How this instrument works
CFM, cubic feet per minute, is the airflow rate a fan, duct, or piece of ventilation equipment moves. Rather than measuring an airflow you already have, this instrument works in the design direction: given a room's volume and a target air-changes-per-hour (ACH) rate, it returns the CFM that equipment needs to deliver to hit that target — the number you'd hand to a contractor or use to size a fan before anything is installed.
The relationship is the same ACH math run in reverse. Since ACH equals (CFM x 60) / room volume, solving that equation for CFM gives CFM = (room volume x ACH) / 60 — multiply the room's cubic footage by the target ACH, then divide by the 60 minutes in an hour to convert from a per-hour rate to the per-minute rate CFM is expressed in.
Target ACH figures come from wherever the ventilation requirement is defined: a building code, a design standard, or a specific use case. ASHRAE's residential 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. Plugging any of those targets into this instrument, along with the room's volume, converts the requirement into a concrete CFM specification.
If the direction is reversed — an airflow is already known or measured, and the question is what ACH it produces — this site's air changes per hour calculator runs the identical relationship the other way, solving for ACH instead of CFM.
- Enter the space's volume into Room volume (cubic feet) — length x width x ceiling height, not floor area.
- Enter the target ventilation rate into Air changes per hour (ACH) — pull this from the applicable code, standard, or design guide.
- Read Required airflow (CFM) beneath the inputs — the airflow a fan, duct, or ventilation system needs to deliver.
- Already have a measured airflow and want the ACH it produces instead? Use this site's air changes per hour calculator to run the relationship in reverse.
Worked example — 1,200 ft³ room targeting 4 ACH
Enter 1200 into Room volume (cubic feet) and 4 into Air changes per hour (ACH). The instrument multiplies 1200 by 4 to get 4,800, then divides by 60: 4800 / 60 = 80. Required airflow (CFM) reads 80.00 CFM.
That 80 CFM figure is exactly what a fan or duct serving this room needs to deliver to hit 4 air changes per hour — hand that number to whoever is sizing or specifying the ventilation equipment, or check it against a fan's rated CFM output before installing it.
Questions
What's the difference between this and the ACH calculator?
They run the same relationship in opposite directions. This CFM calculator starts from a target ACH you need to hit and a room's volume, and returns the airflow required to get there — useful when specifying or sizing equipment before installation. The air changes per hour calculator starts from an airflow you already have, measured or from an equipment spec, and returns the ACH it actually produces — useful for verifying an installed or planned system.
Where do I get the target ACH to enter?
From whatever code, standard, or design guide governs the space. ASHRAE's residential 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 — there is no single universal number, so confirm the target that actually applies before using it here.
Why does the formula divide by 60?
Because ACH is a per-hour rate and CFM is a per-minute rate, so converting between them means converting between hours and minutes. Multiplying room volume by the target ACH gives the total cubic feet of air that need to move through the room each hour; dividing that figure by the 60 minutes in an hour converts it down to the per-minute CFM figure equipment specifications are expressed in.
Does a bigger room always need a bigger CFM for the same ACH?
Yes — CFM scales directly with room volume for any fixed ACH target, since the formula multiplies the two together. Doubling the room's cubic footage while holding the target ACH constant exactly doubles the required CFM, which is why a small closet and a large warehouse hitting the identical ACH target need very different airflow rates.
How do I know if a fan delivers enough CFM for my target?
Compare the fan's rated CFM output — from its spec sheet, not just its nameplate horsepower or size — against the Required airflow (CFM) this instrument returns for the room and target ACH. If the fan's rated output falls short of the computed requirement, it won't hit the target ventilation rate in that room even if it's correctly installed and running.
Does this account for duct losses or static pressure?
No — it returns the theoretical CFM needed at the room based purely on volume and target ACH. Real ducted systems lose some airflow to duct friction, fittings, and static pressure along the way, so the airflow a fan needs to produce at its own outlet, before those losses, is typically somewhat higher than the room-level CFM figure this instrument returns.