Keep the units distinct
| Quantity | Typical unit | Meaning |
|---|---|---|
| Volume flow | L/s, m³/s or CFM | Air volume passing in a given time. |
| Velocity | m/s or feet/minute | Air speed through an area. |
| Area | m² or square feet | Size of the opening being considered. |
CFM means cubic feet per minute. One cubic metre per second equals 1,000 litres per second and approximately 2,119 CFM.
Read the basic relationship
For a defined opening, volume flow equals average velocity multiplied by area. Use compatible units: m/s multiplied by m² gives m³/s.
Arithmetic example only: an opening of 0.40 m² with an average velocity of 0.50 m/s gives 0.20 m³/s, or 200 L/s. These are illustrative numbers, not a prescribed operating position or acceptance limit.
Our face-velocity guide covers the interpretation of that measurement.
Account for the complete cupboard
Air may enter through bypasses or other designed openings as well as the sash opening. A simple face-area calculation therefore does not automatically establish the total exhaust duty for every cupboard.
Use the manufacturer’s data and the approved design information. The bypass guide and variable-volume guide explain why the control arrangement matters.
Carry the duty into equipment selection
- Record airflow with its unit and operating condition.
- Identify the source of the specified duty.
- Keep sash position and relevant configuration with test readings.
- Pair required airflow with the system pressure requirement.
- Check conversions before comparing equipment schedules.
The fan selection guide helps turn those figures into an equipment enquiry.