Duct Size Calculator
Sizes ductwork from airflow. Enter the CFM and the velocity you are designing to, and it gives the round diameter plus the rectangular ducts that carry the same air at the same pressure loss.
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The formula
area = airflow ÷ velocity diameter = √(4 × area ÷ π) equivalent round diameter of a × b = 1.30 × (ab)^0.625 ÷ (a+b)^0.25
Airflow divided by velocity gives the area the duct must present, and the rest is the geometry of a circle. The third expression is ASHRAE’s: a rectangular duct has more wall rubbing against the same volume of air than a round one, so it has to be larger in area to carry the same flow at the same pressure loss.
What to watch out for
Duct sizing is a trade between three things:
- Velocity sets noise. This is the main constraint in a house. Above roughly 900 ft/min in a trunk, or 700 in a branch, occupants start to hear the air. Commercial buildings accept 1,000–1,500 because the plant room is further away and there is more background noise.
- Round beats rectangular. A round duct has the least wall per unit of area, so it loses the least pressure. Rectangular ducts exist because they fit between joists, not because they perform better — and the flatter the duct, the worse the penalty.
- This sizes one duct, not a system. A full design also accounts for the pressure lost in every bend, takeoff and register, and balances the branches against each other. Use this to size a run, not to design a house.
Frequently Asked Questions
What size duct do I need for 400 CFM?
At 800 ft/min, 400 CFM needs about 9.6 in of round duct, so a 10 in size. As a rectangular duct that is roughly 8 × 10 in or 6 × 14 in — both carry the same air at the same pressure loss.
What is a good air velocity for residential ductwork?
700–900 ft/min for main trunks, 600–700 for branches and 500–700 for returns. Those limits come from noise rather than performance — the duct will move the air faster quite happily, you just have to listen to it.
Is round or rectangular duct better?
Round, for the same cross-sectional area. It has less surface rubbing against the air, so less friction and less pressure loss. Rectangular duct is used where depth is limited, and the flatter it gets the more it costs you in pressure.