CFM Calculator
Airflow in CFM for a room from air changes per hour, or a duct.
About this tool
Choose room or duct. For a room, enter its dimensions in feet or metres and the air changes per hour you want, or pick a room type. For a duct, enter its size and air velocity. The result is in CFM, m³/h and L/s.
How to calculate CFM
CFM means cubic feet per minute, the standard airflow rating for fans, extractors, range hoods, air purifiers and HVAC systems. For a room, CFM = volume (ft³) × air changes per hour ÷ 60. A 15 × 12 × 8 ft room is 1,440 ft³, so 6 air changes an hour needs 144 CFM. The presets use common rule-of-thumb rates: around 4 for living rooms, 8 for bathrooms and 10 or more for kitchens. For a duct, CFM = area (ft²) × velocity (ft/min); an 8-inch round duct at 700 ft/min carries about 244 CFM, and a 10 × 8 in rectangular duct at 600 ft/min about 333 CFM. One CFM is 1.70 m³/h or 0.47 L/s. Honest limits: rated fan CFM is measured with almost no resistance, so real airflow through ducts, filters and grilles is often 20–40% lower; building codes and ASHRAE 62.1 and 62.2 set actual ventilation requirements. This is not engineering advice. The BTU calculator and the air density calculator are related.
Frequently asked questions
How do I calculate CFM for a room?
Multiply length × width × height in feet to get cubic feet, multiply by the air changes per hour, then divide by 60.
How many CFM do I need for a bathroom?
A common rule is 1 CFM per square foot of floor (minimum 50 CFM), or about 8 air changes per hour.
How do I convert CFM to m³/h?
Multiply CFM by 1.699. For litres per second, multiply by 0.472.
How much air does a duct carry?
Multiply its cross-sectional area in square feet by the air velocity in feet per minute.
Is my data sent anywhere?
No. Everything is calculated in your browser, and nothing is stored or sent.