessing, not more force
You have no key	Nothing can be removed in the normal way
The door is fire-rated, or an escape route such as a flat entrance door onto a shared corridor	Hardware must preserve fire protection and safe escape, which may include opening from inside without a key; unauthorised alterations could compromise it
A multipoint mechanism has failed, or the hardware is unfamiliar	Specialist systems may need a more detailed assessment
The door is out of line	Working open but binding closed suggests hinges, frame or strike plate need attention
Your insurer specifies a standard and you are not sure the lock meets it	Only your policy can answer that, so check it

Stop too if the replacement does not operate smoothly when tested, or if you are not confident the door can be secured and opened reliably. Appearing to fit and being suitable for the door are not the same thing. A locksmith can assess whether the job needs a compatible replacement, an adjustment or more extensive work, and the price will depend on the lock, its condition and any further fitting or repair. Put simply, paying for a fitted change usually costs less than paying for a lock change plus a repair to a door that has been cut wrongly. If that is where you are, see having the lock replaced for you.

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?

For a simple lock replacement, have a tape measure, the correct screwdriver and a new lock compatible with the existing fitting. Additional tools may be necessary for some models. Review the manufacturer's fitting guidance beforehand so the equipment matches the requirements of the lock being installed.

How long does it take to replace a door lock?

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?

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?

Your circumstances and tenancy agreement determine how to approach a lock change. Review the terms first, then discuss the proposed replacement with the landlord or letting agent before making changes. Where a faulty lock creates an urgent safety risk, notify them without delay.

Can I replace a door lock and keep the same key?

A matching replacement can sometimes preserve use of the same key. If no compatible match is available, expect the new lock to have different keys. Establish compatibility before purchase; a locksmith can advise on rekeying for your particular type. Where locks support keyed-alike use, compatible models can be supplied or sometimes rekeyed for one key. Conventional internal locks often need only basic tools when the replacement is suitable. Take greater care around fire-rated doors, specialist fittings and requirements affecting escape.

Can I replace an internal door lock myself?

For many ordinary internal locks, yes, provided the new fitting is compatible and basic tools are available. A matching replacement is usually manageable for conventional mortice locks or nightlatches. Damage, poor alignment and door alterations may require locksmith help. Specialist hardware, fire-rated doors and specific escape requirements need additional attention. Check compatibility before buying and discuss rekeying your lock type with a locksmith. Suitable locks can be supplied or sometimes rekeyed as keyed alike. Your existing key may work with a compatible replacement matched to it; otherwise, expect different keys.

Do I need to replace the door handle when changing a lock?

The current handle may be reused if it suits the new lock and remains serviceable. Check its operation before deciding, along with the spindle and the locations used for fixing it. A compatible handle in good condition does not need changing simply because the lock changes.

What happens if the replacement door lock is the wrong size?

A size mismatch can leave the lock unable to fit its recess or align with the handle, strike plate or keyhole. Confirm that the replacement is suitable before applying force or making door alterations. Compatibility checks belong before purchase. For shared key use, compatible locks supporting keyed-alike operation may be supplied matched or sometimes rekeyed.

Why is my new door lock stiff after fitting?

The replacement may be stiff because it has been fitted incorrectly, sits out of alignment or is under frame pressure. Make the first test with the door open. Ongoing resistance warrants professional assistance and a compatibility check; avoid force. Identify the mechanism type and measure accurately to assess whether the chosen replacement fits appropriately.