Μάι 21, 2026

Euro Cylinder Sizes Explained: 30/30, 35/45, 40/40 and More

You find a smart lock you like. Then the product page asks you to check your Euro cylinder size: 30/30, 35/45, 40/40, or something similar.

Suddenly, buying a smart lock cylinder for a European door feels more complicated than it should.

This is especially true if you are upgrading from a traditional key cylinder to a WELOCK smart lock or another Euro profile smart lock. Before you choose fingerprint access, passcode entry, RFID cards, Bluetooth app control, or remote unlocking, the first thing to understand is whether the cylinder size fits your door.

The good news is that Euro cylinder sizes are not difficult once you understand the logic. The numbers do not describe the height of the lock. They do not simply mean the total door thickness either.

A Euro cylinder size tells you how long the cylinder is on each side of the fixing screw hole.

For example, a 35/45 Euro cylinder means one side measures 35 mm from the fixing screw hole, and the other side measures 45 mm. The total length is 80 mm.

This guide explains what common Euro cylinder sizes mean, why 35/45 is not the same as 40/40, and what to check before choosing a replacement cylinder or a WELOCK smart lock cylinder.

How to measure the cylinder

Quick Answer: What Do Euro Cylinder Sizes Mean?

A Euro cylinder size such as 35/45 means the cylinder measures 35 mm from the fixing screw hole to one end and 45 mm from the fixing screw hole to the other end.

The two numbers show how the cylinder length is divided on both sides of the center fixing screw hole.

This matters when replacing a traditional cylinder or choosing a WELOCK smart lock cylinder because the smart lock must match both the Euro profile cylinder size and the available space around the door handle.

So:

30/30 = 30 mm + 30 mm = 60 mm total length
35/45 = 35 mm + 45 mm = 80 mm total length
40/40 = 40 mm + 40 mm = 80 mm total length

A 35/45 cylinder and a 40/40 cylinder may both be 80 mm long, but they are not the same size in practice. The fixing screw hole sits in a different position.

What Is a Euro Cylinder Lock?

A Euro cylinder lock is the removable lock cylinder commonly used on doors across Europe. You will often see it on front doors, apartment doors, office doors, rental properties, and holiday homes.

It is called a Euro profile cylinder because of its standard shape. In many doors, the cylinder can be replaced without changing the entire lock case or door handle.

This is why Euro cylinders are also popular for smart lock upgrades. Instead of adding a bulky device over the existing lock, a Euro profile smart lock replaces the old cylinder directly.

A WELOCK smart cylinder is designed for this kind of door setup. It replaces the traditional key cylinder and turns the door into a keyless entry smart lock, with access methods such as fingerprint, passcode, RFID card, Bluetooth app control, and remote control when used with a compatible gateway.

But before choosing any Euro cylinder or smart cylinder, the first thing to understand is the size.

What do 30/30, 35/45, and 40/40 mean?

Euro cylinder sizes are normally written as two numbers.

Examples include:

30/30
35/35
35/45
40/40
45/45
40/50

These numbers are measured in millimeters.

The first number refers to one side of the cylinder.
The second number refers to the other side.

Both are measured from the center of the fixing screw hole.

This fixing screw hole is the center point used to hold the cylinder in the door. You can usually see the fixing screw on the edge of the door when the door is open.

So when you see 35/45,

It means:

35 mm from the fixing screw hole to one end
45 mm from the fixing screw hole to the other end
80 mm total cylinder length

The important point is this:

The total length alone is not enough. You need to know both side measurements.

This is especially important when choosing a smart lock for Euro-profile doors, because the inside and outside parts must sit correctly on the door.

Why 35/45 Is Not the Same as 40/40

This is the part many people get wrong.

A 35/45 cylinder and a 40/40 cylinder both have a total length of 80 mm.

But they do not fit the same way.

Size	Side A	Side B	Total Length
35/45	35 mm	45 mm	80 mm
40/40	40 mm	40 mm	80 mm

The total length is the same, but the fixing screw hole is not in the same position.

A 40/40 cylinder is balanced. Both sides are equal.

A 35/45 cylinder is offset. One side is shorter, and the other side is longer.

If your door needs 35 mm on the inside and 45 mm on the outside, replacing it with a 40/40 cylinder can leave one side too long and the other side too short.

That can affect how the cylinder sits in the handle plate, how neat it looks, and whether it fits correctly. For a WELOCK electronic door lock cylinder, the wrong side length may also affect how the smart lock body aligns with the handle area.

So never choose a Euro cylinder based only on total length.

Common Euro Cylinder Sizes Chart

Here is a practical chart of common Euro cylinder sizes in both millimeters and inches.

The inch values are approximate, because Euro cylinder sizes are normally manufactured and sold in millimeters.

Euro Cylinder Size	Side A	Side B	Total Length	Approx. Total Length in Inches	Type	Common Use
30/30	30 mm	30 mm	60 mm	2.36 in	Equal	Thin doors or simple internal doors
35/35	35 mm	35 mm	70 mm	2.76 in	Equal	Standard residential doors
40/40	40 mm	40 mm	80 mm	3.15 in	Equal	Balanced thicker doors
45/45	45 mm	45 mm	90 mm	3.54 in	Equal	Thicker doors or larger fittings
30/40	30 mm	40 mm	70 mm	2.76 in	Offset	Doors with uneven handle plates
35/45	35 mm	45 mm	80 mm	3.15 in	Offset	Common offset front door setup
40/50	40 mm	50 mm	90 mm	3.54 in	Offset	Thicker or asymmetric doors
45/55	45 mm	55 mm	100 mm	3.94 in	Offset	Thick doors or special hardware

A simple conversion to remember: 25.4 mm = 1 inch

However, when buying a Euro cylinder or a fingerprint smart lock cylinder, always use the millimeter size. The inch value is only for easier understanding.

Equal Euro Cylinders vs Offset Euro Cylinders

Euro cylinders can be divided into two main types by size: equal and offset.

Equal Euro Cylinders

An equal Euro cylinder has the same length on both sides.

Examples:

30/30
35/35
40/40
45/45

These are easier to understand because both sides match.

A 40/40 cylinder, for example, has 40 mm on one side and 40 mm on the other side.

Offset Euro Cylinders

An offset Euro cylinder has different lengths on each side.

Examples:

30/40
35/45
40/50
45/55

Offset cylinders are very common in real doors. Many front doors have different thicknesses on the inside and outside because of handle plates, security escutcheons, or decorative fittings.

This is why you should not assume your cylinder is equal just because the door looks normal.

For a traditional lock, an offset mistake may make the cylinder sit badly. For a WELOCK smart cylinder, it can also affect the positions of the inner and outer smart lock parts.

Why the Inside and Outside Direction Matters

For equal cylinders, direction is usually not a problem. A 40/40 cylinder is the same on both sides.

For offset cylinders, direction matters a lot.

A 35/45 cylinder has one short side and one long side. If you reverse it, the wrong side may stick out too far or sit too deep.

This is especially important when replacing a cylinder or installing a WELOCK smart lock cylinder.

Before removing your current cylinder, it is useful to note which side faces indoors and which side faces outdoors.

You can write it down like this:

Inside: 35 mm
Outside: 45 mm

Or:

Inside / Outside = 35/45

This small step can prevent a lot of confusion later.

What Happens If You Choose the Wrong Euro Cylinder Size?

Choosing the wrong size does not always mean the cylinder will be completely impossible to install. Sometimes it may still go into the door.

But “it fits into the hole” does not mean it is correct.

A wrong cylinder size can cause several problems.

The cylinder sticks out too far

If the cylinder is too long on one side, it may protrude beyond the handle plate or escutcheon.

This looks untidy and may also reduce security. A cylinder that sticks out too far can be easier to grip with tools.

The cylinder sits too deep.

If the cylinder is too short, it may sit too deep inside the handle plate.

This can make it harder to use and may cause alignment problems.

The door looks uneven.

Even if the cylinder works, the wrong size can make the door hardware look unbalanced.

This is especially obvious on front doors, apartment doors, and rental properties where appearance matters.

A smart ock body may not align correctly

For smart lock cylinders, size matters even more.

The inside and outside smart lock bodies need enough clearance around the handle area. If the cylinder size or direction is wrong, the installation may feel awkward or may not sit as intended.

That is why it is better to check the correct Euro cylinder size before buying a WELOCK fingerprint smart lock or any other smart lock cylinder.

How Much Should a Euro Cylinder Stick Out?

A euro cylinder should sit as flush as possible with the door handle plate or security escutcheon.

A very small amount of visible cylinder may be normal, depending on the door hardware. But it should not stick out far.

As a general rule:

Do not choose a longer cylinder “just to be safe”.

A longer cylinder is not safer. It is often the opposite.

The best size is the one that matches the actual door and fittings.

For security and appearance, the cylinder should be close to flush on both sides. This is also important for a smart lock cylinder for European doors, because the smart lock body should sit cleanly and firmly on the door.

Euro Cylinder Size vs Door Thickness

Euro cylinder size is related to door thickness, but they are not the same thing.

Door thickness only tells you how thick the door leaf is.

Euro cylinder size also depends on the following:

Inside handle plate thickness
Outside handle plate thickness
Security escutcheon thickness
Decorative door fittings
Whether the cylinder should sit flush
Whether one side of the door needs more length than the other

For example, two doors may both be 70 mm thick, but they may not use the same cylinder size.

One door may need 35/35.
Another may need 30/40 because the outside fitting is thicker.

This is why measuring only the door thickness can lead to the wrong size.

If you are buying a Euro profile smart lock, door thickness is only one part of the check. The cylinder length, handle clearance, and door edge distance also matter.

Euro Cylinder Sizes and Smart Lock Compatibility

When buying a smart lock, many people first look at features:

Fingerprint unlock
Passcode access
App control
RFID cards
Remote unlocking
Battery life
Alexa or smart home integration

These features are important, but physical compatibility comes first.

A WELOCK smart lock cylinder replaces your existing euro-cylinder. That means the lock must match your door type, cylinder size, and available space around the handle area.

For a traditional cylinder, the wrong size may only affect appearance or operation. For a smart cylinder, the wrong size can also affect how the inside and outside smart lock bodies sit on the door.

That is why WELOCK recommends checking both:

Your Euro cylinder size
Your inside and outside cylinder direction
Your door handle clearance
Your door edge distance

before choosing a smart lock for Euro-profile doors.

A WELOCK electronic door lock cylinder is designed for compatible European doors and can usually be installed without drilling or major door modification when the door meets the product requirements.

This makes it a cleaner option than many bulky retrofit smart locks, especially for homes, apartments, rental properties, and holiday homes that already use a Euro profile cylinder.

Before Buying a Smart Lock Cylinder

Before buying a WELOCK smart lock cylinder, check these points:

Checkpoint	Why It Matters
Your door uses a Euro profile cylinder	A WELOCK smart cylinder must match the lock type
You know both side measurements	Total length alone is not enough
You know inside and outside direction	Important for offset cylinders such as 35/45
The cylinder does not stick out too far	Better security and appearance
Handle plate has enough clearance	Important for fingerprint smart lock cylinder installation
Door edge distance is suitable	Helps avoid installation problems
The smart lock model supports your door	Different WELOCK smart lock models may have different requirements

Common Mistakes When Choosing Euro Cylinder Sizes

Mistake 1: Only Looking at the Total Length

An 80 mm cylinder is not always the same size.

It could be:

40/40
35/45
30/50

These all have different side lengths.

The total length is useful, but it is not enough.

Mistake 2: Assuming 35/45 and 40/40 Are Interchangeable

They are not.

Both are a total of 80 mm, but the fixing screw hole is positioned differently.

If your door needs an offset cylinder, an equal cylinder may not sit correctly.

Mistake 3: Guessing From Door Thickness

Door thickness helps, but it does not include handle plates, escutcheons, or fittings.

Always check the actual cylinder size.

Mistake 4: Ignoring Inside and Outside Direction

This is a common issue with offset cylinders.

If the long side is installed on the wrong side, the cylinder may stick out where it should not.

Mistake 5: Buying a Longer Cylinder Just in Case

A longer cylinder is not a safer choice.

If it protrudes too much, it may look poor and may reduce security.

Choose the correct size, not the largest size.

Which WELOCK Smart Lock Should You Consider?

Once you understand your Euro cylinder size, the next step is to choose a smart lock based on how the door is used.

Different users need different access methods. A family home may need fingerprint access for daily use. An Airbnb host may care more about passcodes and guest access. A holiday home owner may want remote control when paired with a gateway.

Here is a simple way to think about it:

Use Case	Suggested WELOCK Direction
Everyday home use	WELOCK U81 or U71 smart lock cylinder
Fingerprint and keypad access	WELOCK U71 fingerprint smart lock
Airbnb, rental, or guest access	WELOCK ToucA51 Pro smart lock cylinder
Budget-friendly keyless access	WELOCK ToucA51
Remote control and smart home use	Pair a compatible WELOCK smart lock with WiFiBox3
Face recognition and auto-locking	WELOCK A91 auto lock smart lock

The right choice depends on both the door size and the access scenario. A good smart lock for Euro profile doors should fit the cylinder position properly and make daily access easier without making the door look bulky.

Final Thoughts

Euro cylinder sizes look confusing at first, but the idea is simple.

A size like 35/45 means the cylinder measures 35 mm from the fixing screw hole to one side and 45 mm to the other side.

The most important thing is not just the total length. You need to understand how the length is divided on both sides of the fixing screw hole.

That is why 35/45 is not the same as 40/40, even though both are 80 mm in total.

If you are replacing a traditional cylinder or buying a smart lock cylinder for European doors, take a few minutes to check the size properly. It can help you avoid installation problems, poor fit, and unnecessary returns.

For European doors, a correctly sized smart cylinder can be a clean and practical upgrade. It gives you keyless access while keeping the door setup simple and familiar.

A WELOCK smart lock cylinder is designed for this kind of Euro-profile door upgrade. Instead of replacing the whole door lock system, it replaces the existing cylinder and adds modern access methods such as fingerprint, passcode, RFID card, Bluetooth app control, and remote access when paired with a compatible gateway.

If you are not sure which size your door needs, check your current cylinder size first and use the WELOCK smart lock compatibility checker before ordering.

FAQ of Euro Cylinder Sizes

What does "30/30" mean on a Euro cylinder?

A 30/30 Euro cylinder means the cylinder measures 30 mm from the fixing screw hole to one end and 30 mm to the other end. The total length is 60 mm, or about 2.36 inches.

What does "35/45" mean on a Euro cylinder?

A 35/45 Euro cylinder means one side is 35 mm and the other side is 45 mm, measured from the centre of the fixing screw hole. The total length is 80 mm, or about 3.15 inches.

Is 40/40 the same as 35/45?

No. A 40/40 cylinder and a 35/45 cylinder both have a total length of 80 mm, but they are not the same. A 40/40 cylinder is equal on both sides, while a 35/45 cylinder is offset.

Why do Euro cylinders have two numbers?

Euro cylinders have two numbers because the cylinder is measured from the fixing screw hole to each end. The two sides may be equal or different depending on the door and handle fittings.

Should a Euro cylinder be flush with the door?

Ideally, the euro cylinder should sit as flush as possible with the handle plate or escutcheon. It should not stick out too far, because excessive protrusion can affect appearance and security.

Can I replace a 35/45 cylinder with a 40/40 cylinder?

Usually, you should not replace a 35/45 cylinder with a 40/40 cylinder without checking the fit. They have the same total length, but the side measurements are different.

How do I know what Euro cylinder size I need?

The best way is to check your existing cylinder size by measuring from the centre of the fixing screw hole to each end. If you are buying a smart lock cylinder, also check door clearance and compatibility.

Are Euro cylinder sizes measured in inches or millimetres?

Euro cylinder sizes are normally measured and sold in millimetres. You may see inch conversions for reference, but the actual product size should be chosen in millimetres.

Can I install a WELOCK smart lock cylinder on any Euro profile door?

Not always. Your door must use a compatible Euro profile cylinder, and the cylinder size, door thickness, handle area, and clearance must match the WELOCK smart lock model. Always check compatibility before buying.

Is a WELOCK smart cylinder better than a retrofit smart lock?

For many European doors, a WELOCK smart cylinder can be a cleaner option because it replaces the existing euro cylinder directly. It does not require a large retrofit device mounted over the lock. However, the best choice depends on your door type, size, and access needs.

Does a WELOCK smart lock cylinder need WiFi?

Not always. Many WELOCK smart lock cylinders can work with local access methods such as fingerprint, passcode, RFID card, or Bluetooth. WiFi or a compatible gateway is usually needed for remote control, notifications, or smart home integration.