Field calculator
Air changes per hour calculator (ACH to CFM)
Air changes per hour is a quick way to size ventilation or exhaust for a space, and it converts straight to airflow. The CFM needed equals the room volume times the target ACH divided by 60, and the volume is the floor area times the ceiling height. Enter the area, the height, and the target air change rate. One air change is a full replacement of the room's air, so spaces that are specified in ACH (restrooms, commercial kitchens, mechanical and battery rooms, labs, parking garages) translate directly into a fan or supply CFM with this formula. To run it backward and check an existing system, ACH equals CFM times 60 divided by the volume. Treat ACH as a rule-of-thumb target rather than a design in itself: a real ventilation design uses a load calculation or a ventilation-rate method such as ASHRAE 62.1, which sets outdoor air by the number of people and the floor area, so confirm the required rate against the code and the application before sizing equipment.
Result
Airflow from air changes per hour: CFM = room volume × ACH / 60, where the volume is floor area × ceiling height. Enter the area, the height, and the target ACH. An air change is one full replacement of the room volume, so ACH is a quick way to spec ventilation or exhaust for spaces rated that way (restrooms, kitchens, mechanical rooms, labs). To reverse it, ACH = CFM × 60 / volume. Treat ACH as a rule-of-thumb target: a real design uses a load calculation or a ventilation-rate method such as ASHRAE 62.1, which sets outdoor air by occupancy and floor area, so confirm the required rate against the code and the application.
anvilfield.com/calculators/air-changes-per-hour-cfm-calculator · Free field calculators and FieldOS. A planning estimate, verify against the code, the manufacturer, and the engineer of record.
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ACH to CFM FAQ
How much exhaust does a restroom need?
A commercial restroom is exhausted per fixture, commonly 70 CFM per water closet or urinal for an intermittent fan and 50 CFM where it runs continuously, per the IMC. Add the fixtures up for a multi-stall room, and confirm the rate against the adopted code edition and the project specification.
Why does the building need makeup air?
Every cubic foot exhausted has to be replaced or the building goes negative and pulls unconditioned air through doors, shafts, and flue vents. Makeup air, ducted outside air or transfer air from adjacent spaces, gives the exhausted air a planned way back in and keeps the pressure relationships and the appliance venting correct.
What is an upblast exhaust fan?
An upblast exhaust fan is a roof-mounted centrifugal fan that throws the air stream straight up and away from the roof. It keeps the discharge off the membrane and away from people, so it suits dirty, fume-laden, or greasy exhaust. A downblast fan, which discharges toward the roof, is only for clean air.
How do you ventilate a parking garage?
Ventilate an enclosed garage with mechanical exhaust sized around 0.75 CFM per square foot at full output, run on CO and NO2 demand control with a low standby rate near 0.05 CFM per square foot. Sensors stage the fans up as gas climbs. Verify the rates, setpoints, and sensor count against the adopted code.
How far must an exhaust outlet be from an air intake?
The IMC commonly requires an environmental air exhaust outlet at least 10 ft from a mechanical air intake, 3 ft from operable openings for most occupancies, and 3 ft from a property line. Direction and prevailing wind matter too, so add real separation when the geometry is tight rather than just meeting the dimension.