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Duct Size Calculator

A duct size calculator gives the round duct diameter (and a rectangular equivalent) needed to carry a given airflow at a sensible air velocity. Get it wrong and the system is either noisy and starved or oversized and sluggish.

Your measurements

Result

Enter the CFM the run carries and pick a target velocity, and it sizes the duct. Lower velocities are quieter but need bigger ducts — the calculator shows the trade-off so you can round to the next standard size.

How it is calculated

Duct cross-sectional area = CFM ÷ velocity (in feet per minute). Diameter = √(4 × area ÷ π). Residential supply runs are typically sized around 700-900 fpm (lower for returns) to balance noise and friction. This velocity method is a quick guide; a full design uses Manual D with total friction loss.

Steps

Enter the airflow (CFM) the duct must carry.
Pick a target velocity — lower for quiet, higher for trunks.
Read the round duct diameter needed.
Round up to the next standard duct size.
Use the rectangular equivalent if you are running rectangular duct.

Worked example

400 CFM at 800 fpm needs about 72 in² of duct — roughly a 10 in round duct (or about 12 × 6 in rectangular).

Common questions

What size duct for 400 CFM?

About a 10 in round duct at a typical 800 fpm supply velocity. The calculator sizes any airflow.

How do I size HVAC ductwork?

Divide CFM by the target velocity for the area, then convert to a diameter. Round up to a standard size and keep runs short.

Round or rectangular duct?

Round moves air with less friction for the same area; rectangular fits tight spaces. The calculator gives both.

Why does velocity matter?

Too fast is noisy and high-friction; too slow needs oversized ducts. 700-900 fpm is a good residential supply range.