ACH Calculator

Understanding ACH: what it measures and what it doesn't

Air Changes per Hour (ACH) measures how many times the air volume in a space is theoretically replaced in one hour, calculated as airflow divided by room volume, scaled to an hourly basis. It's a simple, intuitive metric widely used to communicate ventilation intensity, especially in healthcare, laboratory, and infection control contexts where dilution of airborne contaminants is the primary goal.

It's important to understand what ACH does not tell you: it says nothing about how much of that airflow is fresh outdoor air versus recirculated air. A room can have a very high ACH using 100% recirculated air with zero outdoor air, providing excellent mixing and filtration but no actual fresh air ventilation. Outdoor air requirements come from ASHRAE Standards 62.1 and 62.2 and are set separately from total ACH, so use the ventilation rate calculator for that calculation.

ACH for infection control and healthcare design

Higher ACH rates are specified in healthcare and laboratory settings primarily to dilute and remove airborne contaminants quickly, reducing the time any infectious particles or hazardous substances remain in the breathing zone. Canadian healthcare facilities follow CSA Z317.2 (the current edition is CSA Z317.2:24), which sits alongside ASHRAE 170: 6 ACH total for general patient rooms, 12 ACH for airborne infection isolation rooms with negative pressure, 12 ACH for protective environment rooms with positive pressure, and 20 ACH for operating rooms. Rates this high usually mean dedicated air handling with pressure control and terminal HEPA filtration, not just a larger fan, because the filter and fan have to be sized together.

Choosing a target ACH for your space

Start with the space type in the reference table, then confirm the number against the standard that governs the building. For health care, the CDC's Guidelines for Environmental Infection Control in Health-Care Facilities explain that most health care ventilation rates are expressed as room ACH, and they call for at least 12 ACH and negative pressure in new or renovated airborne infection isolation rooms. For offices, classrooms, and shops, ACH is usually an outcome rather than a target: the outdoor air requirement and the cooling load set the supply rate, and the resulting ACH should land near the range in the table. Use the Find Airflow for Target ACH tab when you are starting from a required ACH and need the CFM.

As a worked case, a 20 ft by 15 ft room with a 9 ft ceiling holds 2,700 cubic feet. A target of 6 ACH needs (6 × 2,700) ÷ 60 = 270 CFM, and each complete air change takes 60 ÷ 6 = 10 minutes. The same room supplied with 450 CFM runs at 10 ACH, one change every 6 minutes.

Calculating room volume correctly

Room volume should reflect the actual conditioned air volume, not just floor area times a nominal height. For rooms with sloped ceilings, mezzanines, or significant furniture and equipment volume displacement, adjust your volume calculation accordingly for accurate ACH results. This calculator uses simple length × width × height; for complex geometries, calculate volume separately and use the direct airflow comparison.

From required airflow to duct and diffuser size

Once the calculator returns a required CFM, divide it across the supply outlets in the room and check that each branch is sized for it. Pushing the 270 CFM from the example above through a single 8 inch round duct means roughly 775 feet per minute, which is a useful sanity check before you commit to a layout. The duct sizing calculator handles that step, including friction rate and velocity for each run.