What ACH means and why it matters
Air changes per hour is a simple idea: how many times the entire volume of air in a room gets replaced with new air over the course of an hour. An ACH of 6 means a room's full air volume is swapped out six times an hour, once roughly every 10 minutes. It matters because ventilation is how a room clears odor, excess moisture, and airborne pollutants; a room with too few air changes can feel stuffy, hold onto cooking smells or bathroom humidity, or let indoor air quality degrade over time, especially in spaces that generate a lot of one of those (kitchens, bathrooms, garages).
The formula, in plain terms
ACH is airflow divided by room volume, with a unit conversion in between: ACH = (CFM × 60) / room volume in cubic feet. CFM is cubic feet per minute, and there are 60 minutes in an hour, so multiplying CFM by 60 gives cubic feet per hour, which then divides cleanly into the room's volume in cubic feet. Room volume itself is just length times width times height, all in feet. For an oddly shaped room, a closet nook, an L-shaped layout, split it into rectangular sub-volumes, compute each one's length × width × height, and add them together before dividing.
Sizing to ACH versus sizing to a duct friction target
This page and the duct sizing pages on this site answer different questions. The duct pages (round, rectangular, flex) and the grille calculator answer "given a CFM I already need, what size duct or grille delivers it without excess friction or noise?" This page answers a question one step earlier: "given a room's size and a ventilation target, how much CFM do I actually need in the first place?" Once you have that CFM number from an ACH target, hand it to the duct and grille calculators to size the path that delivers it, that's the intended handoff between this page and the rest of the site.
Typical ACH by room type, as guidance
Different rooms are commonly ventilated at different rates because they generate different loads. Bedrooms and living rooms are often cited around 4 to 6 air changes per hour, enough for comfort without excess energy use. Kitchens run higher, commonly 7 to 8, to clear cooking odors, moisture, and combustion byproducts. Bathrooms are similar, roughly 6 to 8, mainly for moisture control after a shower. Garages and workshops often target 6 to 9 or more, especially where fumes or dust are a concern. Specialized spaces, labs, certain commercial kitchens, cleanrooms, run far higher again, governed by their own specific codes. Every one of these numbers is orientation guidance, not a computed result and not a substitute for your actual local building or mechanical code, or ASHRAE 62.1 (residential) and 62.2 (commercial) where they apply; those documents, or a licensed professional, govern the number that actually matters for a real project.
Exhaust, supply, and balanced ventilation
ACH describes total air replacement, it doesn't by itself say how that air gets in and out. Exhaust ventilation pulls air out of a room (a bathroom fan, a kitchen hood) and relies on air infiltrating back in from elsewhere; supply ventilation pushes conditioned air in and relies on it finding a way out; balanced systems supply and exhaust at matched rates. Which approach is right depends on the room and the system design, but the ACH math is the same regardless, it's the rate of total replacement, independent of the specific mechanism moving the air.
How these numbers are derived
Everything on this page is the same small, public relationship: airflow in cubic feet per minute, converted to cubic feet per hour, divided by (or multiplied into) room volume in cubic feet. There's no manufacturer data, no proprietary table, and no duct friction physics involved, that's a separate question answered by the duct and grille pages on this site. Room-type ACH figures are stated as commonly cited conventions for orientation, explicitly not a reproduced code table, always confirm the number that actually applies to a real project against local code or a licensed professional.
Frequently asked questions
How do you calculate air changes per hour?
ACH = (CFM x 60) / room volume in cubic feet, where room volume is length times width times height in feet. The 60 converts CFM (cubic feet per minute) into cubic feet per hour so it matches the room volume's time unit.
How many air changes per hour do I need?
It depends on the room and, often, your local code. Commonly cited orientation figures are roughly 4-6 ACH for bedrooms and living areas, 7-8 for kitchens, 6-8 for bathrooms, and 6-9 for garages or workshops. These are typical ventilation guidance, not a computed requirement, always check local code or ASHRAE 62.1/62.2 for the figure that actually governs your space.
What is a good ACH for a bedroom?
Roughly 4 to 6 air changes per hour is commonly cited as typical guidance for bedrooms and other living spaces. Bedrooms don't generate much moisture, odor, or pollutant load compared to kitchens or bathrooms, so they're usually ventilated at a lower rate.
What CFM do I need for a room?
Multiply your target ACH by the room's volume in cubic feet, then divide by 60: CFM = (ACH x volume) / 60. The calculator above does this directly from room length, width, and height, or you can start from a known CFM and see what ACH it produces instead.
Is a higher ACH always better?
Not necessarily. More air changes mean more ventilation, but also more energy spent conditioning that air, and past a certain point, more noise and draft. Specialized spaces (labs, some commercial kitchens, cleanrooms) run much higher ACH than living spaces because they specifically need it; a bedroom doesn't benefit from lab-level ventilation.