Air Changesper Hour Calculator
Plan your hvac & plumbing project with our free air changesper hour calculator. Get precise measurements, material lists, and budgets.
Reviewed by Abdullah, Technical Content Specialist
Formula
ACH = (CFM x 60) / Volume (cu ft) | CFM = (ACH x Volume) / 60
Air changes per hour is calculated by multiplying the airflow in CFM by 60 to convert to cubic feet per hour, then dividing by the room volume. To find the required CFM for a desired ACH, multiply the target ACH by the room volume and divide by 60.
Worked Examples
Example 1: Find ACH from Known CFM
Problem:A 2,700 cu ft room has an air handler delivering 250 CFM. What is the ACH?
Solution:ACH = (CFM x 60) / Volume\nACH = (250 x 60) / 2,700\nACH = 15,000 / 2,700 = 5.56
Result:5.56 air changes per hour
Example 2: Find Required CFM for Target ACH
Problem:A 4,000 cu ft space needs 8 ACH. What CFM is required?
Solution:CFM = (ACH x Volume) / 60\nCFM = (8 x 4,000) / 60\nCFM = 32,000 / 60 = 533.3
Result:534 CFM required
Frequently Asked Questions
What does air changes per hour (ACH) mean?
ACH is a measurement of how many times the total air volume of a room is completely replaced with fresh air in one hour. An ACH of 6 means the entire room air is replaced 6 times per hour, or once every 10 minutes. This metric is fundamental to HVAC engineering because it determines ventilation adequacy, contaminant removal rates, and thermal comfort in occupied spaces.
References
Reviewed by Abdullah, Technical Content Specialist · Editorial policy