Free tool
Round ↔ rectangular duct equivalent
Equal-friction conversion between rectangular and round duct using the standard formula De = 1.30 × (a×b)^0.625 / (a+b)^0.25. Works in both directions: rectangular to round, or round to rectangular with one side fixed.
Rectangular → round
Equivalent round
Ø 16.8"
Round → rectangular
Other side
10.4"
16" × 10.4" carries the same air as Ø 14"
Reference values for equal friction rate. Final duct sizing belongs to your mechanical designer.