How to Measure a Mortice Lock: Step-By-Step Guide

Measuring up a mortice lock can be confusing to a new Locksmith, and a refresh is occasionally needed for those who require it. This practical guide will help you confidently measure both mortice sash locks and deadlocks, ensuring you get the right replacement every time.

What You’ll Need

1. Tape measure or ruler
2. Pen and notepad to record measurements

Understanding Mortice Locks

A mortice lock is installed within a pocket (the mortice) cut into the edge of a door. You will typically encounter:

• Sash locks: Include a latch operated by handles and a deadbolt operated by a key.
• Deadlocks: Only include a key-operated deadbolt without a latch or handles.

For deadlocks, measure up to Step 4 only.

How to Measure a Mortice Lock

All measurements apply to a mortice sash lock with handles. To measure a dead lock, it is the same as but only up to step 4.

To measure a mortice lock, you’ll need these measurements:

1. Case Depth – Measure from the front edge of the lock (where the faceplate sits) to the back of the lock case inside the door.
2. Backset – Measure from the front of the faceplate to the centre of the keyhole or spindle follower (where the handle spindle goes through). This ensures the handle or keyhole aligns correctly with the door furniture.
3. Body Height – Measure the height of the lock body from top to bottom.
4. Positioning Zone (PZ) – Measure from the centre of the spindle follower (handle spindle hole) to the centre of the
5. Forend Height – Measure the front plate of the lock from top to bottom, excluding the visible faceplate.
6. Faceplate Width – Measure the width across the faceplate of the lock.
7. Screw Centres – Measure the distance between the centres of the screw holes on the faceplate.

Tip: Check out our Locksmith terminology guide if you’re unsure about any of these parts or terms.

Things to Consider When Choosing a Mortice Lock

Deadlock vs. Sash Lock

Generally, deadlocks are used on external front doors for security, while sash locks are commonly fitted to internal doors for functionality with handles. A mortice deadlock provides higher security, while a sash lock combines a latch with locking capability.

Door Stile Measurements

Door stiles vary in thickness, so it’s important to measure the door carefully before fitting a mortice lock to ensure a snug fit without excessive chiselling.

Security Standards

For external doors, you should use a lock that meets British Standards (BS3621:2007 or equivalent). A 3-lever mortice lock does not meet these standards, so a 5-lever British Standard mortice lock is required for external doors to meet insurance and police recommendations.

Pro Tip: For enhanced security, consider using a British Standard mortice deadlock in conjunction with a night latch on external doors.

Common Mistakes to Avoid

• Mixing up backset and case depth measurements
• Forgetting to measure the PZ on sash locks
• Not checking the door swing direction before installation
• Assuming all locks will fit standard cut-outs without checking dimensions

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