How to Measure a Door Cylinder Correctly: A Simple Guide by Importa Hardware

Choosing the right door cylinder isn’t just a detail—it’s essential for your door’s security, smooth functionality, and overall fit. At Importa Hardware, we ensure each cylinder is perfectly matched to your hidden door system, but knowing how to measure your cylinder correctly helps you avoid costly mistakes or mismatches.

Here’s a quick and reliable guide to help you choose the correct size cylinder for your door.

What Is a Door Cylinder?

A door cylinder—also known as a euro cylinder—is the core locking mechanism that fits into your door handle or lock body. It must match your door’s thickness and lock placement precisely.

If it’s too long or short on either side, it could stick out (a security risk) or sit unevenly, which may affect functionality and aesthetics.

How to Measure a Door Cylinder in 3 Simple Steps

To get the correct size, all you need is a tape measure and a little care. Here’s how to do it:

1.Measure the Internal Side

Measure from the center of the fixing screw hole to the inside face of the cylinder (usually the handle side).

2️.Measure the External Side

Now measure from the same center point to the outside face (usually the key side).

3️.Add Both Sides Together

This gives you the total cylinder length.

Cylinder Size Formula

Total Cylinder Size = Internal Size + External Size

Example:

If your internal measurement is 50mm and the external side is 35mm:

Total Size = 50mm + 35mm = 85mm

This would be written as:

50K/35K (key on both sides)

50T/35K (thumbturn inside, key outside)

depending on the cylinder type

Types of Cylinders Offered by Importa

Cylinder Type                           Notation       Description

Euro Cylinder                           50K/35K       Key on both sides

Thumbturn Cylinder                 50T/35K       Thumbturn inside, key outside

Symmetrical                           35K/35K       Equal internal and external measurements

Short Thumbturn               35T/35K       Compatible with smaller door thickness

Common Mistakes to Avoid

Only measuring total length – Always break it into internal and external parts.

Guessing based on door thickness – Use a tape measure and reference the center screw hole.

Installing thumbturn on the wrong side – Always check that the thumbturn goes on the inside.

Importa Expert Tips

Always use a millimeter-accurate tape.

Not sure about your measurement? Take a clear photo like the one in our guide and send it to our team.

Double-check if your door needs a symmetric (e.g., 35/35) or offset (e.g., 50/35) cylinder.

Still Unsure? Let Us Help!

At Importa Hardware, we’re here to make sure you get the right hardware for every door. If you’re in doubt about your cylinder size or which type fits your hidden door system, contact our support team. Send us a photo and your measurements—we’ll guide you to the perfect match.

Ready to order or ask a question? Contact us now or chat with our experts via WhatsApp.