How to Calculate Ventilation Requirements
Proper ventilation removes CO2, moisture, cooking fumes, and airborne contaminants. Too little ventilation leads to poor air quality; too much wastes energy and can create uncomfortable drafts. ACH (Air Changes per Hour) is the standard metric used to size fans and HVAC systems.
The ACH Formula
ACH = airflow rate (m³/h) ÷ room volume (m³). Required airflow (m³/h) = target ACH × room volume. When selecting a fan, add 20% to the calculated airflow to account for duct losses and aging. CFM × 1.699 = m³/h for US-rated products.
ACH Reference Values by Room Type
Living rooms / bedrooms: 3–5 ACH. Offices / classrooms: 5–10. Bathrooms: 10–15 (IBC/ASHRAE 62.2 minimum). Kitchens: 15–30 (with range hood). Workshops: 10–20. Storage: 2–4. Higher ACH is needed where contaminants are generated rapidly.
ASHRAE & Building Code Guidelines
ASHRAE 62.2 (residential) requires a minimum whole-house ventilation rate of 0.35 ACH or 7.5 CFM per person (whichever is greater). Local building codes may require exhaust fans in bathrooms (50 CFM / 85 m³/h minimum) and kitchen range hoods (100 CFM / 170 m³/h minimum).
Frequently Asked Questions
For a kitchen under 100 sq ft (9 m²), a 200–300 CFM (340–510 m³/h) range hood is typical. For a 4-burner gas range, aim for at least 400 CFM (680 m³/h). Add 20% for duct length beyond 6 feet. Island hoods may need 50% more than wall-mount hoods of the same size.
Run the exhaust fan for at least 20–30 minutes after a shower to fully clear moisture and prevent mold. A timer switch or humidity-sensing fan makes this automatic. The goal is to reduce relative humidity back below 60% as quickly as possible.
Exhaust fans pull air out, creating negative pressure that draws fresh air in through gaps and vents. Supply systems push filtered outdoor air in, creating positive pressure. Balanced (HRV/ERV) systems do both simultaneously and are most energy-efficient because they recover heat from outgoing air.