BOTANVÈDuct / CFM Calculator
Work out how much air a room needs and what duct diameter (100–315 mm) carries it at a sensible velocity.
A. Airflow required for a room (air-change method)
B. Duct size for a given airflow
How to size ventilation airflow and ducts
Ventilation is usually specified either by air changes per hour (ACH) — how many times the room's air volume is replaced each hour — or by a fixed litres-per-second-per-person figure for occupied spaces. For extract fans in bathrooms, kitchens and small commercial spaces the ACH method is the quickest way to reach a design airflow. Multiply the room volume (m³) by the ACH to get m³/h, then divide by 1.699 to convert to CFM, which is how most inline fans are rated.
Once you know the airflow, the duct size follows from the velocity you can tolerate: duct area = airflow ÷ velocity. Lower velocities (600–800 FPM) are quieter and suit bedrooms and offices; main trunks and industrial ducting run at 1,000–1,500 FPM. Long duct runs, bends and grilles add static pressure, which is why centrifugal fans such as the VENTS VKMz series are chosen over compact mixed-flow fans for anything beyond a short, straight run.
| Space | Typical ACH | Notes |
|---|---|---|
| Bedroom, office | 4–6 | Comfort ventilation |
| Living room, retail shop | 6–8 | Higher occupancy |
| Bathroom, toilet | 8–10 | Extract fan, run-on timer |
| Domestic kitchen | 12–15 | Extract near hob |
| Commercial kitchen, gym | 15–30 | Check local authority requirements |
Local requirements (Dubai Municipality, ADQCC, Sharjah Municipality and Civil Defence for kitchens) may set minimum rates that override rules of thumb — always check the applicable code for commercial projects.
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FAQs
How do I convert m³/h to CFM?
Divide m³/h by 1.699 (or multiply CFM by 1.699 to get m³/h). 1 CFM ≈ 0.472 L/s.
Which VENTS fan size do I need for a bathroom?
A 2.5 m × 2 m × 2.8 m bathroom at 10 ACH needs about 140 m³/h (82 CFM); a 100 mm inline fan such as the VENTS TT 100 or VKMz 100 is the usual choice. Check the fan's airflow at the static pressure of your duct run.
Round or rectangular duct?
Round duct has less pressure loss and is easier to seal; rectangular duct is used where ceiling void height is limited. The calculator shows an approximate 2:1 rectangular equivalent.
What velocity should I design to?
600–800 FPM for residential branches, 800–1,000 FPM for commercial, up to 1,500 FPM for main trunks where noise is less critical.