Home Improvement

How to Measure a Mortice Lock the Right Way

Do you have a lock that needs to be replaced? Or you need to put up brand new doors and locks in your house?

If you answered “Yes” to one of these questions, then this post is for you. This task isn’t a complicated one. Measuring and buying the right size mortice lock is important and can be done by you, regardless of whether a professional locksmith will come to do the actual work.

This article concerns anyone who wants to know how to measure a mortice lock. Keep reading to learn about the importance of mortice lock sizes and dimensions and how to adequately take them down. But first, let’s start with answering…

Are mortice locks a standard size?

Not entirely. The sizes could vary based on the manufacturer, the lock’s type, whether it’s a sashlock or a deadlock, and the lock’s additional specifications. Therefore, it’s important to get the precise mortice lock sizes and door dimensions before proceeding to buy a lock.

It is often a mistake to buy the lock first and then find out that your door doesn’t have the space for it. There are also some other crucial factors to consider, but more about them later on.

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How to precisely measure a mortice lock

Mortice lock dimensions aren’t difficult to measure regardless of the type of mortice lock type – sash lock or deadlock. Mortice lock set measurements matter the most for getting the right replacement for your pre-drilled door. When the door is brand new and there are no cutouts, the size isn’t as important to get right and precise.

You will make cutouts on the door when fitting the lock. But when you’re replacing an existing lock, it is crucial to get a tight dimension so you can get the right-sized lock.

Tools you need:

  • A screwdriver (to remove an existing lock)
  • Tape measure or ruler 
  • A blank page and a pencil 

If you’re measuring an existing lock or existing holes, you won’t have to remove the lock. 

Measuring a mortice sash lock

  1. Measure the backset first (A) – This means measuring the distance between the centre of the doorknob and the edge of the door on which it will be positioned.
  2. Proceed with the centre (B) – The centre is called the square hole from which the spindle passes through for the handle to the centre of the keyhole.
  3. Case depth (C)– This is one of the main dimensions and it’s often called the ‘lock size’. It is the depth of the entire lock and it shows the depth of the hole in the door you’ll need.
  4. Case height (D) – The height of the case is the height of the back part of the lock which lays inside the door. 
  5. Forend length (E) – This is the front of the lock and it’s the part of it that you see when you open the door.
  6. Forend width (F) – Lastly, this is also self-explanatory like the forend length – the width of the metal part that you see on the door.

Measuring a mortice deadlock

The same goes here as with the sash lock. The two types are almost identical in terms of required measurements but the sash lock has one additional measurement. 

  1. Backset (A) – As with the sash lock, here it’s also the distance from the face of the lock to its end.
  2. Case depth (B) – The case is the hidden part of the lock, so the depth is the space it will require inside of the door. The most common sizes of case depths in deadlocks are 64mm and 76mm.
  3. Case height (C)– With deadlocks, you have almost the same size case height as its case depth. The reason is that this type doesn’t have a follower. 
  4. Forend length (D) – The name says it – it’s the length of the visible part of the lock.
  5. Forend width (E) – You need to precisely measure the forend width as it can be a problem if the door isn’t wide enough.

Does measuring a mortice lock sound like a complicated task? No worries, have a skilful locksmith deal with the task. More about our experise, prices and areas we cover, you can find here.

What to consider before buying a mortice lock?

It is important to know the exact mortice lock dimensions so you can buy the exact lock size. What is more, this is one of the most common mistakes, which can result in either wasting your time or damaging your door. 

#1. Door stile width and thickness

Regardless of the type of mortice lock, or any other type of lock for that matter, they all have different lengths and depths. This means that you can never be 100% sure that a certain lock will fit effortlessly into your door. That’s why measuring the door and the lock’s dimensions is crucial for getting the right lock and installing it the right way without risking damaging the door. 

#2. The mortice lock type: sash lock vs deadlock 

First and foremost, you need to be aware of the type of mortice lock that you need, regardless if you’re replacing an old one or need to install a new one. As mentioned before, there are two types of mortice locks – a sash lock and a deadlock. Knowing the difference between them will help speed up the process and eliminate any confusion that may occur either with measuring or buying the lock. 

A mortice deadlock is often used on front doors, while a sash lock is found on the back and internal doors. Normally, it’s easier to measure a deadlock because you don’t have to worry about the handle alignment measurements. 

#3. Door type

One thing that most people forget to take into account is whether the doors’ lock will be on the right or the left. The good thing about mortice locks is that they are reversible so the doors’ orientation doesn’t matter.

The most important difference between measuring a sash lock and a deadlock is that the sash lock has an extra dimension you need to take and it’s the most crucial one. This important part is called “centres’’. All other dimensions are the same.

Furthermore, not all locks are suitable for all door materials. A wooden door requires a different lock compared to a uPVC or a composite door. For example, a 5-lever mortice deadlock is not suitable for uPVC and composite doors. 

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#4. Security level (grade/how many levers) – the more levers, the better security

Another thing to consider when choosing locks is what locks you need on which doors. For the front door, you’ll want a very secure lock that is tough to break or pick. It’s also important that it locks on both sides. The same goes for the back door. 

As we said above, a sash lock is used on the back and internal doors, while deadlocks are for front doors. There are also two types of deadlocks – a 5-lever and a 3-lever mortice locks. The major difference between these two types of deadlocks is that the 3-lever one is less secure. That’s because it has fewer levers, as their name implies, and that’s a higher risk of key duplication.

In the UK, for a lock to be thief-proof, it needs to conform to BS3621. This is a lock standard by the British Standards Institute. If the lock has this mark, then it’s safe to get it for a front or a back door.
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#5. Is the lock still on the door?

If there’s an existing lock on the door, you can measure the visible parts of the lock – the forend length and width, the keyspace, the handle space, etc. This makes it easier to envision how the new lock will look. 

#6. Purpose 

When measuring for a mortice lock, it’s also important to take into account the purpose of the lock and the door it will be on, for example, if it is on a domestic property or a commercial one. This makes a difference in the type of security level you will need from your lock and also its quality. 

#7. Preferred mortice lock position on the door

This may not sound like a big concern and it usually isn’t. However, we know that in England it’s common to place locks below the centre of the door. Usually, the position of the lock doesn’t play any role in installing it or drilling the holes for it. What is important to consider, however, more specifically for internal doors is whether small children and pets live in the house. 

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If in doubt, get professional help for measuring your mortice lock

As we said at the beginning of the article, you don’t have to do all of the work yourself. If all of this information is too much for you and you don’t think you’ll have the time or energy to deal with all of the steps, you should hire a professional locksmith to measure and fit your locks.

The pros of turning to a real professional are endless and far outweigh the only con – that you’ll pay for the locksmith service. For people who want the job to be done the right way and have a guarantee that everything will work as it should, hiring a locksmith is the best option. 

Looking for professional locksmith services today?

Book an experienced lock specialist to measure and replace your mortice lock in a fast and efficient way.

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  • It’s crucial that you measure pre-drilled holes on your doors.
  • Think about security and check if the locks conform to the British standard.
  • Do not be ashamed to ask for help.
  • Measure locks and doors before purchasing anything. 


Did you find this article helpful? Do you know other tips that might help our readers to measure a mortice lock properly? Please, share your thought!

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