The basic formula is: CFM = Room Volume (cubic feet) × Air Changes per Hour ÷ 60. Room volume = length × width × ceiling height. Air changes per hour (ACH) varies by room type: bedrooms 4–6 ACH, kitchens 15+ ACH, bathrooms 8 ACH minimum. Example: a 10×12×8 ft bedroom = 960 cu ft × 5 ACH ÷ 60 = 80 CFM.

The IRC (International Residential Code) requires bathroom fans to provide at least 1 CFM per square foot of floor area, with a minimum of 50 CFM. For a 50 sq ft bathroom, 50 CFM minimum. For a 100 sq ft bathroom, 100 CFM. HVI (Home Ventilating Institute) recommends 8 air changes per hour for bathrooms.

Range hood CFM requirements depend on cooking intensity. Electric cooktops: 100 CFM minimum. Gas cooktops (up to 40,000 BTU): 200–300 CFM. High-BTU commercial-style ranges: 400–1,200 CFM. A common rule: 100 CFM per 10,000 BTU of gas cooktop output. Island hoods need 50% more CFM than wall hoods due to open sides.

ACH represents how many times per hour the entire volume of air in a room is replaced with fresh or conditioned air. Higher ACH = more ventilation. Recommended ACH by space: bedrooms 4–6, living rooms 4–6, kitchens 15+, bathrooms 8, hospitals 6–15, labs 6–12. Most residential HVAC systems provide 4–8 ACH throughout the house.