Enter the airflow and choose how to size: by friction rate, like a duct calculator wheel, or by air velocity. The calculator gives the round duct size, a rectangular equivalent and the velocity and friction at that size.
Round duct size
Exact diameter
Velocity at this size
Friction rate at this size
Equivalent rectangular duct
How the duct is sized
Air loses pressure as it flows along a duct. The equal friction method picks one friction rate for the whole system and sizes every duct so that it loses that much pressure per 100 ft (or per meter). The calculator finds the round diameter from the Darcy-Weisbach equation, the same physics behind the duct calculator wheels used in HVAC design:
Δp / L = f · (1 ÷ D) · ρ · v² ÷ 2
De = 1.30 · (a · b)0.625 ÷ (a + b)0.25
f
friction factor for galvanized steel duct (roughness 0.09 mm)
ρ
standard air, 1.204 kg/m³ (0.075 lb/ft³)
v
air velocity
De
round diameter with the same friction as a rectangular duct a × b
The standard size picked is the smallest one whose friction or velocity stays within 10% of your design value.
Typical sizes at 0.08 in. w.g. per 100 ft
Airflow
Round duct
Velocity
100 CFM
6 in
about 510 fpm
200 CFM
8 in
about 570 fpm
400 CFM
10 in
about 730 fpm
800 CFM
13 in
about 870 fpm
1,200 CFM
16 in
about 860 fpm
Galvanized round duct, standard air. Check against your design manual (ACCA Manual D, ASHRAE Fundamentals) for the final layout.
Recommended velocities
In homes, supply trunks usually run at 700 to 900 fpm (3.5 to 4.5 m/s) and branches at 500 to 600 fpm. Commercial main ducts can run at 1,500 to 2,000 fpm (8 to 10 m/s) where noise allows. Faster air means smaller ducts but more noise and fan power.
Questions and answers
How do you size a duct?
With the equal friction method you pick a friction rate, typically 0.08 to 0.1 in. w.g. per 100 ft for homes, and find the round diameter that carries the airflow at that rate. The calculator does this with the Darcy-Weisbach equation for galvanized steel duct, the same physics a duct calculator wheel is based on.
What size duct for 400 CFM?
At 0.08 in. w.g. per 100 ft, about a 10 in round duct; at 0.1, the size is just under 10 in. A rule of thumb for homes is roughly 100 CFM per 6 in branch and 400 CFM per ton of cooling on the main trunk.
How do I convert round duct to rectangular?
Use the equivalent diameter De = 1.30 × (a × b)^0.625 ÷ (a + b)^0.25 (ASHRAE). A rectangular duct with the same De has the same friction loss at the same airflow, but a lower velocity. Keep the aspect ratio below about 4:1.
Does this apply to flexible duct?
Flex duct has much higher friction than sheet metal, especially when compressed or sagging. Size flex one step larger or use the manufacturer's friction chart.