About this tool
The CFM from Air Changes per Hour (ACH) calculator is a free, in-browser ventilation tool that links a room's air-change rate to the airflow, in cubic feet per minute, needed to achieve it. Air changes per hour is how many times the entire volume of air in a room is replaced each hour. The core relations are simple: room volume V = length × width × ceiling height, then required CFM = (ACH × V) ÷ 60, and in reverse ACH = (CFM × 60) ÷ V.
All of the math is done locally in your browser, so nothing you enter is uploaded. Enter the room's three dimensions in feet, then leave either the ACH or the CFM field blank and the tool solves for it. The divide-by-60 converts the per-hour air change into a per-minute airflow, since CFM is defined per minute.
Different spaces call for different air-change rates: bathrooms are commonly ventilated at about 8 ACH, kitchens around 15, bedrooms 5–6, offices about 6, and workshops roughly 10, with kitchen exhaust hoods higher still. Results report the required airflow, the resulting ACH, the room volume, and the flow in cubic feet per hour, making it easy to size an exhaust fan or check whether an existing fan meets a target ventilation rate.