Converter
Flat-oval duct size calculator
Flat-oval duct fits the low-profile cavities round duct can't, while keeping much of round duct's efficiency. Most calculators skip it; this one treats it as a first-class shape and gives you its equal-friction equivalent round diameter.
Enter the major and minor dimensions to get the area, perimeter, and equal-friction round diameter.
How flat-oval is sized
A flat-oval section is a rectangle capped with semicircular ends. From its major
axis Ax and minor axis a, the cross-sectional area is
the rectangle plus the two half-circles, and the perimeter is the two straight
sides plus the circular ends. The equal-friction equivalent round diameter then
comes from the Heyt & Diaz correlation
Dₑ = 1.55 · A^0.625 / P^0.25,
using that area A and perimeter P.
With the equivalent diameter in hand you can drop flat-oval into a run wherever a round or rectangular duct would otherwise go, at the same pressure drop. As with every shape conversion, this matches friction and capacity — not velocity — so check the actual velocity for the section in the console. To compare shapes directly, see the equivalent diameter tool.