CFM to Duct Size Chart: What Size Duct Do I Need for 400 CFM?
Duct sizing · CFM chart · Updated 2026-07-22
400 CFM needs a 9.7 in. round duct - install the 10 in. stock size - or a rectangular equivalent of 14x6 or 10x8, at a design friction rate of 0.10 in. wg per 100 ft in galvanized sheet metal. That works out to about 785 FPM at the design diameter, which sits in the normal residential velocity band.
Every number below is computed at that same stated friction rate, using the Darcy-Weisbach equation with a Colebrook friction factor and the ASHRAE absolute roughness for galvanized duct, 0.0003 ft - the value the ASHRAE round-duct friction chart is drawn at. That matters more than it sounds: a duct size is meaningless without the friction rate it was computed at, and almost none of the charts floating around the trade publish theirs.
CFM to duct size chart (0.10 in. wg per 100 ft)
| CFM | Round dia. required (in.) | Nearest stock round | Rectangular equivalents | Velocity (FPM) | Velocity band |
|---|---|---|---|---|---|
| 50 | 4.5 | 5 in. | 8x4, 6x6 | 460 | Quiet |
| 75 | 5.2 | 6 in. | 8x4, 8x6 | 510 | Quiet |
| 100 | 5.8 | 6 in. | 10x4, 8x6 | 550 | Quiet |
| 150 | 6.7 | 7 in. | 10x4, 8x6 | 610 | Residential |
| 200 | 7.5 | 8 in. | 14x4, 8x6 | 660 | Residential |
| 250 | 8.1 | 9 in. | 16x4, 10x6 | 700 | Residential |
| 300 | 8.7 | 9 in. | 12x6, 10x8 | 730 | Residential |
| 400 | 9.7 | 10 in. | 14x6, 10x8 | 785 | Residential |
| 500 | 10.5 | 12 in. | 18x6, 12x8 | 830 | Residential |
| 600 | 11.3 | 12 in. | 20x6, 14x8 | 870 | Residential |
| 700 | 11.9 | 12 in. | 22x6, 12x10 | 900 | Transitional |
| 800 | 12.5 | 14 in. | 18x8, 14x10 | 935 | Transitional |
| 1,000 | 13.6 | 14 in. | 22x8, 16x10 | 985 | Transitional |
| 1,200 | 14.6 | 16 in. | 24x8, 20x10 | 1,030 | Transitional |
| 1,400 | 15.5 | 16 in. | 28x8, 22x10 | 1,070 | Transitional |
| 1,600 | 16.3 | 18 in. | 24x10, 20x12 | 1,110 | Transitional |
| 1,800 | 17.0 | 18 in. | 26x10, 22x12 | 1,140 | Transitional |
| 2,000 | 17.7 | 18 in. | 28x10, 24x12 | 1,170 | Transitional |
Rectangular equivalents are matched on equivalent diameter, not on cross-sectional area - a rectangle always needs more area than a round duct to move the same air, because it has more wetted perimeter per unit of area. Below roughly 150 CFM the listed rectangles are larger than strictly required, simply because 4 in. is about as shallow as anyone fabricates. See rectangular to round duct conversion for the Huebscher equation behind that column.
Why three charts give you three different answers
Pick up three supplier charts and a 6 in. metal duct will be rated at 93 CFM, 100 CFM, and 115 CFM. None is wrong; they were computed at different friction rates. Duct size is not a property of CFM alone - it answers "what diameter loses exactly this much pressure per 100 ft at this airflow."
| Friction rate (in. wg / 100 ft) | Round dia. required (in.) | Velocity (FPM) | Typical use |
|---|---|---|---|
| 0.06 | 10.7 | 640 | Long runs, quiet jobs, high total effective length |
| 0.08 | 10.1 | 715 | Conservative residential default |
| 0.10 | 9.7 | 785 | Standard residential equal-friction default |
| 0.14 | 9.0 | 900 | Short, compact systems with high available static |
| 0.18 | 8.6 | 1,000 | Light commercial, tight chases |
That is a spread of more than two inches on the same 400 CFM - 10.7 in. required at one end of the band, 8.6 in. at the other, with a 360 FPM difference in velocity. This is why ACCA Manual D does not hand you a fixed chart: it has you calculate the friction rate first, from the blower's available static pressure divided by the total effective length of the longest run, and only then size the duct. 0.10 is the residential default the equal-friction method is built around, not a law.
Worked example: a 400 CFM bedroom branch
- Start from the room load, not the register: Manual J says the room needs 400 CFM. That is the number the duct has to carry, before any fitting losses.
- Confirm the friction rate: Blower external static 0.50 in. wg, minus 0.20 for coil and filter, leaves 0.30 available. Longest run total effective length is 300 ft. Friction rate = 0.30 / 300 x 100 = 0.10 in. wg per 100 ft.
- Read the chart: 400 CFM at 0.10 gives 9.7 in. required. Round up to the 10 in. stock size - never down. Rounding down to 9 in. pushes the friction rate to roughly 0.14.
- Check the velocity: 785 FPM at the design diameter, 733 FPM in the installed 10 in. Comfortably inside the residential band, so no whistle at the register.
- Convert if the joist bay is shallow: 10 in. round will not fit an 8 in. joist bay. 14x6 has an equivalent diameter of 9.8 in. and clears it. 10x8 works if the framing allows depth.
Velocity is the second constraint
Friction rate tells you whether the air will arrive. Velocity tells you whether anyone can sleep in the room it arrives in, which is why the chart carries a velocity column. As a working rule, keep residential supply branches under about 700 FPM, trunks under about 900 FPM, and returns quieter still. Recommended maxima vary by source and by whether the run is lined, so treat the bands as guidance rather than code - the duct velocity chart breaks them out by supply, return, and branch.
Eight legal sizes for one CFM, not one. Type 400 CFM and a friction rate limit into Ductulator and the Valid Sizes grid returns eight duct sizes that satisfy it - round, rectangular, and flex - each annotated with its own velocity and friction, the optimal pick highlighted in steel blue, and any over-velocity or over-friction option flagged in safety red. Equivalent diameter runs the Huebscher equation from ASHRAE Fundamentals Ch. 21 Eq. 1B; friction runs full Darcy-Weisbach with Colebrook, citation printed under the number. Learn more about Ductulator.
From the chart to a sized system
A chart row is a starting point. The real job is total effective length, which is why fitting equivalent lengths usually dominate the straight duct in a residential system. Ductulator carries that through: the eight valid sizes with velocity and friction on each, a SMACNA Table 1-4 gauge and reinforcement recommendation for the size you choose, and the five-band velocity classifier. Chain the segments into the multi-segment static pressure budget - blower, trunk, elbow, transition, damper, branch, diffuser - and cumulative pressure drop renders against your available static, green while you have headroom and red when you do not. Save the job with photos and export an inspector-ready PDF with the SMACNA and ASHRAE citations on every page.
Frequently asked questions
What size duct do I need for 400 CFM?
9.7 in. round at 0.10 in. wg per 100 ft, so you install the 10 in. stock size. Rectangular equivalents are 14x6 or 10x8. At a lower design friction rate of 0.06 the same 400 CFM needs 10.7 in.; at 0.18 it needs only 8.6 in.
What size duct for 1200 CFM?
14.6 in. round at 0.10 in. wg per 100 ft - install 16 in. round, or 15 in. where it is stocked - or 24x8 or 20x10 rectangular. Velocity lands near 1,030 FPM, which is fine for a trunk but too fast for a branch feeding a bedroom.
How many CFM can a 6 inch duct handle?
About 110 CFM in galvanized metal at 0.10 in. wg per 100 ft, roughly 560 FPM. Published figures range from 93 to 115 CFM because different charts assume different friction rates and different roughness values. Flex duct of the same nominal diameter carries meaningfully less, about 94 CFM fully extended and less still if it sags.
What friction rate should I use for residential duct sizing?
0.10 in. wg per 100 ft is the common default, but ACCA Manual D has you calculate it: available static pressure divided by the total effective length of the longest run, times 100. Systems with high-MERV filters or long runs often land nearer 0.06, and every duct gets larger as a result.