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 That the Lock Is Actually the Problem

Here is the cheapest check in this whole guide. A stiff or unreliable door lock does not always need replacing, because faults in the hinges, frame or strike plate can produce similar symptoms to a failing lock. So, if the door opens normally, open it all the way and operate the lock gently while it stands open. Key and bolt moving freely there, then dragging once the door is closed, means misalignment may be contributing. Although this provides helpful evidence, the lock could still have worn or damaged internal parts. Working out where the problem lies before you buy may save unnecessary work and expense.

If the trouble only shows with the door shut

Then the bolt may not be lining up correctly with its keep or strike plate. Other clues that alignment may be wrong:

You must lift, push or pull the door before the key turns.
Closing the door takes more effort than before, or it catches on the frame.
Marks from scraping appear near the strike plate or at the bolt's contact point with the frame.
The space around the door appears to vary in width against the frame.

Alignment can be affected by hinges that have loosened or worn, a shifting frame, or timber changing with moisture. These problems may remain even after the lock itself has been replaced.

Problems that also occur with the door open

Unreliable operation of the parts inside a lock may make repair or replacement necessary. Common signs:

Bolt does not move when the key turns: an internal component may be worn, damaged or disconnected.
Lock jams intermittently: a worn or damaged mechanism may prevent the bolt from moving consistently.
Key snapped inside: the broken fragment may obstruct normal operation and require professional removal.
Key difficult to insert or turn: wear, damage or a problem inside the lock may be causing resistance.

These symptoms do not automatically mean the complete lock needs replacing; some faults can be resolved by repairing or replacing an individual component, depending on the lock's design and condition.

When the lock has no fault

Moving into a new property, losing keys or uncertainty over who retains access may still give you security reasons to replace a functioning lock. That is a perfectly good reason to carry on with this guide. If you are weighing up a fix against a new lock, 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.

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.

Now Remove the Old Lock

Two conditions before the screwdriver comes out: the door stands open and is properly supported, and you have a way to lock the property if the job cannot be finished. Both matter more than speed, because an empty mortice is an open door. Proceed only after confirming that the replacement matches your current lock.

Removing a mortice lock, in order:

Sashlocks first lose their handles and spindle.
Next come the screws through the faceplate.
The lock body is then eased, not levered, out of the mortice.
Resistance at this point usually means looking for a second screw under the faceplate, not pulling harder.

Removing a nightlatch, in order:

The case on the inside face is unscrewed.
The case comes off the door.
The connecting screws are removed.
The cylinder is drawn out from the outside face.

Put the original screws, handles and fittings to one side together, ready to compare with what came in the new box.

If any part resists, stop rather than drill, lever or force it. If a key breaks off during removal, stop as well, because a fragment in the mortice makes the job much harder to finish.

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.

Before You Shut the Door, Test It

The door stays open until the lock has proved itself. That is the instruction that matters most in this guide: a latch, bolt or handle that has gone in slightly wrong can lock you out of your own home the first time the door swings shut. So run the checks below with the door fully open, before you use the lock normally.

Open-door checks

Handles and latch. If you fitted a sashlock, press the handles and watch the latch retract, then check both handles spring back to their normal position. Follow the manufacturer's instructions when checking a nightlatch's key from outside, knob or lever from inside, and any extra locking controls.
Key operation. Turn the new key gently in the locking direction and the unlocking direction. Expect smooth movement with no sticking and no excessive force.
Deadbolt travel. For a mortice deadlock or sashlock, use the key to throw and withdraw the deadbolt completely. It should move freely and not catch within the case.
Inside release. Verify that the intended method of unlocking and opening the door from within works, with particular care where the door is on an escape route. Stop at this point if the new lock has altered a required escape function.
Frame alignment. Still with the door open, hold it where it would sit when closed and compare the latch and bolt positions with the keeps on the frame. Watch for height differences or anything that looks likely to catch.

The first closed-door test

Wait until all checks have passed before shutting the door, with either another responsible person present or a separate safe way to gain access. Check for proper engagement of the latch, normal operation of the key and easy reopening of the door. Next, check each remaining key and try every available function from both sides of the door. Manufacturer's fitting instructions come first if your lock has additional testing requirements, and Yale's official lock fitting instructions give model-specific guidance for their products. A lock that is fine with the door open but stiff or reluctant to engage once closed may be suffering from alignment; avoid forcing the key or handle.

Then settle the keys

Make sure every key supplied operates the lock, and decide who should hold one. A spare belongs somewhere secure, not hidden outside. If you changed the lock after moving home, old keys should no longer give access through it, which is useful when copies held by previous occupants or contractors cannot be traced. Consider whether a missing or stolen key could identify your address, along with which other locks, if any, it operates. Some compatible locks can be supplied keyed alike or rekeyed to share a key, depending on design, so consider this before buying, since not every lock supports it. Hold on to useful old components securely until they are no longer needed. The point of the job is control: access to your home through the keys you intend to keep using.

Stop and Call a Locksmith: Knowing When

Is this a job to finish yourself? Often, yes. A door in good condition and a new lock matching the existing fittings generally make a simple replacement manageable. Where the work means altering the door, coping with damaged components or tackling an unfamiliar locking problem, a professional may be safer. These are the moments to put the screwdriver down.

The door is part of an escape route or is fire-rated.

Replacement hardware has to preserve its required fire protection and safe escape functions; an unauthorised alteration could compromise its performance. A flat entrance door onto a shared corridor is a typical example, since the lock may need to open from inside without a key.

You are not sure the lock meets an insurance standard.

If your insurer specifies one and you are uncertain, check your policy.

There is no key to work with.

Conventional removal depends on having one.

Specialist hardware is fitted.

Multipoint locking systems, including a failed multipoint mechanism, and mechanisms you do not recognise may need a more detailed assessment.

The old lock will not come out as it should.

Seized screws, damaged fittings or a jammed mechanism point to specialist assessment rather than more force.

The door binds once closed.

Working with the door open but not when shut suggests the hinges, frame or strike plate need attention.

The replacement does not fit.

If case, faceplate or fixing holes do not align, forcing it or cutting into the door could cause further damage.

Testing raises doubts.

If the lock does not operate smoothly, or you are unsure the door can be secured and opened reliably, stop.

Bear in mind that a lock can look as if it fits without being suitable for the door. To establish whether you need a compatible replacement, an adjustment or more extensive work, a locksmith can carry out an assessment. The cost will depend on the lock, its condition and any additional fitting or repair, and it is usually less than the combined cost of a lock change and a repair to a door that has been cut wrongly.

Discuss Your Lock Problem

Getting a Lock Changed in Castleford Instead

Read the guide and decided this one is not for you? J.K Locksmiths Castleford supplies and fits locks across Castleford, from the straightforward swap to the doors this page tells you to leave alone.

When you call, tell us what the door is made of, where the keyhole is and whether you still have a key. That is enough for a price on the phone. If anything on site would change it, you hear about it before work starts. Attending is free at every hour of the day or night, with no call-out charge.

Get a Price for a Lock Change

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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?

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?

Matching the existing lock and door fittings usually makes replacement manageable, particularly for conventional mortice locks or nightlatches. Budget extra time for an unfamiliar design, incompatible parts or problems with the door. A locksmith may be required for damaged hardware, alignment faults or door alterations. A suitable screwdriver, tape measure and compatible lock are normally the basics. Follow the manufacturer's fitting guidance, checking before starting whether further tools are specified.

Can I replace two door locks with one key?

Yes, where both fittings support using keyed-alike locks. Check compatibility before ordering: suitable locks can be supplied or sometimes rekeyed to share one key. Your current key may be retained through a compatible replacement matched to it; otherwise, new keys normally come with the lock. Seek a locksmith's advice on rekeying your type. Basic tools can replace many compatible conventional internal locks. Fire-rated doors, specialist hardware and escape needs demand care. Incorrect sizing may affect the recess, keyhole, handle or strike plate. Confirm suitability before forcing the fitting or making alterations to the door.

Can I change the locks on a rented property?

A rented property's lock change needs consideration of both the tenancy terms and the situation. Check the agreement and contact the landlord or letting agent before any alteration. If the reason is a faulty lock posing an urgent safety concern, report that problem straight away.

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

You may retain the key if a suitable replacement can be keyed to match it. Check compatibility before ordering, and ask a locksmith whether rekeying is possible for the lock type involved. Without matching, different keys normally accompany the new lock. Keyed-alike arrangements also let compatible locks share a key, through supply or sometimes rekeying. Many conventional internal locks are manageable with basic tools if the replacement fits. Extra care is necessary with specialist hardware, fire-rated doors and doors governed by specific escape requirements.

Can I replace an internal door lock myself?

DIY replacement is often possible for conventional internal locks with compatible fittings and basic tools. Matching the current arrangement generally makes mortice lock or nightlatch changes straightforward. Damaged hardware, misalignment or necessary door alterations may call for a locksmith. Extra care applies to fire-rated doors, specialist mechanisms and escape requirements. Before purchasing, establish compatibility. Compatible keyed-alike locks may be supplied or sometimes rekeyed for shared key use. Ask a locksmith about rekeying your type. Keeping the present key needs a suitable replacement matched to it; otherwise, new locks normally bring different keys.

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?

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?

Stiffness can result from poor alignment, an installation error or pressure exerted by the frame. Begin by trying the lock while the door is open. If resistance continues, check that the replacement is compatible and get professional help instead of forcing it. Identifying the lock type and measuring carefully help establish whether the fitting is suitable.