Duct Sizing Chart
At the standard 0.1 in. w.g. per 100 ft,a 10 inch round duct carries 400 CFM and a 14 inch carries 1000 CFM. But the right chart depends on which side of the system you are on: returns are sized by velocity and come out larger, and flex is rougher than sheet metal and comes out larger again. The quick table is below and the three detailed charts are linked from it.
Pick the chart you actually need
- CFM duct sizing chart for supply duct in galvanized sheet metal. This is the default chart and the one most people mean.
- Return duct sizing chartfor return air. Held to 700 fpm, because a return grille sits at head height in a hallway and noise runs out before pressure does. Comes out one size larger than supply.
- Flexible duct sizing chartfor flex. Rougher than metal, so one to two sizes larger for the same air, and only valid if the flex is pulled tight.
- Printable PDFs of all three, one page each, no email required.
Quick reference
Common airflows for supply duct in sheet metal at 0.1 in. w.g. per 100 ft. The full chart has every step from 25 to 4000 CFM.
| CFM | Round | Velocity fpm | Friction in. w.g./100 ft | Rectangular | Low profile |
|---|---|---|---|---|---|
| 100 | 6" | 509 | 0.082 | 6 x 6 | 12 x 4 |
| 200 | 8" | 573 | 0.071 | 8 x 8 | 16 x 4 |
| 300 | 9" | 679 | 0.083 | 10 x 10 | 18 x 6 |
| 400 | 10" | 733 | 0.084 | 10 x 10 | 24 x 6 |
| 600 | 12" | 764 | 0.073 | 12 x 12 | 24 x 6 |
| 800 | 14" | 748 | 0.058 | 14 x 14 | 30 x 8 |
| 1000 | 14" | 935 | 0.087 | 14 x 14 | 30 x 8 |
| 1200 | 16" | 859 | 0.063 | 16 x 16 | 30 x 8 |
| 1600 | 18" | 905 | 0.060 | 18 x 18 | 30 x 10 |
| 2000 | 18" | 1132 | 0.091 | 18 x 18 | 30 x 10 |
Supply, return, and flex are three different answers
The same airflow gives three different duct sizes depending on what you are running:
| CFM | Supply sheet metal | Return sheet metal | Supply flex |
|---|---|---|---|
| 200 | 8" | 8" | 8" |
| 400 | 10" | 12" | 12" |
| 600 | 12" | 14" | 14" |
| 1000 | 14" | 18" | 16" |
| 1600 | 18" | 22" | 18" |
If you have been using one chart for all three, the return side is the one that has most likely been undersized, and undersized returns are what most of the noise complaints in this trade turn out to be.
What a chart does and does not do
A sizing chart answers one question: given this airflow and this friction rate, how big is the pipe. That is the job a Ductulator does in your hand, and for a branch run or a replacement it is usually all you need.
What it does not do is work out the friction rate in the first place. That comes from the static pressure the equipment has available divided by the total effective length of the worst run, including every fitting, and that calculation is what Manual D is. Using 0.1 as a default is the everyday convention and it is usually close enough on a normal house, but on a long or complicated system it can be well off.
These charts are free and always will be. If you also need to turn the job into a signable quote and an invoice, that is the paid tool this site pays for itself with. See what it costs.
Duct sizing questions
Which duct sizing chart do I need?
Supply duct in sheet metal is the standard chart. Returns need their own chart because they are sized by velocity rather than friction rate, so they come out larger. Flex needs its own because it is rougher than metal and needs a bigger size for the same air. Links to all three are on this page.
What size duct for 1000 CFM?
A 14 inch round supply duct at 0.1 in. w.g. per 100 ft. The return for that same 1000 CFM needs to be 18 inch, and in flex you would need 16 inch.
Is a duct sizing chart the same as Manual D?
No. A chart sizes one duct run at one friction rate, which is the job a Ductulator does. Manual D works out the friction rate itself from the available static pressure and the total effective length of the system, room by room, including every fitting. The chart is the everyday tool; Manual D is the design.
Why do duct charts disagree with each other?
Almost always because they assume different things and do not say so: a different friction rate, a different duct roughness, or rectangular sizes converted by equal area instead of equal friction. The assumptions for these charts are printed above each table and on the face of each PDF.