Return Duct Sizing Chart
Return ducts are sized by velocity, not by friction rate, so they come out one size larger than the supply duct carrying the same air.400 CFM needs a 12 inch return, 1000 CFM needs 18 inch.Every row below is held at or under 700 fpm, which is the point where a return in a hallway stops being something the homeowner notices.
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Why this chart is not the supply chart
On the supply side the thing you are managing is pressure drop, because you have a fixed amount of static to spend across the whole run. On the return side you have that too, but something else runs out first: noise.
A return grille is usually in a hallway or a living room, at head height, with nothing between it and the person sitting in the room. A supply register at the same velocity is quieter because it is smaller, further away, and the air has already lost energy through the boot and the takeoff. So the return gets a velocity ceiling, and on almost every row of this chart the velocity ceiling is what decides the size, not the friction rate.
The practical result: 10 inch supply and 12 inch return for 400 CFM, 14 inch supply and 18 inch return for 1000 CFM. If you have been running the same size both ways, the return is the one that has been undersized.
Return duct size by CFM
Assumptions:
- Velocity at or below 700 fpm, and friction at or below 0.1 in. w.g. per 100 ft, whichever is stricter
- Galvanized steel, absolute roughness 0.0003 ft
- Standard air, 70°F at sea level
- Rectangular sizes are Huebscher equivalents, never smaller than required, 4:1 aspect ratio or less
| CFM | Round | Velocity fpm | Friction in. w.g./100 ft | Rectangular | Low profile |
|---|---|---|---|---|---|
| 25 | 4" | 286 | 0.049 | 4 x 4 | — |
| 50 | 5" | 367 | 0.057 | 6 x 4 | 8 x 4 |
| 75 | 6" | 382 | 0.049 | 6 x 6 | 12 x 4 |
| 100 | 6" | 509 | 0.082 | 6 x 6 | 12 x 4 |
| 125 | 7" | 468 | 0.058 | 8 x 8 | 16 x 4 |
| 150 | 7" | 561 | 0.081 | 8 x 8 | 16 x 4 |
| 175 | 8" | 501 | 0.056 | 8 x 8 | 16 x 4 |
| 200 | 8" | 573 | 0.071 | 8 x 8 | 16 x 4 |
| 250 | 9" | 566 | 0.060 | 10 x 10 | 18 x 6 |
| 300 | 9" | 679 | 0.083 | 10 x 10 | 18 x 6 |
| 350 | 10" | 642 | 0.066 | 10 x 10 | 24 x 6 |
| 400 | 12" | 509 | 0.035 | 12 x 12 | 24 x 6 |
| 450 | 12" | 573 | 0.043 | 12 x 12 | 24 x 6 |
| 500 | 12" | 637 | 0.052 | 12 x 12 | 24 x 6 |
| 600 | 14" | 561 | 0.034 | 14 x 14 | 30 x 8 |
| 700 | 14" | 655 | 0.045 | 14 x 14 | 30 x 8 |
| 800 | 16" | 573 | 0.030 | 16 x 16 | 30 x 8 |
| 900 | 16" | 645 | 0.037 | 16 x 16 | 30 x 8 |
| 1000 | 18" | 566 | 0.026 | 18 x 18 | 30 x 10 |
| 1200 | 18" | 679 | 0.036 | 18 x 18 | 30 x 10 |
| 1400 | 20" | 642 | 0.028 | 20 x 20 | 30 x 12 |
| 1600 | 22" | 606 | 0.023 | 22 x 22 | 30 x 14 |
| 1800 | 22" | 682 | 0.028 | 22 x 22 | 30 x 14 |
| 2000 | 24" | 637 | 0.022 | 22 x 22 | 30 x 18 |
| 2500 | 26" | 678 | 0.023 | 24 x 24 | 30 x 20 |
| 3000 | 30" | 611 | 0.016 | 28 x 28 | 30 x 26 |
| 3500 | 32" | 627 | 0.015 | 30 x 30 | — |
| 4000 | 34" | 634 | 0.015 | — | — |
The velocity column is well under 0.1 in. w.g. per 100 ft on most rows. That is expected: the velocity limit is doing the work, and the friction headroom is a side effect you can spend on a longer run.
The part the chart cannot size for you
Getting the trunk right does not help if the air cannot get back to it. A bedroom with the door closed and no return path pressurizes, and the air leaves through whatever gaps the house has instead of going back to the furnace. Transfer grilles, jump ducts, or undercut doors are what fix that, and none of them show up in a sizing table.
Filter grilles are the other one. A return that is correctly sized as bare duct can still be strangled by a filter with too little face area behind it, and that shows up as a high static reading rather than as anything you can see.
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Return air duct questions
What size return duct do I need for 400 CFM?
A 12 inch round return, or 12 x 12 rectangular. That is one size larger than the 10 inch supply duct for the same 400 CFM, because the return is held to 700 fpm to keep it quiet rather than only to the friction rate.
Why is the return duct bigger than the supply duct?
Because the limit that governs is different. Supply ducts are sized by friction rate. Returns are sized by velocity, since a return grille sits in a hallway or a living room at head height with nothing between it and the person in the chair. At 700 fpm a return is quiet. At 900 fpm it is the thing the homeowner calls you about.
What velocity should return air duct be?
At or below 700 fpm for a residential return trunk, and lower still at the grille itself, where 500 fpm is a common target. This chart holds every row to 700 fpm, so the velocity column shows what you actually get, not the limit.
Can I use one big return instead of several small ones?
Sizing wise yes, and this chart gives you the size for the total CFM. In practice a single central return only works if the doors stay open or the rooms are relieved somehow. A bedroom with the door shut and no return path pressurizes, which pushes the air out through whatever gaps exist instead of back to the furnace.