Estimate duct size, airflow velocity, CFM, round duct diameter, and rectangular duct dimensions. Built for HVAC learners, technicians, contractors, engineers, and homeowners.
Results are estimates only. Actual duct design requires professional analysis including static pressure, friction rate, fittings, and local codes.
Where Area is in ft² and Velocity in FPM.
An HVAC duct calculator estimates the required duct size, airflow velocity, and CFM based on basic input parameters. It helps professionals and homeowners quickly size round or rectangular ducts for heating and cooling systems.
Divide the airflow (CFM) by the desired velocity (FPM) to obtain the required duct area in square feet. Convert to square inches and solve for round diameter or rectangular dimensions using standard geometry.
CFM (Cubic Feet per Minute) measures airflow volume. FPM (Feet per Minute) measures air speed. Duct area (in ft²) connects the two: a larger duct allows lower velocity for the same CFM, reducing noise and pressure drop.
Round ducts are more efficient for airflow and easier to seal. Rectangular ducts are often used where space is limited. Equivalent area estimates help compare round and rectangular performance.
Low velocity may cause poor air distribution, while high velocity increases noise and pressure loss. Typical residential supply velocities range from 600 to 900 FPM, but exact values depend on system design.
This calculator provides basic estimates only. Real duct design involves friction rate, static pressure, duct length, fittings, elbows, equipment capacity, and balancing. For professional HVAC design, consult Manual D, manufacturer data, and local codes.