Work out duct airflow (cfm) instantly with clear inputs, formula shown and shareable results.
Airflow follows directly from the sensible heat a duct must carry: CFM = BTU/h / (1.08 x ΔT). Once airflow is known, dividing by the design velocity gives the cross-sectional area, and the equivalent round diameter follows from the area of a circle.
Airflow
CFM = sensible BTU/h / (1.08 x ΔT)
Duct size
Area ft² = CFM / velocity fpm; diameter in = sqrt(4 x area / pi) x 12
CFM = sensible BTU/h / (1.08 x ΔT). Airflow follows directly from the sensible heat a duct must carry: CFM = BTU/h / (1.08 x ΔT).
Residential trunks are typically 700-900 fpm and branches 600 fpm to keep noise down. Commercial mains run 1200-2000 fpm where acoustic treatment allows.
This calculator takes 3 inputs: Sensible heat load, Supply-to-room temperature difference, Design duct velocity. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.