Home Improvement

Guide to Mortice Lock Sizes

Image source: Zoltan Fabian / Shutterstock.com

Whether you’re securing your home or business, understanding mortice lock sizes in the UK is crucial for optimal performance and security.

So if you:

  • Don’t know the first thing about this kind of lock;
  • Want to install one or several on your premises;
  • Have a hard time deciding between different sizes;

Then read along. In this post, you will learn everything you need to know about mortice locks, their sizes, and how to measure them, plus the essential factors to consider when choosing.

Mortice locks explained

First of all, let’s answer the question, “what is a mortice lock?” A mortice lock is a special kind of lock that’s designed to fit snugly inside a precisely cut cavity or “mortice” in the edge of a door. This cavity is carefully crafted to perfectly house the lock, so it sits nice and flush with the door’s surface.

What really sets mortice locks apart from other types of locks is how they’re installed. Unlike locks that are attached on the surface of the door, like rim locks or padlocks, mortice locks become an integral part of the door itself. This not only provides better security but also looks more elegant. In contrast, surface-mounted locks are visibly bolted onto the door’s surface, which can make them easier to tamper with and potentially compromise the door’s security.

You may also like:
Home Improvement
What Is a Mortice Lock: All You Need to Know

Common mortice lock sizes and measurements

Understanding mortice lock size measurements is crucial for selecting the right lock for your needs. Here are the main things to consider:

  • Length – The length of a mortice lock is typically measured from the faceplate (the part visible on the edge of the door) to the end of the lock body. This measurement helps ensure that the lock can securely engage with the door frame or strike plate when locked. A lock that is too short might not provide adequate security, while one that is too long could pose installation challenges.
  • Width – The width measurement refers to the thickness or diameter of the lock body. It’s essential to know this dimension to ensure that the lock fits correctly within the prepared mortice cavity in the door. A lock that is too wide might not fit, while one that is too narrow may not offer sufficient security.
  • Depth – The depth measurement indicates how deep into a door or frame a lock can be installed. This measurement is critical because it determines whether the lock can be securely fitted into the door without protruding or falling short. An improperly sized lock may compromise the door’s structural integrity and security.
You may also like:
Home Improvement
Multi-point Locks Guide: How They Work and Where to Install Them

Now that you know how to measure a mortice lock, you should become familiar with the standard sizes. Here are all the mortice lock sizes explained:

  • 2.5-inch mortice lock – This size is typically used for internal doors and is well-suited for thinner doors or those with a lower security requirement.
  • 3-inch mortice lock – A 3-inch mortice lock is a common size for internal and external doors. It provides better security than the 2.5-inch version and is suitable for most standard doors.
  • 4-inch mortice lock – This size is often used for external doors that require a higher level of security. It provides increased bolt throw and strength.
  • 5-inch mortice lock – The 5-inch mortice lock is used for heavy-duty external doors, such as those in commercial or industrial settings. It offers enhanced security and durability.
  • 2.375-inch (60mm) and 2.75-inch (70mm) backset – The backset is the distance from the edge of the door to the center of the lock case. These common backset measurements are important to ensure the lock fits properly in the door.
  • Euro profile mortice lock – Euro profile mortice locks have standardized dimensions and are commonly used in uPVC and composite doors. They often come in sizes such as 60mm and 70mm backsets.
  • Oval profile mortice lock – Oval profile locks are also used in uPVC and composite doors, with typical backset measurements like 44mm and 57mm.
You may also like:
Home Improvement
uPVC Door Handle Loose – How to Fix It

What is the most common mortice lock in the UK?

When it comes to mortice locks in the UK, several types are commonly used, with the most prevalent being the 5-lever mortice lock.

This type of lock is known for its robust security features and compliance with British standard BS3621. To adhere to this standard, a mortice lock must pass various tests, including resistance to picking, drilling, and other forms of attack. Choosing the appropriate size for your door is essential to ensure effective performance and security.

You may also like:
Home Improvement
How to Fit a Mortice Lock Like a Pro

How to know what size mortice lock you need

Ok, now you wonder, “what size mortice lock do I need?” Well, choosing the right size is like finding the perfect puzzle piece for your door, isn’t it? Here’s a breakdown of the key factors to consider when determining the size you need:

Door material and thickness considerations

Different doors have varying requirements for strength and durability:

  • Wooden doors vs. metal doors – Wooden doors and metal doors have different needs in terms of strength and security, so your choice should align with the material of your door.
  • Thickness variations in doors – It’s crucial to measure your door’s thickness to select the appropriate length and depth for your mortice lock. An ill-fitting lock might compromise security and functionality, so don’t go with that.

Backset measurement

The backset measurement is the distance between the edge of your door and where you install your lock. Common backset measurements include standard sizes like 2.375 inches and 2.75 inches. It’s important to measure this distance accurately so your lock fits snugly.

Lock case dimensions

Lock case dimensions encompass the height, width, and depth of the lock body we mentioned already. These measurements should align perfectly with your door’s mortice pocket to ensure a proper fit. Paying attention to these dimensions is essential, as a misaligned lock can lead to security vulnerabilities.

Security level requirements

The security level you need depends on various factors:

  • Types of locks available – In the UK, mortice locks come in various security levels, including standard, medium, and high-security options. Each level offers a different degree of protection.
  • Location and needs – Consider where the lock will be installed and your specific security needs. For example, high-security locks may be more suitable for external doors, while standard locks may suffice for internal doors.

Choosing the right mortice lock size ensures it fits seamlessly, provides the necessary security, and maintains the door’s structural integrity.

Don’t know which mortice lock to choose?

A Fantastic locksmith can help.

Learn more

Don’t know which mortice lock to choose?

A Fantastic locksmith can help.

Learn more

How to measure a mortice lock

This is an essential step when choosing the right one for your door, so here is how to measure mortice lock sizes:

  1. Measure the backset – The backset is the distance from the centre of the keyhole or spindle to the edge of the door. Common backset measurements include 2.375 inches (60mm) and 2.75 inches (70mm).
  2. Determine the length – Measure the length of the lock case from the faceplate to the end of the lock body.
  3. Check the width – Measure the width of the lock body. This dimension helps you select a lock that fits snugly into the door’s mortice pocket.
  4. Measure the depth – Determine how deep into the door or frame the lock can be installed. This measurement ensures that the lock doesn’t protrude or fall short of the door’s surface.

For more detailed instructions on measuring different mortice lock types, check out our how to measure a mortice lock guide.

Not sure how to install your mortice lock?

Book professional lock fitting today!

Add a valid postcode e.g. SE1 2TH


  • Mortice locks are designed to fit snugly inside a precisely cut cavity in the edge of a door, offering robust security for both homes and businesses;
  • Common mortice lock sizes in the UK include 2.5-inch, 3-inch, 4-inch, and 5-inch variants;
  • When choosing the right size mortice lock, consider factors such as door material and thickness, backset measurement, lock case dimensions, and security level requirements;
  • The 5-lever mortice lock is the most common type in the UK, known for its robust security features and compliance with British standards;
  • Measuring a mortice lock involves assessing backset, length, width, and depth to ensure a perfect fit, security, and structural integrity.
0 0 votes
Article Rating
Notify of
Inline Feedbacks
View all comments
Would love your thoughts, please comment.x