How it works
The equal-friction method picks one friction rate (inches of water per 100 feet) for the whole system, then sizes each duct so it carries its airflow at that rate. That balances the system without much calculation, and it is what the standard ductulator wheels and charts do.
Rectangular duct is sized by finding the round duct that has the same friction and airflow, using Huebscher's equivalent-diameter formula. Fix one dimension (usually height, set by the joist bay) and the calculator solves for the other. Round duct is also the more efficient shape, so use it where you have room.
Worked example
400 CFM at a 0.10 in. w.g. per 100 ft friction rate, with an 8-inch-high rectangular duct:
- D = (0.109136 × 400^1.9 ÷ 0.1)^(1 ÷ 5.02) = 9.83 in
- Velocity = 183.35 × 400 ÷ 9.83² = 759 fpm
- Rectangular, 8 in high: width 10.14 in (equivalent diameter 1.30 (a b)^0.625 ÷ (a + b)^0.25 = 9.83 in)
| Input | Value |
|---|---|
| Airflow | 400 cfm |
| Friction rate (design) | 0.1 in. w.g./100 ft |
| Rectangular duct height (fixed dimension) | 8 in |
| Result | Value |
|---|---|
| Round duct diameter | 9.83 in |
| Air velocity in the round duct | 759 fpm |
| Rectangular duct width for the height you entered | 10.14 in |
| Rectangular aspect ratio (long side ÷ short side) | 1.3 : 1 |
Assumptions and limits
- Standard air (0.075 lb/ft³) in clean galvanized steel duct. Flex duct, fiberglass duct board, and rough liners have different friction, and flex duct must be stretched fully straight to hold its rating.
- Ducts are sized for friction only. Fittings, dampers, and coils add pressure loss that you must account for separately.
- Total system airflow, room loads, and the friction rate come from a load calculation (Manual J/D). This tool only turns those numbers into duct sizes.
- Results are estimates. Verify against the equipment manufacturer's data and local code.
Common questions
What size duct do I need for 400 CFM?
At a 0.10 in. w.g. per 100 ft friction rate, a round duct of about 9 to 10 inches. The exact figure updates with your friction rate above.
What friction rate should I use?
Set it from your blower's available static pressure divided by the total equivalent length of the longest run, per 100 ft. Many residential designs land between 0.06 and 0.10.
Is a rectangular duct the same as a round one of equal area?
No. A rectangular duct with the same cross-sectional area has more friction. The equivalent diameter formula matches friction and airflow, which is what matters for sizing.
Why is my duct noisy?
Usually velocity is too high, or the ducts are very flat with poor fittings. Larger ducts move the same air more slowly and quietly.
Sources
- ASHRAE Handbook, Fundamentals, Duct Design chapter: Huebscher (1948) equivalent diameter, D_e = 1.30 (ab)^0.625 / (a+b)^0.25 (Eq. 25 and Table 2).
- Round-duct friction fit for standard air and galvanized steel, Δp = 0.109136 Q^1.9 ÷ D^5.02 (in. w.g. per 100 ft; Q in cfm, D in inches), the form used in ASHRAE-based ductulators.
Updated 2026-09-30