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Order until 5PM next day delivery on selected products

Lock Shop Direct measurement guide

How to Measure a Door Cylinder Correctly

Door cylinders are measured in two halves from the centre of the fixing screw. The physical measurements are straightforward, but the order in which manufacturers display the two numbers is not universal. Always record the inside and outside measurements separately before choosing a product size.

Most important rule: write the measurements down in words first — INSIDE: ___ mm and OUTSIDE: ___ mm. Do not assume that a size written as 35/45 always means 35mm inside and 45mm outside. Some ranges use the opposite order.

In this guide

1. Identify the cylinder type

First identify the cylinder profile and how it operates. Common versions include euro profile, oval profile, double key cylinders, key-and-thumbturn cylinders, and single or half cylinders.

Common door cylinder profiles and operating types

Cylinder type: compare the profile and operating arrangement before measuring. A euro cylinder and an oval cylinder may have similar lengths but are not interchangeable.

Common door cylinder features and security options

Features: note whether the existing cylinder is key/key, key/thumbturn or single-sided, together with any security marks, snap lines, manufacturer names or key references.

2. Mark the inside and outside before removal

Before taking the cylinder out of the door, identify which end faces the protected interior and which end faces outside.

Inside: the side facing the interior of the property.
Outside: the side exposed to the external or attack side of the door.
On a thumbturn cylinder, the turn is normally fitted on the inside, but do not use this as the only way of deciding the size order.
Place a small removable label on the outside end, or take a clear photograph before removing the cylinder.

Do not lose the orientation: an offset anti-snap cylinder may physically fit when reversed but place the sacrificial or reinforced side on the wrong side of the door.

3. Measure each half from the centre fixing screw

Open the door and locate the long cylinder retaining screw in the edge of the door. Measure from the centre line of that screw to each end of the cylinder body.

Measure from the centre fixing screw to the inside end of the cylinder body.
Measure from the same centre point to the outside end.
Do not include the key, thumbturn knob or decorative handle projection.
Add the two measurements only as a cross-check of the overall cylinder length.

Measuring a double euro cylinder from the centre fixing screw

Measure both sides independently from the centre fixing point. Keep the two measurements labelled as inside and outside.

Measuring a single or half euro cylinder

For a single or half cylinder, check which side is the operating side and follow the selected product’s diagram; some ranges describe the unused side as 10mm.

Measurement	Where to measure	Your measurement
Inside	Centre of fixing screw to the inside end of the cylinder body.	_____ mm
Outside	Centre of fixing screw to the outside end of the cylinder body.	_____ mm
Overall	Inside plus outside, used only as a cross-check.	_____ mm
Operation	Key/key, key/thumbturn, single or another arrangement.	__________

4. Check the manufacturer’s size order

There is no universal rule that the first number is always inside or always outside. Manufacturers and product ranges use different conventions, and the letters A and B are also not standardised.

Example: your door measures 35mm inside and 45mm outside

If the product is listed inside/outside, select 35/45.
If the product is listed outside/inside, select 45/35.
If a thumbturn product is listed turn side/key side, and the turn is inside, select 35/45.

The same door can therefore require a differently written option depending on the selected manufacturer or product page.

Alternative door cylinder size order used by some product ranges

Alternative order: some Lock Shop Direct ranges use the opposite first/second dimension from other ranges. Treat the labels on the individual product page and its diagram as the controlling information.

Example convention	How the size is presented	What to do
Inside / outside	First number is the inside; second is the outside.	For 35mm inside and 45mm outside, choose 35/45.
Outside / inside	First number is the outside; second is the inside.	For 45mm outside and 35mm inside, choose 45/35.
Turn side / key side	Used on some thumbturn-cylinder selectors.	Match the turn side and key side explicitly; do not assume A/B.
Body / cam / body	A technical drawing may show three figures, such as 30/10/40.	Follow the drawing. A 30/10/40 body can correspond to nominal 35/45 measurements from the screw centre.
A side / B side	A and B mean only what the manufacturer’s diagram says.	Never assume A is inside or outside without checking the legend.

Before adding to basket: check the wording next to the size selector. The selected product page may say internal/external, external/internal, turn side/key side, snap side, A/B, or show a three-part body/cam/body dimension.

5. Check security features and orientation

For anti-snap or 3-star cylinders, identify the side marked EXT, external, snap side or otherwise shown as the protected external end.
Make sure that side faces outside when installed.
On a key-and-thumbturn cylinder, the thumbturn normally belongs on the inside.
Do not assume that a key/key cylinder is reversible merely because both ends accept a key; some high-security features are fitted on one side only.
Equal-size cylinders such as 35/35 remove the length-order issue, but the security orientation may still matter.
Check whether the cylinder must work when a key is left in the other side; this may be described as emergency, double-clutch or key-override operation.

Choosing a door cylinder that fits the door furniture correctly

The cylinder should suit the complete door-and-furniture thickness and should not project unnecessarily beyond the handle or security escutcheon.

6. Remove the cylinder to confirm the size

Where safe to do so, removing the cylinder is the most reliable way to check its size and markings.

Keep the door open.
Mark or photograph the outside end.
Remove the cylinder retaining screw from the door edge.
Insert the key and turn it slightly until the cam aligns with the cylinder body.
Slide the cylinder out without forcing it.
Measure both halves and inspect the body for manufacturer, size, EXT, snap-side and security markings.
Refit the cylinder and test the lock with the door open before closing it.

Ordering check: inside ___ mm; outside ___ mm; profile __________; operation __________; product page order __________; selected option __________; external security side confirmed: yes / no.

Still not sure which cylinder you need?

Send clear photographs and the two labelled measurements to our team before ordering. Please include:

A photograph of the cylinder fitted in the door, showing which side is outside.
A photograph of the complete removed cylinder beside a ruler or tape measure.
The inside and outside measurements written separately.
Any brand, code, EXT mark, snap line, Kitemark or key reference.
Whether you require key/key, key/thumbturn, keyed alike, master keyed or restricted keys.

Telephone: 0330 174 0851
Email: info@lockshopdirect.co.uk

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Cylinder terminology, nominal sizes, A/B labels and first/second measurement conventions vary by manufacturer and sometimes by product range. The individual product page, current technical drawing and manufacturer’s fitting instructions take precedence over any general example. If the cylinder is security-handed, has a thumbturn, contains a sacrificial snap section or forms part of a fire, escape or access-control doorset, confirm the complete specification before ordering or fitting it.