Converter
Equivalent diameter calculator
The equivalent diameter is the round size that loses the same friction and carries the same air as a given rectangular or flat-oval duct. It's the common currency that lets you compare any two shapes on equal terms.
Enter the duct's dimensions to get its equal-friction equivalent round diameter.
The two relationships
For rectangular duct, ASHRAE uses the Huebscher circular-equivalent relationship
Dₑ = 1.30 · (a·b)^0.625 / (a+b)^0.25,
which equates friction and capacity. For flat-oval duct, the Heyt & Diaz
correlation
Dₑ = 1.55 · A^0.625 / P^0.25
does the same job from the section's area A and perimeter P.
Both give equal friction, which is what you want when you're matching a run's pressure drop. They are not equal-velocity conversions — the actual velocity follows the real cross-sectional area, which the console reports for every shape. If you instead want the round duct with the same cross-section (and therefore the same velocity), switch the relationship to equal area, which returns Dₑ = √(4A/π). To turn an equivalent diameter into specific rectangular sides, use the round-to-rectangular converter.