How is ACH calculated?

• A. (Volume × 60) / CFM
• B. (CFM × 60) / room volume
• C. (room volume) / (CFM × 60)
• D. CFM / room volume

Answer: (B)

Air changes per hour (ACH) tells you how many complete air replacements occur in one hour. To find ACH, use the ventilation rate in cubic feet per minute (CFM) and the room’s volume in cubic feet. Since CFM is cubic feet per minute, multiply by 60 to convert to cubic feet per hour, then divide by the room volume. This gives how many times the room’s air is replaced each hour. So the correct expression is (CFM × 60) / room volume. For example, 500 CFM in a 3,000 ft³ room yields ACH = (500 × 60) / 3000 = 10. The other forms would represent different quantities: trying to get time per air change or a per-minute rate instead of per hour.