Airflow CFM Calculator
Work out the airflow in cubic feet per minute from a heat load and temperature difference, from room volume and air changes, or from tonnage.
Air carries the heat
In a forced-air system, heating and cooling are delivered by moving air, so the airflow, in cubic feet per minute, determines how much heat reaches each room. The link is the sensible heat equation: BTU/h = 1.08 × CFM × ΔT, where ΔT is the difference between the supply air temperature and the room temperature. The constant 1.08 comes from standard air: a density of 0.075 lb/ft³ times a specific heat of 0.24 BTU/lb·°F times 60 minutes in an hour. Rearranged, CFM = BTU/h ÷ (1.08 × ΔT).
Worked example
A zone with a sensible cooling load of 24,000 BTU/h, supplied with air 20 °F cooler than the room, needs 24,000 ÷ (1.08 × 20) = 1,111 CFM. If the supply air is only 15 °F cooler, the same load needs 1,481 CFM, a third more air. That is why airflow and coil temperature have to be designed together: a coil that cannot produce a large temperature drop forces bigger ducts and a faster fan. For heating with a gas furnace, the temperature rise is usually 35 to 70 °F, listed on the furnace data plate, so much less air carries the same heat.
The 400 CFM per ton rule
For central air conditioning, the traditional starting point is 400 CFM per ton of cooling: 1,200 CFM for a 3-ton system. It is a rule of thumb, not a law. In humid climates, installers often set 350 CFM per ton, because slower air over the coil makes it colder and removes more moisture. In dry climates, 450 CFM per ton increases sensible capacity. A common misconception is that more airflow is always better; too much air in a humid climate leaves the house cool but damp.
Air changes per hour
For ventilation and exhaust, airflow is often specified in air changes per hour, ACH: the number of times the volume of air in a space is replaced in an hour. CFM = volume × ACH ÷ 60. A 2,400 ft³ workshop needing six air changes per hour requires 240 CFM, one complete change every 10 minutes. Recommended rates vary from about 0.35 ACH of fresh air for whole houses to 10 or more for kitchens, bathrooms during use and workshops with fumes.
How to use it
- Choose the method: heat load and temperature difference, room volume and air changes, or system tonnage.
- For heat load, enter the sensible BTU/h and the supply-to-room temperature difference.
- For air changes, enter the room volume in cubic feet and the target ACH.
- For tonnage, enter the system size and the CFM per ton suited to your climate.
Questions from the job site
How do you calculate CFM from BTU?
Divide the sensible heat load in BTU/h by 1.08 times the temperature difference between supply air and room air. A 24,000 BTU/h load with a 20 °F difference needs 24,000 ÷ 21.6 = 1,111 CFM. The 1.08 factor assumes standard sea-level air.
How many CFM per ton of air conditioning?
About 400 CFM per ton is the usual starting point, so a 3-ton system moves about 1,200 CFM. Humid climates often use around 350 CFM per ton for better dehumidification, and dry climates up to about 450 CFM per ton.
What does the 1.08 in the CFM formula mean?
It is the product of standard air density, 0.075 pounds per cubic foot, the specific heat of air, 0.24 BTU per pound per degree Fahrenheit, and 60 minutes per hour. At high altitude the air is thinner, so the factor is smaller and more CFM is needed.
How do I convert air changes per hour to CFM?
Multiply the room volume in cubic feet by the air changes per hour and divide by 60. A 2,400 cubic foot room at 6 air changes per hour needs 240 CFM. Measure volume as floor area times average ceiling height.
Disclaimer: HVAC results are estimates from simplified methods and the values you enter. They are not a Manual J, S or D design and must not replace one for equipment selection or permits. Gas, electrical and refrigerant work must be carried out by licensed technicians to local codes, and combustion appliances need working carbon monoxide alarms.