Duct Size Calculator
Size a round duct by friction rate or velocity, pick the standard size, and find rectangular ducts with the same capacity.
What a ductulator does
A duct has to carry its airflow without too much resistance or noise. HVAC designers size ducts with a ductulator, a slide rule that links airflow, duct size, friction loss and velocity. The calculation behind it is the ASHRAE friction equation for smooth galvanised round duct: friction in inches of water column per 100 ft = 0.109136 × CFM^1.9 ÷ diameter^5.02. Because diameter is raised to the fifth power, a small increase in size makes a large difference: going from an 8 in. to a 10 in. duct cuts friction to about a third for the same airflow.
Friction rate and velocity
There are two common ways to choose a size. The equal-friction method picks a friction rate, typically 0.06 to 0.10 in. w.c. per 100 ft in houses, based on the pressure the blower can provide and the total effective length of the duct run, including the equivalent length of every elbow and fitting. A 400 CFM trunk at 0.08 needs a calculated 10.3 in. diameter; a standard 10 in. duct then runs at 733 ft/min with a friction of 0.092, within 15% of the target. The velocity method instead caps air speed to control noise: about 600 to 900 ft/min for residential supply branches and 700 to 1,000 for trunks. At 700 ft/min, 400 CFM needs 10.2 in. A 1,200 CFM main at 0.08 needs 15.6 in., so a 16 in. duct, running at 859 ft/min.
Rectangular ducts
Where a round duct will not fit between joists, a rectangular duct of equivalent capacity is used. Equivalent does not mean equal area: a rectangle has more wall surface per unit of area, so it needs to be larger. The Huebscher equation gives the equivalent diameter: De = 1.30 × (ab)^0.625 ÷ (a + b)^0.25. A 10 in. round duct is equivalent to about 11 × 8 in., 15 × 6 in. or 9 × 10 in. Very flat ducts, with an aspect ratio over about 4:1, become inefficient and noisy, so it is better to go deeper than wider where space allows.
Flexible duct and common mistakes
The friction equation applies to smooth metal duct. Flexible duct has a corrugated inner liner and much higher friction, and if it is compressed, kinked or allowed to sag between supports, friction can double or triple. A common installation fault is a long, sagging flex run to the furthest room, which then never gets enough air. Designers often size flex at least one size larger than metal and keep runs short, straight and fully stretched. Another frequent mistake is sizing each branch for its own airflow without checking the total effective length of the longest run, which sets the friction rate the whole system can afford.
How to use it
- Enter the airflow for the duct section in CFM.
- Choose whether to size by friction rate or by velocity, and enter the target.
- Read the calculated diameter and the smallest standard round size within 15% of the target.
- Use the rectangular equivalents where a round duct will not fit.
Questions from the job site
How do I size a duct for a given CFM?
Choose a design friction rate, often 0.08 in. w.c. per 100 ft for houses, and solve the ASHRAE friction equation for diameter, or choose a maximum velocity and divide airflow by velocity to get the area. 400 CFM at 0.08 needs about a 10 in. round duct.
What friction rate should I use for residential ducts?
Typically 0.06 to 0.10 in. w.c. per 100 ft. The right value depends on the static pressure the blower can supply after the coil, filter and registers, divided over the total effective length of the longest duct run including fittings, as in ACCA Manual D.
How do I convert a round duct to a rectangular one?
Use an equivalent diameter, not equal area. The Huebscher equation, De = 1.30 × (ab)^0.625 ÷ (a + b)^0.25, gives the round size with the same friction. A 10 in. round duct matches roughly an 11 × 8 in. or 15 × 6 in. rectangle.
Does flexible duct need to be larger than metal duct?
Usually yes. Flex duct has higher friction than smooth metal, and much higher if it is compressed, kinked or sagging. Many designers size flex one standard size up and insist on short, straight, fully extended runs supported at regular intervals.
Disclaimer: HVAC results are estimates from simplified methods and the values you enter. They are not a Manual J, S or D design and must not replace one for equipment selection or permits. Gas, electrical and refrigerant work must be carried out by licensed technicians to local codes, and combustion appliances need working carbon monoxide alarms.