CFM calculator
CFM is cubic feet of air per minute. This calculator finds it three ways: from the size of a cooling system, from the heat a furnace delivers and how much it warms the air, and from the volume of a room and how often its air should be replaced.
- Cooling, 3.0 tons1,200 CFM
- Heating, 50 °F rise1,110 CFM
- Room, 6.0 air changes/h144 CFM
Heating airflow: 60,000 BTU/h ÷ (1.08 × 50 °F) = 1,111 CFM. Use the furnace output, not its input rating.
Room airflow: 1,440 cubic feet × 6.0 air changes ÷ 60 minutes = 144.0 CFM.
Results update as you type. Nothing you enter leaves your browser.
CFM by air conditioner size
A central air conditioner or heat pump is set up to move a fixed amount of air across the indoor coil for each ton of capacity. The standard figure is 400 CFM per ton. Installers lower it toward 350 in humid climates, so that the coil runs colder and removes more moisture, and raise it toward 450 in dry climates.
| System size | 350 CFM/ton | 400 CFM/ton | 450 CFM/ton |
|---|---|---|---|
| 1.5 tons | 525 | 600 | 675 |
| 2 tons | 700 | 800 | 900 |
| 2.5 tons | 875 | 1,000 | 1,125 |
| 3 tons | 1,050 | 1,200 | 1,350 |
| 3.5 tons | 1,225 | 1,400 | 1,575 |
| 4 tons | 1,400 | 1,600 | 1,800 |
| 5 tons | 1,750 | 2,000 | 2,250 |
One ton of cooling is 12,000 BTU per hour, so a 36,000 BTU/h system is 3 tons and needs about 1,200 CFM.
CFM from furnace output and temperature rise
The air a furnace must move follows from the heat it puts into that air:
CFM = BTU/h ÷ (1.08 × temperature rise in °F)
The temperature rise is the supply air temperature minus the return air temperature. Every furnace lists an allowed rise range on its rating plate, and the airflow must keep the rise inside that range.
| Heat output (BTU/h) | 40 °F rise | 50 °F rise | 60 °F rise | 70 °F rise |
|---|---|---|---|---|
| 40,000 | 925 | 740 | 615 | 530 |
| 60,000 | 1,390 | 1,110 | 925 | 795 |
| 80,000 | 1,850 | 1,480 | 1,235 | 1,060 |
| 100,000 | 2,315 | 1,850 | 1,545 | 1,325 |
| 120,000 | 2,780 | 2,220 | 1,850 | 1,585 |
Use the output of the furnace. A furnace rated at 100,000 BTU/h input and 80% efficiency puts 80,000 BTU/h into the air.
Where 1.08 comes from
Air at sea level weighs about 0.075 pounds per cubic foot and takes 0.24 BTU to warm one pound by one degree Fahrenheit. Multiply those by the 60 minutes in an hour and the result is 1.08. Air is thinner at altitude, so the factor falls: it is roughly 0.90 at 5,000 feet, and the same heat needs more CFM.
CFM for a room from air changes per hour
Ventilation is specified as the number of times per hour the air in a room is replaced:
CFM = room volume in cubic feet × air changes per hour ÷ 60
A 15 by 12 foot room with an 8 foot ceiling holds 1,440 cubic feet. At 6 air changes per hour it needs 144 CFM. The right number of air changes depends on the use of the room and on the code or standard that applies to it.
Limits of this calculator
- 400 CFM per ton is a convention. The federal test procedure uses it as the default when a manufacturer has not certified an airflow. The right figure for a house depends on how much of the cooling load is moisture.
- The furnace sets its own limits. The rating plate lists the temperature rise the furnace is approved for. The installer sets the blower speed to stay inside it.
- Air changes are not a code check. Ventilation requirements come from the building code and the use of the room.
Sources
- US Department of Energy test procedure for central air conditioners, 10 CFR 430 Subpart B, Appendix M: standard air of 0.075 pounds per cubic foot, and the default of 400 CFM per 12,000 BTU/h: https://www.ecfr.gov/current/title-10/part-430/subpart-B/appendix-Appendix M to Subpart B of Part 430
- ACCA, what is the correct airflow for an air conditioner: https://hvac-blog.acca.org/hey-ed-what-is-the-correct-airflow-for-an-ac/
- Green Building Advisor, getting the right airflow for an air conditioning system, for the humid-climate and dry-climate ranges: https://www.greenbuildingadvisor.com/article/getting-the-right-airflow-for-an-air-conditioning-system
Related
Once the airflow is known, the duct size calculator gives the duct that carries it. To find the tonnage first, use the AC tonnage calculator.
More sizing calculators
- BTU calculatorBTU for one room or a whole house
- Furnace size calculatorFurnace BTU from floor area and climate
- AC tonnage calculatorCentral AC size in tons from floor area
- Mini-split sizing calculatorBTU per zone for one to four rooms
- Water heater size calculatorTank size from your busiest hour of hot water use
- Tankless water heater size calculatorGPM, temperature rise, and BTU or kW
- Heat loss calculatorHeating load of a house, component by component
- Duct size calculatorRound and rectangular duct size from CFM
- Superheat and subcooling calculatorSuperheat and subcooling from pressure and line temperature
Calculator last reviewed on October 5, 2026. Method and sources.