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CFM Duct Airflow Calculator

Estimate volumetric airflow from the internal section of a duct and its measured average air speed. Choose rectangular or circular geometry.

mm
mm
m/s
Use an average across the section, not a single peak reading.

Includes the waste margin explained below. Check your product label for coverage and sizes.

Airflow from a known duct section

CFM means cubic feet per minute, a volume of air passing a section during a minute. This calculator estimates that flow from the open area of a duct and the average velocity through it. It is useful for checking a measurement or comparing a flow stated in CFM with one stated in cubic metres per hour. It does not size a bathroom fan from floor area or prescribe a ventilation requirement. Use measured internal duct dimensions and a velocity representing the whole section rather than just a fast spot at the centre.

Area multiplied by average velocity

Rectangular area = width × height; circular area = π × diameter² ÷ 4; flow = area × mean velocity

Metric duct dimensions use millimetres and average speed uses metres per second. Imperial dimensions use inches and speed uses feet per minute. The calculation converts both to a consistent internal metre basis, then reports litres per second, CFM and the unit-mode primary result. One CFM equals 0.0004719474432 cubic metres per second. For a round duct, the width entry is its inside diameter and the height is ignored. For a rectangular duct, both dimensions are required. Linings or obstructions that reduce the open section should be accounted for in the measurements rather than using an external casing size.

Example rectangular duct

A duct 400 millimetres wide and 200 millimetres high has an internal area of 0.4 × 0.2 = 0.08 square metres. An average air speed of 3 metres per second gives 0.08 × 3 = 0.24 cubic metres per second. That is 240 litres per second, 864 cubic metres per hour or about 508.53 CFM. In a separate imperial example, a 12 by 8 inch duct has an area of 96 square inches, or two-thirds of a square foot. At 600 feet per minute, its estimated flow is 400 CFM. These examples depend on average velocity, not on a fan's advertised free-air rating.

Interpret the measurement conditions

Air speed often varies across a duct. A proper traverse takes several readings according to the measurement procedure, then uses an appropriate average. A reading near a bend, grille or obstruction may not represent the downstream flow reliably. A grille's free area can also differ from its face dimensions, so use supplied free-area data where relevant. The area-times-speed equation does not estimate static pressure, friction, leakage or fan performance. It cannot prove that a ventilation system meets a room's needs. Zero velocity returns zero flow, while zero geometric dimensions for the chosen shape are rejected. Keep the operating fan setting and measurement location with the result so repeated checks refer to comparable conditions.

Frequently asked questions

What does CFM mean?

It is cubic feet per minute, a volumetric airflow unit. It describes air volume passing a cross-section during one minute.

Can I use a circular duct?

Yes. Select circular and enter its inside diameter in the width field. The unused height field does not affect the result.

Is one centre velocity reading enough?

It can misrepresent mean flow. Use an average obtained with a suitable measurement procedure across the section, especially where velocity is uneven.

Does this select a fan for a room?

No. It converts section and average velocity to flow. Room ventilation needs and a fan operating point require separate information.

How do I convert CFM to m³/h?

Multiply CFM by approximately 1.69901079552. The tool performs that conversion internally and also reports litres per second.

Last reviewed: 2026-10-03 · How we calculate