Guide

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

Most door lock replacements take 30 to 60 minutes with a screwdriver, as long as the new lock matches the hole already cut in the door. The tricky part is not the fitting; it is choosing a replacement that fits first time, and that is where most attempts go wrong. One rule matters more than any other, so it comes first: test the new lock with the door wide open, and do not close the door until every test has passed.

This guide, from Runcorn Locksmith in Runcorn, covers timber doors with mortice locks and nightlatches, from identifying the lock to dealing with the old keys. uPVC and composite doors work differently; see our uPVC door lock service. It applies only to doors you own or have the right to work on. To hand the job over, call 01928 611001.

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Which Lock Are You Actually Replacing?

The door decides what you are replacing, not personal preference, and three questions settle it. Look at your own door as you answer them.

What is the door made of? If it is timber, read on. If it is uPVC or composite, the lock is almost certainly a euro cylinder inside a multipoint mechanism, covered further down.
Where is the keyhole? If it is in the edge of the door, you have a mortice lock. If it is in the face of the door near the handle, it is a euro cylinder. If it is on a box fixed to the inside face, it is a nightlatch with a rim cylinder on the outside.
Does lifting the handle throw bolts up and down the frame? If so, that is a multipoint mechanism, and the cylinder is just one part of it.

A deadlock has a deadbolt only; a sashlock has both a latch and a deadbolt (ERA, a UK lock manufacturer). If your answers point to a timber door, the next two sections apply to you. If they point to uPVC or composite, the job and the measurements change, and lock replacement explains how we handle those doors.

Locks Fitted Into the Edge of a Timber Door

A keyhole in the door edge means a mortice lock, the type this guide covers in full. There are two versions. A mortice deadlock throws a deadbolt only, worked by the key from either side, and is the type insurers most often name for a final exit door; check your own policy for what it actually requires. A mortice sashlock combines a latch with a deadbolt, so it takes a handle as well as a key.

Mortice deadlock	Mortice sashlock
Deadbolt only, key from both sides	Latch plus deadbolt, handle and key

On an external door, five-lever is the usual specification, and some external timber doors carry five-lever mortice locks including models designed to meet BS 3621. The lever count is normally stamped on the faceplate or printed on the key. Before you go anywhere near a shop, write down whether yours is a deadlock or a sashlock.

Locks Fitted Through the Face of a uPVC or Composite Door

A keyhole in the face of the door, level with the handle, is a euro cylinder sitting inside a multipoint mechanism. The cylinder swaps in minutes once you have the right length, but measuring it and the mechanism behind it are a separate job, so this guide stops here. Two parts, often confused:

The cylinder is the part the key turns.
The mechanism is the strip of bolts and hooks that the handle drives.

Replacing the wrong one is the most expensive mistake on these doors. A handle that will no longer lift points to the mechanism, not the cylinder. For this door type,.

Check the Lock Is Actually the Problem

A new lock fixes a faulty lock and nothing else, so spend two minutes finding out which you have. Worn hinges, a shifted frame or a misplaced strike plate can mimic a failing lock. The quickest check: open the door fully and work the lock gently. If key and bolt move freely with the door open but stick once it is shut, misalignment is probably involved, though that does not rule out wear inside the lock. Lock repair covers the adjustment side.

Signs the Lock Itself Has Failed

Faults that point at the lock:

The key turns and turns without throwing the bolt.
The key will not go in, or will not turn, even with the door open.
The turn feels gritty all the way round, which can mean wear that a lock repair may sort out.
A lever lock jams on and off.
A key has snapped off inside; see broken key removal rather than forcing it.

Reasons to replace that are not faults at all:

Keys lost or stolen
Moving into a new home
Keys unaccounted for after building work or a tenancy ends

Never force a jammed lock to see if it will give.

Signs the Door, Not the Lock, Needs the Work

The tell is simple: if the fault only shows once the door is shut, it is alignment. In practice:

You have to lift, pull or lean on the door before the key will turn.
The bolt catches the edge of the keep rather than sliding into it.
There are bright scrape marks on the keep or the strike plate.
The gap around the door is wider at the top than the bottom, or the other way round.

Typical causes are worn or loose hinges, a shifted frame, or timber swelling in wet weather. Adjusting a keep or hinge is often smaller than a lock change, and it is what actually fixes the fault. A heavy door can drop when hinges are loosened, so hinge work is best left to someone with help on hand.

Measure the Lock Before You Buy It

A replacement has to match the hole already in the door, so measure the old lock first and shop second. You are matching a lock, not choosing one, and a lock 12mm out will not go in. A simple swap may need only a screwdriver that fits the existing screws, a tape measure, and a replacement matching the old lock's type, dimensions, fixing positions and the security specification you need; exact tools vary between mortice locks, nightlatches and rim cylinders. If the numbers do not line up, lock installation is the route.

The Five Measurements on a Mortice Lock

Five dimensions describe a mortice lock, and three of them (backset, case depth and centre) decide whether a replacement fits. Mortice deadlocks and sashlocks sit in a recess cut into the door edge, and most are held by screws through the faceplate, though fixing arrangements vary. Some measurements can be taken with the lock in place; others are easier once it is out, with the door safely open. Start with any manufacturer marking or model number on the old lock, record everything in millimetres, and compare with the new lock's specification before buying.

Dimension	What it measures	Common UK size (ERA)
Backset	Centre of the keyhole to the edge of the lock	Commonly 44mm on a 64mm lock, 56mm on a 76mm lock
Case depth	Depth of the lock body routed into the door	Typically 64mm (2½ inch) or 76mm (3 inch)
Centre	Keyhole centre to the centre of the follower (spindle) hole; sashlocks only	Varies by lock; measure yours
Case height	Top to bottom of the lock body	Varies by lock; measure yours
Faceplate	Length and width of the plate visible on the door edge	Varies by lock; measure yours

Sizes above are common, not universal. Write all five down and take the old lock with you to the counter. If you would rather pass the measuring to someone else, how to choose a locksmith explains what to check before you book.

Measuring a Rim Cylinder or Nightlatch

A nightlatch is measured differently, because its case sits on the inside face of the door rather than buried in it. First decide whether you are replacing the whole nightlatch or only the outside rim cylinder. Then record:

Backset: from the door edge to the centre of the cylinder hole. UK nightlatches come in more than one standard backset, so measure yours rather than assuming.
Case size: the height and width of the box on the inside face.
Latch type: a standard nightlatch, or a deadlocking one that can be locked from inside.
Cylinder length: from the outside face of the door through to where the tailpiece comes out on the inside.

The cylinder and the case are often sold separately, so a worn rim cylinder can usually be replaced without touching the case. A nightlatch on its own should not be assumed to satisfy an insurer.

Remove the Old Lock

Before the first screw comes out, make sure you can still secure the property if the job stops halfway. Work with the door open and properly supported, and keep the original screws, handles and fittings together so you can compare them with the new parts.

For a mortice lock:

Open the door, make sure the deadbolt is fully withdrawn, and stop the door swinging shut while you work.
On a sashlock, undo the handle fixings and gently draw out both handles and the square spindle.
Remove the faceplate screws on the door edge.
Ease the lock body out of the mortice without levering. If it will not come, look for a second screw under the faceplate rather than pulling harder.

For a nightlatch:

Undo the case on the inside of the door.
Remove the connecting screws that hold the cylinder.
Withdraw the cylinder from the outside.

If a key snaps during removal, stop, because a fragment makes the job harder; see broken key removal. If anything refuses to come out, stop rather than drill, lever or force it.

Fit the Replacement

The new lock should slide in without persuasion. Anything that needs forcing is the wrong size or the wrong way round.

Offer the new lock up dry, before any screws go in, and check it sits flush in the recess.
Check the bolt and latch face the right way; a latch fitted backwards will not let the door close.
Put the faceplate screws in finger tight, work the lock a few times, then tighten.
On a sashlock, refit the spindle and handles, checking the spindle is long enough for the new case.
Check the keep. If the new lock throws its bolt further or shorter than the old one, the keep may need adjusting, and this is the step that most often goes wrong.

If the new faceplate is bigger than the old recess, the door must be cut, which is where many people should stop and book lock installation. Fire doors are different: cutting a fire door or certified door can change its rating, so leave it to someone qualified.

Test It Before You Shut the Door

Test everything with the door wide open, and do not close it until every check passes. A badly fitted latch, a misaligned bolt or a wrongly connected handle can leave you unable to open the door again.

With the door open, insert the new key and turn it gently both ways. It should move smoothly, without sticking or needing force.
Throw the bolt fully and check it comes out cleanly.
On a sashlock, check the latch springs back when you release the handle.
Still with the door open, hold it in its closed position against the frame and check the bolt lines up with the keep.
Only then close the door and test from outside, using every key in the new set, since keys supplied with a new lock occasionally differ.

Do not carry on if the replacement has changed an escape function the door needs. Keep the old lock until you are satisfied; a spare lock that works is a better fallback than a call-out.

Deal With the Old Keys

A new lock only solves the problem if the old keys stop mattering, so the last step is about keys, not tools.

Write down how many keys came with the new lock, and who holds each one.
If you have just moved in, assume keys exist that you know nothing about, held by former tenants, tradespeople or letting agents.
If a key was lost rather than stolen, the lock still needs changing, because a found key works for whoever finds it. If it was your only key and you are shut out, see locked out.
If you want one key for front and back doors, ask for keyed alike when you buy; it cannot be added later without changing both locks again. [CLIENT TO CONFIRM: whether keyed-alike sets are supplied and in what ranges]
Keep one spare away from the property, not in it.

A key hidden outside is not a spare. It is an unlocked door with extra steps. Only change locks on a property you own or rent.

When to Stop and Call a Locksmith

A simple lock change is usually within reach when the new lock matches the old fittings and the door is sound. Altering the door, damaged parts or an unfamiliar locking problem are where professional help may be safer. Stop if:

The new lock does not fit.

If the case, faceplate or fixing holes do not line up, forcing it or cutting the door can cause further damage.

The door is a fire door or a certified door.

Cutting it changes what it is rated to do.

It is a flat entrance door onto a shared corridor.

The lock has to open from inside without a key, and the wrong choice creates an escape problem.

There is no key at all.

Nothing can be removed in the usual way.

The old lock will not come out normally.

Seized screws, damaged fittings or a jammed mechanism need assessing, not more force.

The lock is part of a failed multipoint mechanism, or the door has dropped.

The hinges, frame or strike plate may need work first.

Your insurer specifies a standard and you are unsure the new lock meets it.

Check your policy wording.

Having a lock fitted usually costs less than a lock change plus a repair to a door that has been cut wrongly. See lock replacement, or what a lock change typically costs in our locksmith price guide.

Discuss Your Lock Problem

Get a Lock Changed in Runcorn Instead

Runcorn Locksmith supplies and fits replacement locks across Runcorn, including the doors this guide suggests leaving alone. When you call, have the door material, where the keyhole sits, and whether you still have a working key and can open the door normally.

There is no call-out charge at any hour. Every job is priced on the call from what you describe, and anything found on site that changes the price is explained before work begins. Emergency calls are answered 24 hours a day, every day of the year. [PROVIDER PACK: DBS and insurance evidence]

Get a Price for a Lock Change: call 01928 611001.

Door Lock Replacement Questions

How long does it take to replace a door lock?

Usually 30 to 60 minutes for a like-for-like change on a timber door, if the replacement matches the existing recess and fixings.

What tools do I need to change a lock?

Normally a screwdriver to suit the existing screws and a tape measure. Needing a chisel suggests the new lock does not match the old recess.

How do I know what size lock to buy?

Match the old one. Look for a model number, measure in millimetres, and compare with the new lock's specification; a small difference can stop it fitting.

Can I change a mortice lock myself?

Often, if the replacement matches the recess and fixings and the door is sound. Stop if the door needs cutting or is a fire door.

Do I need to replace the whole nightlatch or just the cylinder?

If the latch case works properly, the outside rim cylinder can usually be changed by itself.

Why is my new lock stiff when the door is closed?

The bolt is probably not meeting the keep squarely. The keep or hinges may need adjusting rather than the lock.

Should I change the locks when I move house?

It is sensible, because you cannot know how many copies of the old keys exist or who holds them.

What if the new lock will not fit?

Do not force it or cut the door. Recheck your measurements, or call 01928 611001 and we can fit it for you.