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Fireplace opening : flue size ratio
A fireplace should have a minimum round flue diameter of 200mm.
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