How do I work out CFM from duct size and velocity?

Airflow is the duct face area times the air speed. Put the area in square feet and the velocity in feet per minute and the answer comes out in cubic feet per minute — CFM. A 12 by 8 inch duct is 0.667 sq ft; at 900 fpm that is about 600 CFM. This tool does the area for you from the width and height.

How do air changes per hour relate to CFM?

An air change replaces the whole room’s volume once. Multiply the room volume by the changes you want per hour and divide by 60 to get CFM. A 2000 cubic foot room at 6 air changes an hour needs 2000 × 6 ÷ 60 = 200 CFM. Switch this tool to the air-change method and enter the room and the rate.

Does this size a fan or a duct for me?

No — it is airflow arithmetic only. It will not select a fan, size a duct run for a given static pressure, or account for losses along the way. Use it to express a flow you already want or to check one you have measured, and leave the equipment selection to a mechanical designer.

Duct CFM calculator

This is airflow arithmetic — area times velocity, or volume times a change rate. It does not size a fan or a duct, judge static pressure, or account for the losses of a real run. Take it as a flow figure to aim at or to check, not an equipment specification.

Pick a method. The first turns a duct width, height and air velocity into a flow; the second turns a room volume and a target of air changes per hour into the flow that delivers them. Both read out in CFM, litres per second and cubic metres an hour.

Worked example

A 12 × 8 inch duct is (12 ÷ 12) × (8 ÷ 12) = 0.667 sq ft. At 900 feet per minute that carries 0.667 × 900 ≈ 600 CFM, about 283 litres per second. By the room method, a 2000 cubic foot room at 6 air changes an hour needs 2000 × 6 ÷ 60 = 200 CFM.

The formula

duct: CFM = area × velocity · room: CFM = volume × ACH ÷ 60

Flow is a volume passing per unit time. The duct method builds it from the face area and the speed of the air through it. The room method spreads a whole-room volume across the hour at the change rate you set, dividing by 60 to land in CFM.

Questions

How do I work out CFM from duct size and velocity?: Airflow is the duct face area times the air speed. Put the area in square feet and the velocity in feet per minute and the answer comes out in cubic feet per minute — CFM. A 12 by 8 inch duct is 0.667 sq ft; at 900 fpm that is about 600 CFM. This tool does the area for you from the width and height.
How do air changes per hour relate to CFM?: An air change replaces the whole room’s volume once. Multiply the room volume by the changes you want per hour and divide by 60 to get CFM. A 2000 cubic foot room at 6 air changes an hour needs 2000 × 6 ÷ 60 = 200 CFM. Switch this tool to the air-change method and enter the room and the rate.
Does this size a fan or a duct for me?: No — it is airflow arithmetic only. It will not select a fan, size a duct run for a given static pressure, or account for losses along the way. Use it to express a flow you already want or to check one you have measured, and leave the equipment selection to a mechanical designer.

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