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 air density 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–White friction factor (laminar f = 64 ÷ Re below Re ≈ 2,300, turbulent solved iteratively). The constant 9.95×10⁻⁴ converts the Darcy result to inches of water column per 100 ft in IP units. Air density follows ρ = 0.075 · (530 ÷ (T + 460)) lb/ft³, and the rectangular equivalent diameter is the ASHRAE relation that gives the round duct with the same friction loss. Roughness values follow standard ASHRAE/SMACNA duct-design practice.