HVAC
HVAC Duct Size Calculator
Size a duct from the air it has to carry and the velocity you want to hold it to.
Result
Velocity sizing only. It does not account for friction loss along the run, fittings, or the total static pressure the blower can work against, all of which a proper duct design (Manual D) covers. Equivalent rectangular ducts carry slightly less than the same area in round.
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About this calculator
Why does velocity matter?
Noise and resistance. Air moving too fast whistles at the registers and costs the blower more static pressure; too slow and the duct is needlessly large and expensive.
Why lower velocity in bedrooms?
Because you notice the sound at night. Dropping to around 500 FPM in branches serving bedrooms is a cheap way to avoid a complaint that is expensive to fix later.
Is flexible duct the same?
No. Flex has considerably more resistance than smooth metal of the same diameter, and far more again if it is not pulled tight. Sizing flex as though it were metal is a common cause of poor air flow.
The formula
duct area = CFM / velocity; round diameter = sqrt(4 x area / pi)
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