How to Make a Circular to Square Duct

Quite often air ducts need to be converted from circular to square or rectangular and vice versa. This may be because the motor is circular but the space for the ducting requires a rectangle shape, or in my case, I wanted to fit a square fan to a circular pipe.


After a bit of searching, I came across this page. It includes a nice little spreadsheet to help you with dimensions for the conversion section. Basically you end up drawing 2 arcs and then draw some triangles and lines. You may know straight lines that touch the edge of a circle as “chords”. You only draw the chords on the “outer radius”. You measure the “ticks” by a chord, however, you only draw a tick on the inner radius to mark where your triangle lines go. Note that in the spreadsheet it has a diagram for a circular to rectangular section – for a square the longer and shorter lengths are the same.

Time to actually make something! Note that you end up needing quite a large bit of material for something that ends up smallish.


Here is the design for a 75mm circular diameter to 120mm square section. I added tabs to the square end to attach to a fan. With this layout you can’t really add tabs to the circular end – you would have to attach the end to a pipe.


I scored along the lines to be folded to make things come together easier. If making this from metal, you would have to fold the lines as you go. I added a tab to one side so I could join the face together.

nutri grain packs are useful

Once you join the face together it is basically done! I glued and taped the fan into the square end. The circular tube was just taped as it was a bit loose – I didn’t measure it precisely since it was out-of-round.

Once you get your head around the geometry, these are really simple to make! I thought I’d make a nice little article to help anyone else looking to make one of these, as for some reason it took me a while to find a simple solution 🙂 You can also make a large circle go into a small rectangle/square, just note your inner radius will now be the outer radius and vice versa (it’s all on the page I linked to).

I hope you found this useful and interesting, please tell me if you make something interesting!

Wtyl,     <— “write to you later…”

Rennie ♪

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