If you’re planning to replace an old lock or install a new one, this guide is here to help. Even though the process might seem simple to experienced DIYers, accurate measurements are essential to make sure you get the right fit.
When you measure a mortice lock correctly, you’ll avoid common problems like buying the wrong size, damaging your door, or wasting time. Fortunately, measuring is something you can easily do yourself—and we’ll show you exactly how.
Why Accurate Measurement Matters
Choosing the right size mortice lock starts with knowing the correct dimensions. Getting these wrong can lead to issues like:
- A lock that doesn’t fit the existing hole
- Damaged doors
- Wasted time and money
Let’s go through the key things to consider before buying your lock.
Type of Mortice Lock
First, identify which type of mortice lock you need:
- Deadlocks: Typically used on front doors; they only include a bolt and keyhole.
- Sash locks: Include both a latch (for handles) and a bolt; common on back and interior doors.
Deadlocks are usually simpler to measure because they don’t require handle alignment.
Door Stile Thickness and Width
Mortice locks come in various depths and lengths. Not all locks fit all doors, and doors themselves vary in thickness and width. That’s why it’s essential to measure the stile—the vertical edge of your door—before buying a lock.
Door Type and Material
The type and material of your door can also affect your lock choice. Mortice locks are designed for wooden doors only. You can’t install them on uPVC or composite doors.
Also, check which side the lock is on—left or right. Most mortice locks are reversible, so this usually isn’t a major issue, but still worth noting.
Security Level
If you’re installing a lock on a front door, security is key.
Cylinder for locks that meet EN1303 standards. These have been tested for resistance against picking and forced entry.
Purpose and Location
Think about where the lock will be used—residential or commercial property?
The location will influence the level of durability and strength required.
Replacing an Existing Lock
If you’re replacing an old lock, measurement becomes easier. You can simply measure the visible parts of the existing lock without removing the whole unit:
- Forend width and length
- Handle and keyhole spacing
How to Measure a Mortice Lock
Whether you’re installing a new lock or replacing an old one, here’s what you need:
Tools:
- Notepad and pen (to record measurements)
- Ruler or tape measure
- Screwdriver (to remove the existing lock, if needed)
For Deadlocks:
Take these 5 measurements:
- Backset: Distance from the front face of the lock to the center of the spindle hole.
- Case Depth: How deep the lock body is inside the door (commonly 64 mm or 76 mm).
- Case Height: The vertical height of the lock body.
- Forend Width: The width of the visible metal plate on the door edge.
- Forend Length: The length of that metal faceplate.
For Sash Locks:
Same measurements as deadlocks, plus one more:
- Centres (Center Measurement): Distance between the handle spindle hole and the keyhole.
This is especially important because it affects how the handle and key will align with the lock.
Final Tips
- Double-check all your measurements before buying a lock.
- If replacing a lock, ensure the new one matches the existing cut-outs.
- For new doors, carefully choose the right size and type based on material and door width.
- Know whether you need a 3- or 5-lever lock and whether it should be a deadlock or a sash lock.
Write everything down clearly, take your time with measurements, and you’ll be set up for a smooth and secure installation.
