Size round or rectangular HVAC ducts from airflow, target friction rate, or target velocity. Standard air (70°F, sea level).
The Ductulator implements a single-segment sizing model with three modes: solve for duct size given airflow + target friction rate; solve for duct size given airflow + target velocity; or evaluate an existing duct size for resulting velocity, friction rate, and Reynolds number. Round and rectangular ducts are supported.
Pressure drop per unit length follows the Darcy-Weisbach equation, ΔP/L = f · (V²/2D) · ρ, where f is the Darcy friction factor, V is the air velocity, D is the duct diameter (or equivalent diameter for rectangular), and ρ is air density.
The Swamee-Jain explicit approximation of the Colebrook-White equation is used to avoid an inner iterative solve: f = 0.25 / [log₁₀(ε/(3.7D) + 5.74/Re^0.9)]². This is the standard closed-form friction factor for fully turbulent flow, covering essentially all HVAC duct velocities.
Rectangular ducts use the ASHRAE equal-friction-and-capacity equivalent round: De = 1.30 · (ab)^0.625 / (a + b)^0.25. This finds the round duct with the same friction rate and the same airflow capacity, not just the same velocity.
Standard air assumed: 21°C / 70°F at sea level, ρ = 1.204 kg/m³, ν = 1.51×10⁻⁵ m²/s. No temperature, altitude, or humidity correction.
The default load is 1700 m³/h at 0.85 Pa/m on galvanized steel, round duct. The solver returns a diameter of approximately 345 mm at a velocity near 5.05 m/s. With "Round up to standard size" enabled, that snaps to 350 mm at ~4.9 m/s. This matches standard metric duct sizing charts for those inputs.
