What the result means
The calculated diameter is an estimated round-duct size for the airflow and friction rate you enter. Because duct comes in standard sizes, we also show the next size up and the velocity you will actually get in it.
Rectangular duct is sized so its friction matches the standard round size. Enter one side and the calculator solves the other. Round the second side up to the nearest whole inch when ordering.
How it's calculated
- Friction (in. w.c./100 ft) = 0.109136 × Q^1.9 ÷ D^5.02
- D = (0.109136 × Q^1.9 ÷ friction)^(1 ÷ 5.02)
- Velocity (FPM) = Q ÷ (π × (D ÷ 12)² ÷ 4)
- Equivalent round De = 1.30 × (a × b)^0.625 ÷ (a + b)^0.25
Q in CFM, D in inches. Round galvanized steel at standard air (0.075 lb/ft³).
Standard round sizes: 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 20, 22, 24 in.
Worked example
A supply trunk carrying 400 CFM designed at 0.08 in. w.c. per 100 ft.
- D = (0.109136 × 400^1.9 ÷ 0.08)^(1 ÷ 5.02) = 10.27 in
- Next standard size = 12 in
- Velocity in 12 in round ≈ 509 FPM
Use 12 in round, or an 8 in tall rectangular duct about 16 in wide.
Reference table
The values below are computed from the same calculator functions used above. They let you compare common sizes and operating points without replacing the interactive calculation or its assumptions.
| Diameter | 0.06 in. w.c./100 ft | 0.08 in. w.c./100 ft | 0.10 in. w.c./100 ft |
|---|---|---|---|
| 4 in | 28 CFM · 326 FPM | 33 CFM · 379 FPM | 37 CFM · 426 FPM |
| 5 in | 51 CFM · 376 FPM | 60 CFM · 438 FPM | 67 CFM · 492 FPM |
| 6 in | 83 CFM · 423 FPM | 97 CFM · 492 FPM | 109 CFM · 553 FPM |
| 7 in | 125 CFM · 467 FPM | 145 CFM · 543 FPM | 163 CFM · 611 FPM |
| 8 in | 178 CFM · 509 FPM | 207 CFM · 592 FPM | 232 CFM · 666 FPM |
| 9 in | 242 CFM · 549 FPM | 282 CFM · 638 FPM | 317 CFM · 718 FPM |
| 10 in | 320 CFM · 587 FPM | 372 CFM · 683 FPM | 419 CFM · 768 FPM |
| 12 in | 518 CFM · 660 FPM | 603 CFM · 768 FPM | 678 CFM · 863 FPM |
| 14 in | 779 CFM · 729 FPM | 906 CFM · 848 FPM | 1019 CFM · 953 FPM |
| 16 in | 1108 CFM · 794 FPM | 1290 CFM · 924 FPM | 1450 CFM · 1039 FPM |
| 18 in | 1513 CFM · 856 FPM | 1760 CFM · 996 FPM | 1980 CFM · 1120 FPM |
| 20 in | 1999 CFM · 916 FPM | 2325 CFM · 1066 FPM | 2615 CFM · 1199 FPM |
| 22 in | 2571 CFM · 974 FPM | 2991 CFM · 1133 FPM | 3364 CFM · 1274 FPM |
| 24 in | 3235 CFM · 1030 FPM | 3764 CFM · 1198 FPM | 4233 CFM · 1348 FPM |
Friction rates and velocities are calculated using the duct sizing functions in this calculator.
Sources and assumptions
Q in CFM, D in inches. Round galvanized steel at standard air (0.075 lb/ft³). Standard round sizes: 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 20, 22, 24 in. Calculator formulas are published with their units so a technician can check a result instead of treating it as a black box. Where a code table, manufacturer specification or field measurement is required, use the applicable local source.
Last reviewed . Spot an error? Email support@tiermenu.com.
FAQ
What friction rate should I use?
Many residential designs use 0.06 to 0.10 in. w.c. per 100 ft, but the right number comes from the available static pressure and total effective length, as in ACCA Manual D.
Does this work for flex duct?
No. Flex has much higher friction than galvanized, especially when compressed or sagging. Use the manufacturer's flex friction chart.
Why jump from 10 to 12 in?
Those are the standard sizes in this list. Round up so the duct is never smaller than required.
How close is the rectangular size?
It is an approximate equivalent based on friction, not velocity. The rectangular duct is modeled to carry the same air at the same friction as the round duct, but its velocity will differ slightly.
Can I use this for return ducts?
Yes. The math is the same. Many designers choose a lower friction rate for returns to keep them quiet.
Using a reference result in the field
A calculator is most useful when the inputs match the job being checked. Confirm whether a value is nominal or measured, and whether readings were taken at design or actual operating conditions. The unit label beside the result is part of the result; do not carry a number into another calculation without checking its units.
Reference rows are provided for comparison, not as universal design limits. They use the values and conditions stated with each table. Elevation, air temperature, equipment and installation details can change a field result. Where the page calls out a code check, equipment rating or manufacturer table, that check remains part of the job.
Use the interactive inputs for the exact conditions at hand, then review the displayed formula and assumptions. If a value looks unexpected, change one input at a time and compare it with a known point in the reference table. Keep the measured readings and job-specific notes with the work record so another technician can understand how the result was obtained.
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