Round duct size chart (CFM to round duct)
Round galvanized duct sized at the design friction rate above. Velocity is shown so you can spot runs that would be noisy — branches are usually kept near 700–900 fpm.
| CFM | m³/h | Round dia. (in) | Dia. (mm) | Velocity (fpm) | Velocity (m/s) |
|---|
Rectangular duct size chart
Equivalent rectangular sizes at the chosen aspect ratio, matched to the round duct by Huebscher equivalent diameter (same airflow at the same friction, not the same area).
| CFM | Equiv. round (in) | Rectangular (in) | Velocity (fpm) |
|---|
Flexible (flex) duct size chart
Flex duct sized at the same friction rate. Because the bore is rougher, flex is typically a size larger than metal for the same CFM — and this assumes the run is pulled fully taut.
| CFM | Flex dia. (in) | Dia. (mm) | Velocity (fpm) | vs metal |
|---|
Return-air duct size chart
Returns are sized for a lower velocity so they stay quiet, which makes them larger than a supply for the same CFM. This table targets a reduced velocity rather than the supply friction rate.
| CFM | Round dia. (in) | Rectangular (in) | Velocity (fpm) |
|---|
Metal duct gauge & dimensions reference
Typical galvanized sheet-metal gauges used for residential and light-commercial duct. Heavier (lower-number) gauge is used as duct size and pressure increase; check your local code and the relevant sheet-metal trade standards for the exact schedule.
| Gauge | Steel thickness (in) | (mm) | Typical use |
|---|---|---|---|
| 30 ga | 0.0157 | 0.40 | Small round branch, light flex collars |
| 28 ga | 0.0187 | 0.47 | Round branch up to ~12 in, small rectangular |
| 26 ga | 0.0217 | 0.55 | Common residential trunk & branch |
| 24 ga | 0.0276 | 0.70 | Larger trunks, wider rectangular sides |
| 22 ga | 0.0336 | 0.85 | Large commercial trunk duct |
How these charts are computed
Every size in the tables comes from the same public physics as the duct size calculator:
Because nothing is copied from a fixed published table, you can move the friction rate and aspect ratio and the whole chart re-solves. That also means it is an honest computed reference: it gives a flex, round or rectangular duct chart answer from first principles and states the method, rather than reproducing any copyrighted table.
Duct chart FAQ
What friction rate is this duct chart based on?
The round, rectangular and flex tables default to an equal-friction rate of 0.10 in. w.g. per 100 ft, a common residential supply design value; the return-air table targets a lower velocity. Change the friction rate at the top and every table recomputes. Sizes are calculated from Darcy–Weisbach with a Swamee–Jain friction factor and standard air, not copied from any copyrighted chart.
How big a duct for 600 CFM?
At 0.10 in. w.g. per 100 ft in galvanized steel, 600 CFM lands around a 12 in round duct or an equivalent rectangular size such as 14 × 8 in, with a velocity near 800 fpm. Flex would be a size larger. Read the exact figure from the round and rectangular charts above, or use the duct size calculator for a custom CFM and friction rate.
What size return air duct do I need?
Return ducts are sized for a lower velocity than supplies to stay quiet, so for the same CFM a return is a size or two larger. The return-air chart above sizes returns at a reduced velocity target. As a whole-house rule of thumb, total return free area should comfortably handle the system airflow at roughly 400 CFM per ton, split across enough return grilles.
Does flex duct use the same chart as metal?
No. Flexible duct has a rougher bore, so at the same CFM and friction it must be larger than smooth galvanized steel — and only if the flex is pulled fully taut. The flex chart above applies a roughness correction; compressed or sagging flex performs far worse than any chart shows.
Is this duct sizing chart based on standard reference tables?
No. It is not a copy of any published design table. Every size here is computed live from public fluid-flow physics (the Darcy–Weisbach equation with a Swamee–Jain friction factor, air at 0.075 lb/ft³), so you can change the friction rate and watch the chart update. For a stamped design, confirm against the relevant published standards and a professional.
Is the chart free to use and printable?
Yes. It's free, needs no signup or download, and runs entirely in your browser, so you can print or screenshot it offline. The tables are real selectable HTML, not images, so they stay sharp and you can copy values directly.