Air Changes per Hour Calculator

Air Changes per Hour (ACH) measures how many times the entire volume of air inside a room or enclosed space is completely replaced or filtered with clean air in a 60-minute period. The imperial formula is ACH = (CFM × 60) ÷ Room Volume in cubic feet, where CFM is the airflow rate in Cubic Feet per Minute. In metric units, ACH = Airflow (m³/h) ÷ Room Volume (m³).

Recommended ventilation rates vary by occupancy and contamination risk per ASHRAE 62.1/62.2 and CDC guidelines: Residential living rooms and bedrooms need 0.35 to 1 ACH; home kitchens require 5 to 8 ACH; bathrooms need 6 to 8 ACH (or minimum 50 CFM intermittent); home workshops require 10 to 20 ACH; indoor grow tents demand 20 to 60 ACH; and hospital airborne infection isolation rooms (AIIR) require at least 12 ACH.

To determine the necessary airflow capacity to achieve a desired ACH, rearrange the ventilation formula: Required CFM = (Target ACH × Room Volume in cu ft) ÷ 60. For example, a 15 ft × 20 ft bedroom with 8 ft ceilings has a volume of 2,400 cu ft. To achieve 5 ACH for HEPA allergen filtration: CFM = (5 × 2,400) ÷ 60 = 200 CFM Clean Air Delivery Rate (CADR).

Airborne particle clearance follows exponential decay: Time (minutes) = -ln(1 - Removal Efficiency) ÷ ACH × 60. For 99% removal efficiency (-ln(0.01) ≈ 4.6), the required time is 4.6 ÷ ACH × 60 minutes = 276 ÷ ACH minutes. At 6 ACH, a room takes approximately 46 minutes to eliminate 99% of airborne contaminants; at 12 ACH, it takes just 23 minutes.

Fresh air ventilation exchanges stale indoor air with outdoor air through HVAC ducts or exhaust fans to remove CO2, radon, and moisture. Air cleaner or HEPA purifier ACH recirculates existing indoor air through filters to capture smoke, pollen, and dust without introducing outdoor air. Both contribute to equivalent clean air changes (eACH), but fresh air is still required to manage indoor carbon dioxide buildup.