Air changes per hour explained for room ventilation
What ACH means, typical rates for each room, and how to convert an air change target into the CFM a fan must deliver.
What an air change per hour really measures
Air changes per hour, shortened to ACH, describes how completely a room's air is refreshed over an hour. One air change means a volume of fresh air equal to the whole room has passed through, in theory replacing everything inside. Real airflow mixes rather than sweeping the old air out in one clean push, so ACH is a rate target, not a promise that every molecule leaves. Even so it is a practical yardstick, because it ties the fan you choose directly to the size of the space it serves.
Turning ACH into CFM
Because fans are rated in cubic feet per minute but ventilation targets are set in changes per hour, you need a bridge between the two, and that bridge is time. First find the room volume by multiplying length, width, and height in feet. Multiply that volume by your ACH target to get cubic feet of air moved per hour, then divide by 60 to bring it down to per minute. A 320 cubic foot bathroom at 8 ACH needs 320 times 8 divided by 60, which is about 42.67 CFM. That single conversion is exactly what this calculator automates.
Typical rates by room
Different rooms call for different air change rates because of the moisture, odors, and heat they produce. Bathrooms commonly target 6 to 8 ACH to clear steam before it condenses and feeds mildew. Kitchens often sit around 7 to 8 to pull out cooking grease and smells, though a range hood over a stove is usually sized separately. Bedrooms and living areas need less, roughly 5 to 6, since they generate far less moisture and odor. These are starting points, and your local building code or the fan manufacturer may set a firmer number for a given use.
Why real fans deliver less than their label
A fan's rated CFM is measured on a test bench with almost no resistance, but a real installation fights back. Every foot of duct, every elbow, and the grille itself adds static pressure that drags the delivered airflow below the number on the box. A long or twisty duct run can cut effective output by a fifth or more. That is why installers pad the target, choosing a fan rated above the calculated CFM or one whose performance curve still meets the goal at the pressure the ductwork imposes. Use the figure from this tool as the airflow you must actually achieve at the grille, then shop with headroom.