Duct Size Calculator

This calculator compares straight round and rectangular metal ducts at a specified airflow using either a target friction rate or maximum velocity. It rounds the round diameter upward within an illustrative size list and checks rectangular choices against the selected target. Results describe the entered section, not a complete duct network, fan operating point or code-compliant design.

How to use this tool

1. Enter the airflow the duct must carry in CFM.
2. Choose equal friction or maximum velocity and enter the target.
3. Enter the height you have room for if you need a rectangular duct.
4. Read the standard round size, its velocity and friction, and the rectangular options.

Formula

d = (0.109136 × Q^1.9 ÷ f)^(1/5.02); d = 12 × √(4Q ÷ (π V)); De = 1.30 (a·b)^0.625 ÷ (a + b)^0.25

Q is CFM, f is inches of water per 100 ft, d is inches and V is ft/min. The friction expression is an empirical turbulent-flow approximation for galvanized metal duct. Huebscher diameter compares equal flow, friction and length; it is not the diameter of an equal-area circle. Rectangular velocity sizing instead requires actual area a·b/144 ≥ Q/V. Widths are tried in 2-inch steps up to 120 inches.

Example

For 1,000 CFM and 0.10 in. w.c. per 100 ft, the friction fit gives about 13.9 inches. A listed 14-inch duct gives approximately 935 ft/min and 0.097 in. w.c. per 100 ft. A 22 × 8 inch rectangle meets that friction target. For a separate velocity example, 400 CFM at a 700 ft/min maximum needs at least 0.5714 sq ft; a 12 × 8 inch rectangle provides 0.6667 sq ft and runs at 600 ft/min.