ACH Calculator

1. Choose your calculation mode. Use "ACH from CFM" if you know the current airflow and want to see if it's adequate. Use "CFM from ACH" if you have a target ventilation rate and need to know the required airflow.

2. Enter room dimensions. Measure the length, width, and ceiling height in feet. For irregular rooms, approximate the closest rectangular dimensions.

3. Enter airflow or target ACH. In ACH mode, enter the measured or rated CFM from the supply registers. In CFM mode, enter the desired ACH from the reference table.

4. Review the results. Check the color-coded ACH rating against the room type reference table and verify ASHRAE compliance status.

Air changes per hour (ACH) measures how many times the entire volume of air in a room is replaced in one hour. It is the standard metric for evaluating ventilation adequacy and indoor air quality.

Higher ACH means fresher air but also higher energy costs, because you are conditioning more outside air. The goal is to hit the right range for each room type. A bedroom at 4-6 ACH is comfortable and healthy. A commercial kitchen below 10 ACH will have grease buildup, odor complaints, and code violations.

ACH matters for IAQ because it directly controls the dilution of indoor pollutants, CO2, moisture, and volatile organic compounds. Rooms with inadequate ACH develop stale air, condensation problems, and occupant complaints. Rooms with excessive ACH waste energy and can create uncomfortable drafts.

Ventilation system design. Before specifying fans, ductwork, or ERVs, you need to know the target ACH for every room. This calculator converts those targets into CFM values you can use for equipment selection and duct sizing.

IAQ complaint investigation. When a customer reports stuffiness, odors, or moisture issues, measuring the actual CFM and calculating ACH tells you immediately whether the room is under-ventilated. If a bathroom is running at 3 ACH when it needs 7, you have found the problem.

Code compliance verification. Building inspectors and commissioning agents use ACH to verify that mechanical ventilation meets ASHRAE 62.1 (commercial) and 62.2 (residential) requirements. This calculator flags whether the ACH meets those minimums.