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Airflow & duct area calculator

Put airflow, area and velocity in the same calculation.

Start with a room and your air-change target. See the corresponding airflow, the round duct area at your chosen velocity, and the velocity through a diameter you enter.

A volume-and-geometry calculation. Set air-change and velocity targets from your project design; this tool does not select a fan or establish ventilation compliance.

Free to use, with no account and no sign-up. It is a standalone utility for anyone measuring a room, whether or not OrdexisOne is part of how you work.

Define the air volume.

Illustrative values are loaded so you can try the calculation.

Room & air changes

Use the clear internal dimensions of a rectangular space. For an irregular space, calculate its component volumes separately.

The example of 6 ACH is an input, not a recommended rate. Total circulated air and outdoor ventilation air are different quantities.

Round duct comparison

Use internal, unobstructed diameter. All calculated room airflow is assumed to pass through this single duct.

Airflow at your target

Calculated from the example inputs.

Air volume per minute
—CFM
Metric airflow
—L/s
Room volume
—ft³

At the entered velocity

Cross-sectional area
— ft²
Equivalent round diameter
— in

This is the exact geometric diameter at your target, before any commercial size selection.

Through the entered diameter

Average air velocity
— fpm

Calculate to compare velocity with your target.

Diameter changes velocity. It does not establish available fan pressure or actual delivered flow.

Read the result

A useful airflow calculation. One part of the system design.

Room volume × ACH ÷ 60 gives cubic feet per minute when dimensions are in feet. For metric inputs, volume × ACH ÷ 3.6 gives litres per second.

Airflow equals average velocity × duct area. This tool rearranges that relationship to find area, then uses the area of a circle to find diameter. The same relationship gives velocity through the diameter you enter. See TSI’s flow-rate equations (PDF).

Choose the correct air-change basis.

ACH expresses airflow relative to room volume. Outdoor air, total supply air and equivalent clean air are not interchangeable design inputs. The appropriate rate depends on the space and applicable requirements. CDC explains air changes and ventilation.

Check the full duct and fan system.

Duct length, fittings, filters and other resistance affect pressure loss. Fan selection needs the required flow at the system’s pressure, alongside noise and other project criteria. The geometry here does not calculate those losses. Greenheck explains airflow and static pressure.

Bring the service operation together.

Explore how OrdexisOne connects HVAC service requests, assessed quotes, technician appointments and customer billing.

Other free tools: estimate steady-state heating loss, work out pipe capacity from inside diameter, plan an HVAC labor rate, or open the full list of free calculators.

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