Field calculator
Duct airflow (CFM and velocity) calculator
Pick the duct shape and enter the size: a round diameter, or a rectangular width and height in inches. Then enter either the airflow in CFM or the velocity in feet per minute, and leave the other blank to solve it. The calculator works the continuity relationship Q = V x A, where Q is CFM, V is the velocity, and A is the duct free area in square feet. Use it to size a duct for a target velocity, or to check the velocity a given duct carries at a given CFM. Comfort branch ducts commonly run around 700 to 1200 fpm and trunks higher, but confirm the airflow and the velocity and noise limits against the design and the balancing report.
Result
Duct airflow uses continuity: Q = V × A, where Q is the airflow in CFM, V is the velocity in feet per minute, and A is the duct free area in square feet. Enter the duct size and either the CFM or the velocity, and the rest is solved. Round area = pi × (d/2)squared / 144; rectangular area = (w × h) / 144. Verify against the design airflow and the velocity and noise limits.
anvilfield.com/calculators/hvac-duct-airflow-calculator · Free field calculators and FieldOS. A planning estimate, verify against the code, the manufacturer, and the engineer of record.
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Duct airflow 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.