How to measure a door cylinder: how to determine the right size

Do you need a new door cylinder but don't know which size to choose? Or do you want to replace an existing cylinder and first check which size you need?

The correct cylinder size is important for proper lock operation and a neat installation. In this article, we explain how door cylinder dimensions work, how to measure a cylinder and what to consider when choosing the right size.

Door cylinder for different applications and size combinations

Why is the right cylinder size important?

A door cylinder needs to fit correctly with the thickness of the door and the existing security hardware. A cylinder that is too short may not provide enough space for proper installation, while a cylinder that is too long may protrude unnecessarily.

When replacing an existing cylinder, it is therefore important to consider not only the type of cylinder, but also the correct size.

How does door cylinder sizing work?

The size of a euro profile cylinder is determined by measuring from the centre of the fixing screw hole to both sides of the cylinder.

A cylinder size such as 30/35 means that the cylinder measures 30 mm from the centre towards one side and 35 mm towards the other.

There are both symmetrical and asymmetrical cylinders.

For example:

30/30 – both sides are 30 mm;
30/40 – one side is 30 mm and the other 40 mm;
35/45 – one side is 35 mm and the other 45 mm.

The order in which the two measurements are stated may vary depending on the manufacturer or product. When ordering, always check the product description and dimensions of the specific cylinder.

At Javena, we use the inside measurement first in our product coding.

What does 30/30 mean?

A 30/30 cylinder measures 30 mm from the centre of the fixing screw hole to each side.

This is a symmetrical cylinder. With this type, it generally does not matter which side faces the inside or outside, provided both sides have the same configuration.

What does 30/40 or 35/45 mean?

With an asymmetrical cylinder, the two sides have different lengths.

With 30/40, one side is 30 mm and the other is 40 mm. With 35/45, one side is 35 mm and the other is 45 mm.

An asymmetrical size may be required when the thickness of the door and security hardware differs on either side.

Door cylinder dimensions measured from the centre of the fixing screw hole

How do you measure a door cylinder?

The most accurate way to determine the required size is to remove the existing cylinder from the door.

Measure from the centre of the fixing screw hole to both ends of the cylinder. This determines the size of each side separately.

In many situations, it is also possible to measure a cylinder without removing it. A cylinder measuring tool can be used for this purpose.

Want to know exactly how to do this? See our detailed guide:

How to measure a door cylinder

There we explain step by step where to measure and what to look out for.

Cylinder measuring tool for determining the correct door cylinder size

Which door cylinder size do you need?

When choosing the right size, you need to consider more than just the thickness of the door.

Consider factors such as:

the thickness of the door;
the thickness of the security hardware;
the position of the lock;
the available space on both sides;
any differences between the inside and outside.

It is also important to consider how the cylinder will sit in relation to the security hardware.

Want to learn more about different cylinder sizes and how to choose the right one?

See: Which cylinder size?

How far may a cylinder protrude?

A door cylinder should fit correctly with the security hardware. A cylinder that protrudes too far on the outside may negatively affect the security of the door.

As a practical guideline, a cylinder should not protrude more than approximately 3 mm beyond the security hardware.

The exact situation may vary depending on the type of cylinder and security hardware. Always check that the selected cylinder fits correctly with the existing hardware.

Thumbturn cylinder or double cylinder?

The cylinder configuration is also important, in addition to the size.

A double cylinder has key operation on both sides. A thumbturn cylinder has a thumbturn on one side and key operation on the other.

In our product coding, the thumbturn side is indicated by the letter K. The K stands for the thumbturn side, which is located on the inside of the door in our applications.

Examples include:

K30/30
K30/45
K45/30

With this coding, the thumbturn side is listed first. So K30/45 means that the thumbturn side is 30 mm and the other side is 45 mm.

Want to know more about the differences between the two configurations?

See: Thumbturn cylinder vs double cylinder

Thumbturn cylinder with thumbturn operation on the inside of the door

Common mistakes when choosing a cylinder size

Several things are regularly overlooked when replacing a cylinder.

Only measuring the door thickness

The door thickness alone is not enough. The security hardware also affects the required cylinder size.

Misinterpreting the measurement order

Not every manufacturer uses the same order when stating the two measurements. Always check the product specifications.

Mixing up the inside and outside

With asymmetrical cylinders, the position of the different measurements can be important. Check which side of the cylinder needs to be installed where.

Only looking at the existing size

An existing cylinder is a useful starting point, but when replacing it, always check whether the size and configuration are actually suitable for the current situation.

Replacing a cylinder: when do you need a different size?

When replacing an existing cylinder, it is often possible to use the same size. However, a different size may be required if, for example, the security hardware has been changed or the new cylinder has a different configuration.

You should also check the dimensions again when switching from a double cylinder to a thumbturn cylinder.

If you are replacing several cylinders, it can be useful to first record all the required sizes and configurations.

Need help determining the right cylinder size?

Not sure which size you need? For example, send us a photo of the existing cylinder and security hardware. We can help you determine the correct size and configuration.

If you need several cylinders, we can also help you map out the different sizes and configurations clearly.

Frequently asked questions about cylinder sizes

How do you measure a door cylinder?

Measure from the centre of the fixing screw hole to both sides of the cylinder. For the most accurate measurement, you can remove the cylinder from the door.

Read the full guide to measuring a door cylinder

What does 30/30 mean on a cylinder?

A 30/30 cylinder has a length of 30 mm on both sides, measured from the centre of the fixing screw hole.

What does 30/40 mean on a cylinder?

With a 30/40 cylinder, one side is 30 mm and the other is 40 mm. This is an asymmetrical cylinder size.

Which thumbturn cylinder size do I need?

This depends on the door, security hardware and desired position of the thumbturn. Thumbturn cylinders are available in different size combinations, such as K30/30, K30/45 and K45/30.

How far may a cylinder protrude?

As a practical guideline, a cylinder should not protrude more than approximately 3 mm beyond the security hardware. The exact situation may vary depending on the type of hardware and cylinder.

Related information

Which cylinder size?

Learn more about different cylinder size combinations and what to consider when choosing the right size.