Duct Size Calculator: Round and Rectangular Ducts by CFM
Enter the airflow the duct must carry and the friction rate you are designing to. The calculator solves the duct friction equation for the round diameter, rounds up to the next standard size, and gives the air velocity and pressure loss at that size plus rectangular ducts with the same friction.
Your project
From your Manual D friction-rate worksheet; 0.08 is an example.
Equivalent length, fittings included, for the pressure drop of the run.
Your results
- Round duct (next standard size)
- 7 in
- Exact diameter
- 6.08 in
- Velocity in a 7 in duct
- 374 ft/min
- Friction in a 7 in duct
- 0.039 in. w.g./100 ft
- Rectangular, 6 in deep
- 6 × 6 in
- Rectangular, 8 in deep
- 4 × 8 in
- Rectangular, 10 in deep
- 4 × 10 in
Estimates use typical US material and labor pricing. Confirm quantities against the coverage chart on the bag before you buy.
Gear for this job
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Frost King CF55 Foil Backed "No Itch" Natural Cotton Duct Wrap, 12-Inch x 1-Inch x 15-Feet
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How this calculator works
- Friction (in. w.g./100 ft) = 0.109136 × CFM^1.9 ÷ D^5.02
- Diameter D = (0.109136 × CFM^1.9 ÷ friction rate)^(1 ÷ 5.02)
- Velocity (ft/min) = CFM ÷ (π × (D ÷ 12)² ÷ 4)
- Rectangular a × b ≈ round D when 1.30 × (a b)^0.625 ÷ (a + b)^0.25 = D
Round duct size by airflow (0.08 in. w.g./100 ft example)
| Airflow | Exact diameter | Standard size | Velocity |
|---|---|---|---|
| 50 CFM | 4.68 in | 5 in | 367 ft/min |
| 80 CFM | 5.59 in | 6 in | 407 ft/min |
| 100 CFM | 6.08 in | 7 in | 374 ft/min |
| 150 CFM | 7.09 in | 8 in | 430 ft/min |
| 200 CFM | 7.9 in | 8 in | 573 ft/min |
| 300 CFM | 9.21 in | 10 in | 550 ft/min |
| 400 CFM | 10.27 in | 12 in | 509 ft/min |
| 600 CFM | 11.98 in | 12 in | 764 ft/min |
| 800 CFM | 13.36 in | 14 in | 748 ft/min |
| 1,000 CFM | 14.53 in | 16 in | 716 ft/min |
| 1,200 CFM | 15.57 in | 16 in | 859 ft/min |
Galvanized round duct; flex duct has more friction and usually goes up a size. Use your own friction rate in the calculator.
How much air a round duct carries at a given velocity
| Duct | At 500 ft/min | At 700 ft/min | At 900 ft/min |
|---|---|---|---|
| 6 in | 98 CFM | 137 CFM | 177 CFM |
| 8 in | 175 CFM | 244 CFM | 314 CFM |
| 10 in | 273 CFM | 382 CFM | 491 CFM |
| 12 in | 393 CFM | 550 CFM | 707 CFM |
| 14 in | 535 CFM | 748 CFM | 962 CFM |
CFM = velocity × area. The velocities are reference points, not limits.
Worked example: 100 CFM at 0.08
D = (0.109136 × 100^1.9 ÷ 0.08)^(1/5.02) = 6.08 in, so the next standard round size is 7 in. In a 7 in duct 100 CFM moves at 374 ft/min with a friction loss of about 0.04 in. w.g. per 100 ft. A 6 in duct would run slightly over the design rate (0.085).
For a rectangular duct, Huebscher's equivalent diameter does the conversion: 6 × 6 in works out to 6.56 in, enough for the 6.08 in requirement, and 8 × 6 in to 7.55 in, a match for the 7 in round. The calculator lists the narrowest width at 6, 8 and 10 in deep.
Where the friction rate comes from
In ACCA Manual D the friction rate isn't picked from a chart: it is the static pressure the blower has left after the coil, filter and registers, divided by the longest supply-plus-return run (fittings counted as equivalent length), times 100. A long, fitting-heavy system has a lower friction rate and needs bigger ducts.
Whatever the size, ducts outside the insulated envelope lose heat through their walls. Seal the joints with mastic or foil tape and wrap them — the duct insulation calculator works out the wrap.
Keep going
- How much air does the room need first? CFM calculator
- Insulate ducts that run through an attic or crawl space: duct insulation calculator
- Room cooling load in BTU per hour: BTU calculator
Frequently asked questions
How do I size a duct for CFM?
Pick a friction rate and solve the duct friction equation for the diameter: D = (0.109136 × CFM^1.9 ÷ friction)^(1/5.02). At 0.08 in. w.g. per 100 ft, 100 CFM needs 6.08 in — round up to a 7 in duct — and 400 CFM needs about 10.3 in, so 12 in.
How many CFM can a 6 inch duct carry?
It depends on the friction or velocity you accept. At 0.08 in. w.g. per 100 ft a 6 in round duct carries about 97 CFM; at 700 ft/min it moves about 137 CFM (700 × 0.196 sq ft).
How do I convert a round duct to rectangular?
Use the equivalent diameter, de = 1.30 × (a b)^0.625 ÷ (a + b)^0.25. An 8 × 6 in rectangular duct has the same friction as a 7.55 in round duct, so it can replace a 7 in round. Square-ish shapes are more efficient than long flat ones.
What is a typical friction rate for home ductwork?
It comes from the system, not a standard value: Manual D divides the blower's available static pressure by the total effective length of the longest run. This page uses 0.08 in. w.g. per 100 ft as an example; enter your own figure.
How do I calculate air velocity in a duct?
Divide the CFM by the duct's cross-section in square feet. A 7 in duct is π × (7/12)² ÷ 4 = 0.267 sq ft, so 100 CFM moves at 374 ft/min.
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Round duct (next standard size)
7 in