Duct Size Calculator

What size duct do I need?

CFM
in per 100 ft
Duct shape
in

Use about

8inch round duct

Round size from the equal-friction formula for galvanized duct, within about 5%. Rectangular size uses the equivalent-diameter formula. Flex duct and long runs need a proper Manual D design.

8 in round200 CFMFriction: 0.08 in/100 ft
Exact round size7.9 in
Velocity in round duct573 ft/min
Friction rate used0.08 in per 100 ft

Turn an airflow and friction rate into a round duct size or a rectangular size that carries the same air.

How it works

Solves the equal-friction equation for galvanized duct to get a round diameter, then finds the rectangular width whose Huebscher equivalent diameter matches.

Before you order

  • Friction rates near 0.08 to 0.1 in per 100 ft are common in homes.
  • Flex duct has more resistance than sheet metal, so size it larger.
  • Very wide flat ducts have more wall friction, so keep the proportions moderate.

Common questions

Why do I need to know CFM first?

Duct size is set by airflow. Your load calculation decides the airflow, then duct design delivers it.

Is this a full duct design?

No. Real design covers fittings and equivalent lengths under Manual D, so have a professional complete it.