Duct Size Calculator
Work out the duct size to carry a given airflow. Set the airflow (CFM), pick an application (which sets a target air velocity), and get the round duct diameter plus an equivalent rectangular size — using the velocity method.
Rough guide: ~400 CFM per ton of cooling.
Lower = quieter & lower fan energy but bigger ducts; higher = compact but noisy.
Pick a duct depth that fits your ceiling void; the width is sized to match.
to carry 400 CFM at 700 fpm · cross-section 82 sq in.
Round the diameter up to the next standard duct size. Round ducts are the most efficient; the rectangular figure is an equal-friction equivalent for a ceiling void. Proper design uses the equal-friction method across the whole system.
A planning estimate. This is a single-section velocity-method sizing — it ignores fittings, total run length, pressure drop and system balancing. Real ductwork is designed across the whole system (equal-friction / static-regain) to IS 655 and ASHRAE/ISHRAE practice by a qualified MEP/HVAC engineer. Use this to sanity-check a size, not to build from.
Related Guides — Deep-dive reading
HVAC Duct Design: How Conditioned Air Gets Around a Building
The channels that carry cooled air — duct materials (GI sheet, aluminium, pre-insulated, flexible), how ducts are sized (velocity vs equal-friction), why insulation and sealing matter, and the principles behind good ductwork, grounded in IS 655.
HVAC & CoolingAir, Airflow & CFM: How Cooling Actually Travels Through a Building
Air is the delivery medium of comfort — this guide covers what air can carry, the airflow-to-cooling relationship (CFM and the sensible-heat equation), static pressure and why fans work harder than you'd think, and how airflow ties tonnage to duct size.
HVAC & CoolingDining Space Design in India: Where the Family Comes Together
Why a dining room can be too big, centring on the light, casual vs formal dining, table sizes and clearances, and the clever buffet-alcove trick
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