calcs.homes
calcs.homes
Enter your room's dimensions and type to size an exhaust fan — bathroom, kitchen, and laundry rooms each need a different air-change rate.
Estimate only — ducting length and bends reduce a fan's real-world airflow below its rating.
CFM = room volume (length × width × height) × air changes per hour ÷ 60. Bathrooms and laundry rooms target 8 air changes per hour, kitchens target 15 to clear cooking moisture and odors faster, and general rooms target 4. We then round the raw CFM figure up to the nearest common retail fan size — 50, 80, 110, 150, 190, or 250 CFM — since that's what's actually sold, rather than showing a number like 47 that doesn't match a real fan.
The air-change targets follow HVI (Home Ventilating Institute) sizing guidance, which is more accurate than the simpler 1-CFM-per-square-foot rule of thumb because it accounts for ceiling height, not just floor area — two bathrooms with the same footprint but different ceiling heights need different fans. Rounding up to a standard size also builds in a small margin, which matters because duct length and elbow bends reduce a fan's real-world airflow below its rated CFM.
A common rule of thumb is 1 CFM per square foot for bathrooms up to 100 sq ft — this calculator uses the more precise volume-based HVI formula, which accounts for ceiling height too.
Yes — long duct runs and elbow bends add resistance that reduces actual airflow below the fan's rated CFM. For long runs, size up or choose an in-line duct booster fan.