How this instrument works
Air changes per hour, or ACH, counts how many times the complete volume of air inside a room is replaced with new air every hour. A CFM (cubic feet per minute) airflow rate delivered into a room translates directly into ACH once you know the room's volume: multiply CFM by 60 to get cubic feet delivered per hour, then divide by the room's cubic footage. An ACH of 4 means the room's full air volume is theoretically swapped out four times in sixty minutes.
ACH shows up constantly in building codes and ventilation guidance because it scales with room size in a way that a raw CFM number doesn't. A 240 CFM fan sounds identical whether it serves a small closet or a large warehouse, but the ACH it produces is wildly different — which is exactly why codes specify a target ACH rather than a fixed CFM and leave the airflow-sizing arithmetic to whoever is designing the system.
Real-world targets vary by space. ASHRAE's residential ventilation guidance points to roughly 0.35 whole-dwelling air changes per hour, CDC guidance for public indoor spaces during elevated respiratory-virus periods points to about 5 ACH, and hospital patient rooms are commonly designed to 6 ACH or higher under mechanical-engineering codes. None of those numbers is universal — always check the code or guidance that actually applies to the room in question.
This instrument answers one direction of the relationship: given an airflow and a room volume, what ACH results? If you instead know the ACH you need to hit and want to know what airflow to specify, this site's CFM calculator runs the same relationship in reverse, solving for the airflow a fan or duct needs to deliver.
- Enter your fan or duct's delivered airflow into Airflow rate (CFM) — read it off the equipment spec sheet or a balancing report, not a nameplate rating.
- Enter the room's volume into Room volume (cubic feet) — multiply length x width x ceiling height, not the floor area alone.
- Read Air changes per hour (ACH) beneath the inputs — it updates instantly as either field changes.
- Compare the result against the ACH your code, standard, or design guide actually requires for that room type before calling the ventilation adequate.
- Need the airflow instead? If you already know a target ACH, use this site's CFM calculator to solve for the required CFM directly.
Worked example — 80 CFM into a 1,200 ft³ room
Enter 80 into Airflow rate (CFM) and 1200 into Room volume (cubic feet). The instrument multiplies 80 by 60 to get 4,800 cubic feet delivered per hour, then divides by the 1,200 cubic foot room volume: 4800 / 1200 = 4.0. Air changes per hour (ACH) reads 4.0 exactly.
An ACH of 4.0 sits below CDC's 5 ACH public-space guidance and above ASHRAE's roughly 0.35 whole-dwelling residential minimum, which is why context matters more than the raw number: the same 4.0 ACH could be undersized for a waiting room and generous for a bedroom.
Questions
What is a good ACH for a room?
It depends entirely on the space. ASHRAE's residential guidance points to roughly 0.35 whole-dwelling air changes per hour, CDC recommends about 5 ACH for public indoor spaces during elevated respiratory-virus periods, and hospital patient rooms are commonly designed to 6 ACH or more. There is no single universal target — check the code, standard, or design guide that governs the specific room type before judging a result as adequate or not.
How is ACH different from CFM?
CFM (cubic feet per minute) is a raw airflow rate — how much air a fan or duct delivers per minute, regardless of room size. ACH (air changes per hour) scales that airflow against the room's volume, telling you how many times the room's full air content turns over in an hour. The same 200 CFM produces a very different ACH in a small closet than in a large warehouse, which is why codes specify ACH targets rather than fixed CFM values.
Does ACH mean every cubic foot of old air is actually gone?
Not literally. ACH is a theoretical average based on total volume divided by total airflow; it assumes perfect mixing, where incoming air blends evenly through the whole room. In practice, supply air can short-circuit straight to a return grille without ever mixing through occupied space, so the effective air exchange occupants experience can run lower than the calculated ACH suggests. Engineers account for this with a separate mixing-efficiency factor in detailed ventilation design.
I know my target ACH — how do I find the CFM I need?
Run the relationship in reverse: multiply your target ACH by the room volume, then divide by 60. This site's CFM calculator performs exactly that calculation — enter the room volume and the ACH you're required to hit, and it returns the airflow rate to specify for the fan or duct.
Do I need floor area or room volume for this calculation?
Volume, not floor area. Multiply the room's length by its width by its ceiling height to get cubic feet — a 10 ft x 12 ft room with an 8 ft ceiling is 960 cubic feet, not the 120 square feet a floor-area measurement would give you. Using square footage in place of cubic footage will overstate the calculated ACH substantially.
Why do hospital rooms need more air changes than a house?
Because the risk of airborne contamination is higher and the consequences of poor air quality are more serious. Mechanical-engineering codes commonly set patient-room targets at 6 ACH or higher, compared with roughly 0.35 whole-dwelling air changes per hour recommended for an ordinary home — a difference driven by occupancy type and infection-control needs, not by any change in the underlying ACH formula itself.