What Size Mortice Lock Do I Need?

Choosing the right lock for your door is about more than just security; it’s also about ensuring a proper fit and a seamless look. If you’ve decided on a mortice lock, you’ve picked a durable and reliable option. But now you’re facing a new question: what size mortice lock do I need?

A mortice lock that’s too small might not provide adequate security, while one that’s too large might not fit your door at all. Selecting the correct size ensures your door functions smoothly, looks aesthetically pleasing, and provides the security you expect. This guide will walk you through everything you need to know to measure and select the perfect size mortice lock for any door in your home.

Understanding Mortice Lock Sizing

Before you can choose the right mortice lock, you need to understand the key measurements that determine its size. These dimensions ensure the lock fits correctly within your door and aligns perfectly with your door handles and keyholes.

The two most critical measurements for a mortice lock are the backset and the case depth.

What is the Backset?

The backset is the horizontal distance from the edge of the door to the center of the keyhole (or the center of the handle spindle). This measurement is crucial because it determines where your doorknob or handle will be positioned on the face of the door.

A standard backset prevents your knuckles from hitting the door frame when you turn the handle. If the backset is too short, you’ll find it awkward and potentially painful to open the door.

Common backset sizes:

• 44mm (approximately 1.75 inches): Often used for internal doors where the door stile (the vertical frame of the door) is narrow.

• 57mm (approximately 2.25 inches): The most common size for modern internal doors, providing ample space for your hand.

What is the Case Depth?

The case depth is the total depth of the lock mechanism that is embedded—or "morticed"—into the edge of the door. To find this measurement, you measure from the front plate (the part you see on the door's edge) to the very back of the lock case.

The case depth must be less than the width of your door's stile. If the lock case is too deep, it could weaken the door's structure or break through the other side of the stile during installation.

Common case depth sizes:

• 64mm (2.5 inches): This size typically corresponds to a 44mm backset and is a popular choice for interior doors.

• 76mm (3 inches): This size usually has a 57mm backset and is the standard for most modern internal doors.

For external doors or those requiring higher security, larger case depths like 102mm (4 inches) or even 127mm (5 inches) are available. These larger locks are more robust and provide greater security.

How to Measure for a New Mortice Lock

Measuring for a new mortice lock is a straightforward process. You'll need a tape measure and a clear view of your door.

Step 1: Measure the Backset

To measure the backset, hold your tape measure against the edge of the door. Measure horizontally from the edge of the door to the center of the keyhole or spindle hole. Note down this measurement in millimeters or inches. Remember, the most common residential backsets are 44mm and 57mm.

Step 2: Measure the Case Depth

To find the case depth, you'll need to measure the mortice pocket in your door. If you are replacing an existing lock, simply remove it and measure the depth of the lock body from the faceplate to the back.

If you are installing a new lock, measure the width of the door's stile (the vertical wooden frame of the door panel). Your case depth should be comfortably less than this width. For example, if your door stile is 100mm wide, a 76mm (3-inch) case depth would be a safe choice, as it leaves enough wood on either side to maintain the door's structural integrity.

Step 3: Measure the Case Height

While less critical than backset and case depth, the case height is the vertical length of the lock body. If you're replacing an old lock, measuring the height of the existing mortice pocket will help you find a replacement that fits without requiring additional cutting.

Step 4: Check the Forend (Faceplate)

The forend, or faceplate, is the metal plate visible on the edge of the door once the lock is installed. Measure its length and width. Forends can be rounded or square, so make sure your new lock’s forend matches the cutout on your door if you want to avoid extra carpentry work.

Choosing the Right Size for Different Doors

The right mortice lock size depends heavily on the type of door you are securing.

Internal Doors

For most internal doors, such as those for bedrooms and bathrooms, a standard mortice lock is sufficient.

• Backset: 44mm or 57mm. The 57mm backset is generally preferred as it provides more room for your hand.

• Case Depth: 64mm (for a 44mm backset) or 76mm (for a 57mm backset).

These locks are typically mortice sash locks (which have a latch and a deadbolt operated by a key or thumbturn) or mortice latches (which only have a latch operated by a handle).

External Doors

External doors, like your front or back door, require a higher level of security. Therefore, a larger, more robust mortice lock is recommended.

• Backset: While 57mm is common, some high-security locks may have larger backsets.

• Case Depth: 76mm (3 inches) is a good starting point, but sizes like 102mm (4 inches) or 127mm (5 inches) offer enhanced security. A deeper case allows for a more substantial and stronger deadbolt mechanism.

Look for mortice deadlocks that are compliant with insurance standards, often indicated by a British Standard (BS 3621) Kitemark in the UK or similar certifications elsewhere. These locks are designed to withstand common burglary techniques.

Narrow Stile Doors

Some doors, particularly those with glass panels (like French doors) or aluminum-framed doors, have very narrow stiles. For these, a standard-sized mortice lock will be too deep. You will need to find a "narrow stile" or "narrow backset" lock. These can have backsets as small as 20mm or 25mm, with correspondingly shallow case depths. Always measure the stile width carefully before purchasing a lock for this type of door.

Final Checks Before You Buy

You’ve taken your measurements and have an idea of the size you need. Before you finalize your purchase, run through this quick checklist:

• Confirm your measurements: Double-check your backset and case depth. It’s always better to measure twice and cut once.

• Check the door’s thickness: Most mortice locks are designed for standard door thicknesses (around 35-45mm), but if your door is thicker or thinner, ensure the lock you choose will fit.

• Match your door hardware: Make sure the finish of the lock's faceplate (e.g., polished brass, satin chrome) matches your door handles and hinges for a cohesive look.

• Consider the lock type: Do you need a deadlock (key-operated bolt only), a sash lock (latch and bolt), or a simple latch? Your choice will depend on the door's location and security needs.

Your Next Steps

Choosing the correct size mortice lock is a simple but essential step in securing your home and ensuring your doors operate flawlessly. By taking a few careful measurements—focusing on the backset and case depth—you can confidently select a lock that fits perfectly and provides the security you need. Whether you're upgrading an old lock or installing a new one, getting the size right from the start will save you time, effort, and potential frustration.

With your measurements in hand, you're now ready to find the ideal mortice lock for your project.

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