DIY and How to

How to: Sizing vents correctly

How to measure size of vents:

When ordering replacement vents it is important to ensure you know how to choose the correct size vents you wish to order. It seems simple enough but the last thing you want is to have your new vents delivered only to find out they do not fit with the existing duct adaptors or even the existing duct work.

To save time, money and your sanity we’ve written the following as a 'crash course' on measuring and selecting the right sized vents with adaptors which will hopefully save you the time and hassle of having to send back a stack of vents and adaptors to be exchanged for different sizes.

So what do you need to do before clicking the add to cart button?

Well first and foremost you need to measure the vents you wish to replace. For this you need to get hold of a tape measure or ruler. You'll  need access the vents being replaced and to be able to measure them correctly and accurately.  And finally, a note pad and pencil to wright down any measurements you take. Don’t try and remember all the measurements, just wright them down!

When measuring there are three important sizes which must be taken into account. These are the neck size, face size and duct spigot size (AKA pop size)

Neck Size:

Measured off the back of the vent, this is the most important measurement as it determines how much or little air a vent is designed to handle. In the air conditioning industry all vent sizing is determined by this size in particular. The neck size works out to be the approximate hole size which is cut into the ceiling, floor or wall and gives a good indication for what the required vent size may be. 

For square or rectangular vents it is simply a case of measuring the width by height (W x H) off the back of the vent and round vents it is the diameter of the round spigot on the back.

In the images below, the neck sizes of different shaped vents have been highlighted. In the first image, the square neck is shown as A x B and highlighted in green. The second image the neck diameter is show as C and highlighted in blue.

Face size:

The face is the part of the vent which people see when installed in the ceiling, floor or wall. It is also the easiest to measure as you just need a ladder and measuring tape and doesn’t involve having to remove the vent from where it is fitted.

Same as with measuring the neck size, square and rectangular vents are measured as width by height (W x H) and round vents from the front face diameter.

Keep in mind that different vents styles will have different face sizes even if they have the same neck size. For example our MDO 30cm x 30cm style X and beveled face diffuser 30cm x 30cm have exactly the same neck size (and even use the same duct adaptor) but have face sizes of 340mm x 340mm and of 383mm x 383mm respectively. This  is why it is so important to take into consideration the face size, especially if you are changing vent style throughout your home. If the face size does not cover the existing cut out in the ceiling, floor or wall the vent may fall through the hole or not cover it completely! If this is the case you may have to consider a vent style with larger face size or even for a larger neck size.  

In the images below, the face sizes of different shaped vents have been highlighted. In the first image, the square face is shown as D x E and highlighted in purple. The second image the face diameter is show as F and highlighted in yellow.

Duct spigot size:

Also known as ‘pop’ size, this is the size relating to the duct work being attached to the vent. This size is determined by measuring the inside diameter of the ductwork itself.

On Square and rectangular vents the duct adaptors (or floor boots) are a separate optional extra and can be purchased online from here. Round vents have the spigot inbuilt and do not require such an additional duct adaptor all you have to do is match the duct size to the neck size and your away!

In the images below, the duct adaptor size and duct size are highlighter and shoud be matched accordingly. In the first image, the adaptor diameter is shown as G and highlighted in orange. The second image the duct internal diameter is show as H and highlighted in Pink.


As long as you have taken these 3 measurements then sizing new vents and grilles shouldn’t be an issue and will then simply be a case of selecting the correct (or as close to correct) size from our range of vents and adaptors online.

Hopefully this guide will assist you with making your selection for replacement vents and grilles that little bit easier to understand. If you are still not 100% on how to measure please feel free to get into with us at and we'll assist where we can.