How to Measure a Roller Blind Tube Before Buying a Motor

Buying a roller blind motor by tube diameter alone is one of the easiest ways to end up with the wrong parts. For a retrofit, you need a small set of measurements and photos that tell you both whether the motor will physically fit and whether the blind is suitable for motorisation.

The five things to record before choosing a motor

1. Outside diameter of the bare aluminium tube.

2. A clear straight-on photo of the internal tube profile.

3. Overall blind width and fabric drop.

4. Fabric type or approximate weight, especially on large blinds.

5. Photos of the chain end, idler end and existing brackets.

How to measure the tube diameter correctly

Remove the blind and measure the aluminium tube itself. Do not measure over rolled-up fabric. Hold the ruler or callipers straight across the centre of the tube from outside edge to outside edge. If you are between sizes, record the measurement rather than rounding it to the nearest familiar number.

Why internal profile matters

The crown and drive are shaped to lock into a particular tube profile. An outside diameter of roughly 44–45mm does not prove that a tube is S45. A true L45, for example, uses a different internal shape and different adaptors. The profile photo is what removes the guesswork.

Measure the blind as well as the tube

Tube compatibility tells you what will fit; blind size and fabric load help determine how much torque you need. Record the finished blind width and the fabric drop. On wider blinds, the strength of the tube matters too. A more powerful motor can lift more load, but it cannot make a light tube stiffer.

Check the wall and profile before assuming it can be motorised

Some budget and off-the-shelf blinds use thin aluminium tubes designed only for manual chain operation. A diameter that appears suitable does not automatically make the tube motor-ready. If the tube is very thin, has no useful internal profile, is damaged or sags noticeably, replacement is often the better route.

What photos should you take?

• Open end of the tube with the idler removed.

• Ruler or callipers showing the outside diameter.

• Existing chain control and bracket at the control end.

• Full blind so the overall size and installation are visible.

Before you shop for a motor, compare your measurements with the profiles in our roller blind tube identifier. Once you select the tube, the next page shows only the motors confirmed for that profile.

Do you measure the motor or the tube?

For a retrofit, start with the tube you already own. The tube determines which motor body and adaptor system can be used. Motor torque is then selected to suit the blind size and load.

If your tube cannot be identified or is unsuitable, that does not necessarily mean replacing the whole blind. In many cases the fabric can be transferred onto a motor-ready tube and the blind can still be upgraded.