HVAC Duct Sizing & Airflow CFM Calculator
Calculate recommended HVAC round duct diameter, equivalent rectangular duct dimensions, air velocity (FPM), and tonnage airflow capacity using Manual D friction standards.
Calculated Results
Frequently Asked Questions
How many CFM of airflow are required per ton of air conditioning?
In residential HVAC systems, the standard industry rule of thumb is 400 CFM (Cubic Feet per Minute) per 1 ton of air conditioning cooling capacity. A 3-ton system requires 1,200 CFM; a 4-ton system requires 1,600 CFM.
What is the standard duct friction loss rate?
Most residential HVAC duct design (ACCA Manual D) uses a friction rate of 0.10 or 0.08 inches of water column (in. w.g.) per 100 feet of ductwork. 0.08 in. w.g. ensures quieter operation with lower static pressure.
What is the ideal air velocity inside residential ductwork?
For quiet residential systems, main supply trunks should ideally maintain air velocities between 700 and 900 FPM (feet per minute). Branch ducts feeding individual room registers should stay under 600 FPM to prevent whistling and air rush noise.
How do you convert round duct size to rectangular duct size?
HVAC engineers use Huebscher's formula: D = 1.30 * (a * b)^0.625 / (a + b)^0.25, where D is round diameter, and a and b are rectangular dimensions that produce identical fluid friction.