Fireplace opening size

A fireplace should have a minimum round flue diameter of 200 mm. If that condition is met then the flue size should be 15% (or more) of the fireplace opening size.

You can use our fireplace flue calculator to work out the flue size needed for a particular fireplace just click the link below.

Fireplace flue size calculator

Workings out for a round liner

The area of a circle is Π r²

Π r² ≥ area of the fireplace opening * 0.15

r² ≥ area of the fireplace opening * 0.15 / Π

r ≥ √area of the fireplace opening * 0.15 / Π

We want the diameter which is simply 2 x r