How to Measure a Door Cylinder: Complete Guide for Accurate Fitting
Choosing the correct door cylinder is crucial for both security and functionality.
At Importa, we ensure that each cylinder perfectly fits your hidden door system. But how do you know which size to choose? This guide explains how to measure a door cylinder correctly and introduces the cylinder size formula used globally.
What Is a Door Cylinder?
A door cylinder, often called a euro cylinder, is the locking mechanism inserted into the door handle or lock body. Its size must match the door thickness and the lock placement exactly—otherwise, the lock might sit unevenly or leave part of the cylinder exposed (a security risk).
How to Measure a Door Cylinder in 3 Simple Steps
To determine the correct cylinder size, you’ll measure from the central screw hole (used to fix the cylinder) to each side of the cylinder.
Measure the Internal Side
From the center of the screw hole to the face of the cylinder inside the building (handle side).
Measure the External Side
From the center of the screw hole to the face of the cylinder on the outside (key side).
Add Both Sides Together
This gives you the total cylinder length.
Cylinder Size Formula
Total Cylinder Size = Internal Size + External Size
Example:
If the internal size is 50mm and the external side is 35mm, then:
Total Size = 50mm + 35mm = 85mm
This would be written as: 50K/35K or 50T/35T depending on the cylinder type (K = key, T = thumbturn).
Types of Cylinders in the Importa Range
(Shown in the measuring guide image)
Cylinder Type | Notation | Description |
Euro Cylinder | 50K/35K | Key/key operation |
Thumbturn Cylinder | 50T/35K | Thumbturn inside, key outside |
Symmetrical | 35K/35K | Equal internal and external sides |
Short Thumbturn | 35T/35K | Smaller door thickness compatibility |
Common Mistakes to Avoid
- Measuring total length only – You must measure each side separately.
- Guessing based on door thickness – Always use a tape measure and check against center screw hole.
- Ignoring thumbturn direction – If installing a thumbturn, ensure the correct side faces inside.
Tips from Importa Experts
- Use millimeter-accurate tape.
- If you’re unsure, take a picture like the guide above and send it to our team.
Always double-check if you’re choosing a symmetric cylinder (e.g., 35/35) or an offset one (e.g., 50/35).
