Doing this for a whole building? See this step inside a connected AIM Works project — loads, ventilation, equipment, and Design Review in one place.
Book a Demo →This duct sizer finds the round and rectangular duct sizes that carry your airflow at the design intent you choose. Enter the airflow in CFM, the air temperature, and the duct material, then pick a method: equal friction (a target friction rate in in.w.c. per 100 ft) or velocity (a target air velocity in fpm). The tool returns a continuous ideal diameter, the next standard round size, the equal-friction rectangular equivalent, and the resulting velocity and friction for each.
Sizing drives fan energy, noise, and the space the ductwork consumes. Undersized ducts raise velocity, static pressure, and sound; oversized ducts waste material and ceiling space. Material roughness and air temperature both shift the friction, so the calculator corrects for the selected material and for air density at the entered temperature (sea-level pressure) rather than assuming a single fixed value.
| Velocity (round) | V (fpm) = Q (CFM) ÷ [π · (D ÷ 24)²] (D in inches) |
| Friction loss | ΔP₁₀₀ = f · ρ · V² · 9.95×10⁻⁴ ÷ Dh |
| Hydraulic dia. | Dh = 4 · W · H ÷ [2 · (W + H)] |
| Equiv. diameter | Deq = 1.3 · (W × H)0.625 ÷ (W + H)0.25 |
Friction loss uses the Darcy–Weisbach equation with the Colebrook friction factor (laminar f = 64 ÷ Re below Re ≈ 2,300, turbulent solved iteratively). The constant 9.95×10⁻⁴ converts the Darcy result to in.w.c. per 100 ft in IP units. Air density follows ρ = 0.075 · (530 ÷ (T + 460)) lb/ft³ and kinematic viscosity ν = 1.60×10⁻⁴ · ((T + 460) ÷ 530)1.76 ft²/s — sea-level air. At altitude the friction at the same airflow and size is lower, roughly in proportion to density (about 15–17% lower at 5,000 ft), so these results are conservative there. The rectangle is sized from the standard round size through the Huebscher equivalent diameter Deq at a 1.5:1 aspect ratio, each side rounded up to the next 2 in.; its friction is then computed directly from its hydraulic diameter Dh and face velocity, which agrees with reading the round-duct chart at Deq to within a few percent. Absolute roughness: galvanized and galvanized spiral 0.0003 ft and stainless 0.00015 ft (the ASHRAE Fundamentals Ch. 21 medium-smooth and smooth categories); fibrous-glass duct board 0.0025 ft, flexible duct 0.012 ft (fully extended — compressed or sagging flex runs far higher) and concrete 0.005 ft are representative values for rougher surfaces.