Round Duct Air Velocity

Duct velocity is the number that decides whether a system whispers or roars. The area of a round duct is πd²/4, so velocity is 4V̇/πd² — a 14-inch duct carrying 1,000 cfm runs 1,000 ÷ 1.069 ft² ≈ 935 ft/min, or 4.75 m/s on this page. Residential design keeps main trunks near 700–900 fpm and branch runouts under 600 fpm; commercial mains tolerate 1,500–2,500 fpm because they are lined and further from ears. Above roughly 2,500 fpm you start buying noise, and friction loss climbs with the square of velocity, so every doubling of speed quadruples the fan energy.

The speed picker here has ft/s rather than ft/min: divide fpm by 60, so 1,200 fpm is 20 ft/s. The classic trap is using nominal instead of inside dimensions — flex duct is measured to the inner liner, and a 6-inch flex that was never pulled tight behaves like a 5-inch duct with three times the pressure drop, which is the single most common cause of a starved bedroom register. Solve for d when you know the flow and the velocity you can live with: 2,000 cfm at 20 ft/s wants 1.457 ft, or a 17.5-inch duct, so you round up to 18 inches.