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Equivalent round duct diameter calculator
Duct is sized from a friction rate in round dimensions, then converted to a rectangular duct that fits the available space, and the link between the two is the equivalent round diameter. The formula is De = 1.30 times (a times b) to the 0.625 power, divided by (a plus b) to the 0.250 power, with the rectangular sides a and b in inches. Enter the two sides to get the round duct that carries the same airflow at the same friction loss. The point that trips people up is that the equivalent is matched on friction, not on cross-sectional area, so a rectangular duct sized this way actually has a larger area than the round it replaces. The calculator also reports the aspect ratio, the long side divided by the short side. Keep that under about 4 to 1, because a flat, high-aspect duct burns more sheet metal, adds friction and noise, and costs more to fabricate and hang. Size the system from the design friction rate and confirm the final layout with the mechanical engineer.
Result
Equivalent round duct diameter: De = 1.30 × (a × b)^0.625 / (a + b)^0.250, with the rectangular sides a and b in inches. This gives the round duct that carries the same airflow at the same friction loss as the rectangular duct, which is how a friction-rate (Manual D) round-duct sizing is converted to a rectangular duct that fits the space. The key point is that the equivalent round is matched on friction, not on cross-sectional area, so a rectangular duct of the equivalent round actually has more area. Watch the aspect ratio (the long side divided by the short side): keep it under about 4 to 1, because a flat, high-aspect duct uses more sheet metal, adds friction and noise, and costs more to build and hang. Size the system from the design friction rate and confirm the layout with the mechanical engineer.
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Duct equivalent diameter FAQ
What is a friction rate in duct design?
A friction rate is the pressure drop per 100 ft of duct that a Manual D design allows every run to spend, in inches of water column per 100 ft. It equals the available static pressure times 100 divided by the total effective length, and a workable value usually lands between about 0.06 and 0.18.
How do you size a duct with Manual D?
Calculate the friction rate from available static pressure and total effective length, then enter a friction chart or ductulator with each run's CFM and that friction rate to read the duct size and velocity. Size the trunk for full flow, reduce it as air drops off, and check the velocity stays in range.
What is available static pressure?
Available static pressure is the static left for the duct after the components take their share. Start from the equipment's rated external static off the blower table, subtract the filter, coil, registers, grilles, and accessory losses, and what remains is the available static pressure the trunks and branches are designed against, in inches of water column.
What is total effective length in Manual D?
Total effective length is the longest supply run plus the longest return run, with every fitting counted as its equivalent length of straight duct. It is the critical path the system is sized around, not the sum of all the duct, and fittings often outweigh the straight footage on a run.
Why is my return undersized?
Returns get value-engineered down to one grille while the supply gets the design attention, but the return has to pull back everything the supply pushes out. Size the return path for the full system CFM at the friction rate, the same as the supply trunk, and give closed-door rooms a transfer path back.