As an Amazon Associate I earn from qualifying purchases.
R-49 is the common target for the roof or ceiling in cold and mixed climates. That is about 20 inches of blown fibreglass, 15 inches of blown cellulose, 16 inches of fibreglass batt or about 8 inches of closed-cell spray foam. Air seal the ceiling plane before adding any of it.
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.
With these inputs:
| Result | Value |
|---|---|
| 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 |
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.
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
| 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.
| 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.
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.
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).
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.
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.
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.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.