CFM Converter

Airflow is measured in different units depending on region.

Conversion factors:

• 1 CFM = 0.4720 L/s (liters per second)
• 1 CFM = 1.6990 m3/h (cubic meters per hour)
• 1 L/s = 2.1189 CFM = 3.6 m3/h
• 1 m3/h = 0.5886 CFM = 0.2778 L/s

Common HVAC airflow values:

• Bathroom fan: 50-110 CFM (24-52 L/s)
• Kitchen range hood: 100-400 CFM (47-189 L/s)
• Central AC (per ton): 400 CFM (189 L/s)
• Whole house fan: 3,000-6,000 CFM (1,416-2,832 L/s)

Ventilation standards:

• ASHRAE recommends 15-20 CFM per person for office spaces.
• Residential: about 0.35 air changes per hour.

CFM is really a way of asking how fast the air in a space gets replaced. To size a fan, take the room’s volume in cubic feet, multiply by the air changes per hour you want, and divide by 60.

CFM = room volume (cu ft) x air changes per hour / 60

A 600 cubic-foot bathroom, roughly 10 by 7.5 by 8 feet, at the 8 air changes an hour that bathrooms want, needs 600 x 8 / 60 = 80 CFM. A 1,000 cubic-foot master bath at the same rate needs 133 CFM, so the common 110 CFM fan would be a touch undersized for it.

Air changes per hour, by room:

The catch is that a fan’s rated CFM is measured on a test bench with no resistance. Add real ducting, elbows, and a roof cap and the actual airflow can drop well below the box number, sometimes by half on a long, twisty run. That is why experienced installers size up and keep duct runs short and straight rather than trusting the rated figure at face value. If you can only measure one thing, measure the duct: a 4-inch flex run doing the work of a 6-inch rigid one costs more airflow than any fan upgrade will get back.