How to Measure a Cylinder Lock: A Complete Guide for Homeowners & DIYers

Whether you’re upgrading your home security, replacing a faulty lock, or simply enhancing the appearance of your door, measuring a cylinder lock accurately is essential. Choosing the wrong size can lead to poor security, jammed locks, and even increased vulnerability to break-ins. If you’ve ever wondered, “How do I measure a cylinder lock?” this in-depth guide is for you.

We’ll explain everything from the tools you’ll need to the step-by-step process of measuring cylinder locks, including euro cylinder locks, UPVC lock barrels, and more. We’ll also discuss selecting the correct replacement and avoiding common mistakes. Let’s dive in.

What Is a Cylinder Lock?

A cylinder lock, often known as a lock barrel, is a common mechanism found in residential and commercial doors. It’s the component where the key goes in and where the locking/unlocking action happens. One of its most significant advantages is the ability to replace it without changing the entire lock hardware.

➤ Common Types of Cylinder Locks:

Euro Cylinder Locks: Most popular in the UK and Europe, especially in UPVC doors.
Thumbturn Cylinder: Operated by a key from the outside and a thumbturn from the inside.
Double Cylinder: Requires a key on both sides.
Single Cylinder: Key on one side; blank or turn-style on the other.
Rim Cylinder: Found in night latches, mainly on wooden doors.

Getting the correct cylinder lock size is crucial to ensure smooth operation and optimal security. Now, let’s discuss how to measure it.

Tools You’ll Need to Measure a Cylinder Lock

Before you start the measuring process, gather the following tools:

A measuring tape or ruler (millimetres preferred)
A screwdriver (usually a Phillips or flathead)
Pen and paper (to note your measurements)
A flashlight (optional, for low-light areas)

With these on hand, you’re ready to begin measuring your lock barrel like a pro.

Step-by-Step Guide: How to Measure a Cylinder Lock

To accurately measure a cylinder lock, especially a Euro profile lock, follow these steps:

Step 1: Remove the Lock Cylinder

Open the door.
Locate the fixing screw on the edge of the door (usually aligned with the cylinder).
Use the screwdriver to remove the fixing screw.
Insert the key into the cylinder and turn it slightly (usually about 10 degrees) to align the cam.
Pull the cylinder out gently.

If you’re measuring a lock that’s still in place and cannot be removed, you can measure the projection of the barrel from both sides of the door.

Step 2: Measure the Cylinder Lock

Once removed, you’ll need to measure two dimensions:

A (External Length): From the centre screw hole to the external end of the lock.
B (Internal Length): From the centre screw hole to the internal end.

Write down the measurements as A/B (e.g. 40/40mm, 35/45mm).

📌 Pro Tip: The total length = A + B (e.g. 40mm + 40mm = 80mm overall).

Special Notes for UPVC and Wooden Doors

Measuring a UPVC Door Lock:

Most UPVC doors use euro cylinder locks. These can protrude if not measured correctly, creating a serious security vulnerability. Always measure both sides precisely and choose anti-snap models.

Measuring a Wooden Door Lock:

These may use rim cylinders or standard euro cylinders. If the lock sits flush with the handle plate, measure as described above. For rim cylinders, measure the length from the inside edge to the front faceplate.

Cylinder Lock Size Guide (Standard Sizes)

Here are some standard euro cylinder lock sizes for quick reference:

30/30 mm (60 mm overall)
35/35 mm (70 mm overall)
40/40 mm (80 mm overall)
35/45 mm (80 mm overall, offset)
45/50 mm (95 mm overall)

These sizes refer to the length from the centre fixing point outward. Offset sizes are standard in doors with unequal thickness on each side of the lock.

Common Mistakes to Avoid When Measuring

❌ Not accounting for handle thickness – Ensure you’re measuring the full projection beyond the faceplate, not just the door.
❌ Using inches instead of millimetres – Most cylinder locks are sized in mm.
❌ Guessing the size – This often leads to misalignment, difficulty turning the key, or an exposed cylinder (security risk).
❌ Not measuring both sides – Offset locks (e.g. 35/45) are standard, especially in composite or custom doors.

How to Choose the Right Cylinder Lock After Measuring?

After you’ve recorded your measurements, it’s time to find the perfect replacement. But don’t just shop by size, consider security features too.

Key Security Features to Look For:

✅ Anti-Snap Protection: Resists lock snapping (a standard break-in method)
✅ Anti-Bump Technology: Prevents forced entry using bump keys
✅ Anti-Drill Pins: Adds resistance against drilling attacks
✅ High-Security Certifications: Look for British Standard Kitemark or 3-star TS007 rated locks

Trusted Cylinder Lock Brands

If you want reliability and security, go for reputable brands:

Yale
ERA
Ultion
ABS
Mul-T-Lock

These brands offer a range of security levels, from basic to high-end smart locks with remote control access.

What Size Cylinder Lock Do I Need?

Once you’ve taken your A/B measurements, look for the corresponding size when shopping.

Example:

If your lock measures 35mm internal and 45mm external, you need a 35/45 cylinder lock (80mm total length).
For balanced doors with equal projection, a 40/40 or 30/30 ratio may be the best fit.

Top Tip: Never choose a cylinder that extends more than 5mm outside the handle, as doing so makes it easier for burglars to grip and snap it.

FAQs About Measuring Cylinder Locks

“How do I know what size cylinder lock to buy?”

Measure from the central fixing screw to both ends (A/B) in millimetres. Match these values with a lock listing (e.g., 40/40).

“Can I measure a cylinder lock without removing it?”

Yes, but accuracy may be limited. Measure the protruding parts from both sides of the door, including the depth of the handle.

“Are all euro cylinders the same size?”

No. Euro cylinders are available in various lengths and configurations (e.g., single/double, with or without a thumbturn). Always check the A/B values.

“Can a locksmith help me choose the right lock?”

Absolutely. A professional locksmith can not only measure and install your new lock but also advise on the best security features for your property.

Conclusion: Why Accurate Cylinder Lock Measurement Matters

Measuring your cylinder lock accurately is more than a technical task. It’s a key step in protecting your home or business. Using the wrong size can lead to poor functionality, compromised security, or even damage to the door. But the right size, paired with a high-security lock, offers peace of mind.

Whether you’re dealing with euro cylinder locks, UPVC locks, or traditional wooden doors, taking a few extra minutes to measure properly can save you time, money, and potential risk in the long run.

If you’re unsure, don’t hesitate to contact a trusted locksmith in your area. For those in the Central London area, our team at Central London Locksmith is always ready to assist, whether it’s measuring, fitting, or upgrading to high-security locks.