Every heating and cooling system is really two systems: the refrigerant or burner that makes the heat (or removes it), and the air that carries it to the rooms. CFM is how you measure the second one. Get the refrigerant side perfect and the house will still be uncomfortable if the air isn't moving in the right amounts.
CFM in plain English
One CFM is one cubic foot of air passing a point every minute. A 3-ton system at 400 CFM per ton moves about 1,200 CFM, enough to fill a 10 × 12 × 10 foot bedroom roughly every minute.
NIST lists one cubic foot per minute as about 0.000472 cubic meters per second1, which is the conversion you'll need for metric spec sheets.
Two numbers matter:
- System airflow: total CFM across the indoor coil or furnace heat exchanger.
- Room airflow: the CFM each supply register should deliver, so every room gets its share.
How much CFM per ton?
The "400 CFM per ton" figure is everywhere, but it's a starting point. DOE's Building America guide says air conditioners are typically designed for 350 to 400 CFM per ton, recommends 350 in humid climates for better moisture removal, and cites 500 for hot-dry climates where latent loads are small2.
ACCA makes the same point with an example 2.5-ton unit at 63°F entering wet bulb3:
Total capacity barely changed (29,500 to 30,300 Btu/h). What changed was the split between sensible cooling and moisture removal.
| 350 CFM per ton | 15,340 |
|---|---|
| 400 CFM per ton | 17,284 |
| 450 CFM per ton | 18,483 |
Source: ACCA HVAC Blog, Feb. 4, 2021
Lower airflow leaves more capacity for removing moisture. Higher airflow gives more sensible cooling. ACCA's advice is to set airflow from the manufacturer's tables rather than a rule of thumb3.
How do you calculate CFM?
From a load calculation (design)
The designer splits the blower's airflow across rooms by each room's share of the load. The ICC presentation on ACCA's design process works this example4:
| Room | Cooling load | Heating load | Cooling CFM | Heating CFM | Design CFM (larger) |
|---|---|---|---|---|---|
| Room 1 | 4,800 Btu/h | 5,800 Btu/h | 100 | 97 | 100 |
| Room 2 | 19,200 Btu/h | 25,200 Btu/h | 400 | 420 | 420 |
| Room 3 | 24,000 Btu/h | 29,000 Btu/h | 500 | 483 | 500 |
| Total | 48,000 | 60,000 | 1,000 | 1,000 |
Room CFM
Each duct run is then sized for its design CFM at the friction rate Manual D allows.
Free calculatorCFM & duct size calculatorAirflow from tonnage, and the round duct size for a target friction rate.From a temperature rise (field check)
On electric heat or a heat pump with a known heat input, you can back into airflow with the sensible heat formula. The 1.08 constant comes from air's specific heat (0.24), 60 minutes per hour, and air's specific volume of about 13.33 cubic feet per pound5.
Sensible heat formula
Worked example (from ACHR News): electric strip heat drawing 45 A at 230 V is 10,350 W, or about 35,325 Btu/h. With a 32°F rise across the air handler, airflow is 35,325 ÷ (1.08 × 32) ≈ 1,022 CFM5.
Common mistakes: using nameplate kW instead of measured amps and volts, measuring supply temperature too close to the heat strips, and forgetting that 1.08 assumes standard air near sea level.
What does low or high CFM do?
350
CFM per ton below which efficiency starts to drop and charge tests become unreliable
DOE's guide cites lab testing of a 3-ton system: at 250 CFM per ton, sensible capacity fell to 75%, total capacity to 85% and EER to about 92% of the values at 400 CFM per ton2. Too much airflow has its own costs: more fan energy, more noise, and less moisture removal in humid climates.
What moves airflow?
| Lever | Why it works | How to check |
|---|---|---|
| Filter and filter grille size | Filters were the biggest return-side loss in DOE's 62-home study | Pressure drop across the filter |
| Return duct size | An undersized return starves the blower | Return static reading |
| Blower speed tap or ECM setting | Sets the airflow the motor targets | Blower table at measured static |
| Clean evaporator coil | Fouling adds resistance and cuts airflow | Pressure drop across the coil |
| Duct design | Right-sized runs hit design CFM at the room | Flow hood at registers |
See static pressure for how to find the restriction.
How is airflow verified?
ENERGY STAR's commissioning checklist looks up airflow from measured total external static pressure and the fan speed setting, and passes the system if it's within ±15% of the design airflow. Its optional room-by-room balancing step uses the greater of ±20% or 25 CFM of each room's design airflow6.
How CFM shows up in TradesWork
The TradesWork technician app includes offline calculators for duct size from CFM, CFM from duct size, temperature rise and airflow, and temperature split, with inputs in US field units. Each answer shows how it was worked out, so a tech can explain an airflow problem to the customer.
Footnotes
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NIST Special Publication 811, Guide for the Use of the International System of Units, Appendix B.8. https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b8 ↩
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D. Springer and B. Dakin, Measure Guideline: Air Conditioner Diagnostics, Maintenance, and Replacement, U.S. DOE Building America, DOE/GO-102013-3766, March 2013, pp. 8–11. https://www1.eere.energy.gov/buildings/publications/pdfs/building_america/measure_guide_air_cond_diagnostics.pdf ↩ ↩2
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M. Akins, "400 CFM Per Ton, Or Is It?" ACCA HVAC Blog, Feb. 4, 2021. https://hvac-blog.acca.org/400-cfm-per-ton-or-is-it/ ↩ ↩2
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J. Sedine, "An Overview on ACCA's Residential HVAC System Design Process," 2018 ICC Annual Conference. https://iccsafe.org/wp-content/uploads/Residential-HVAC-Design-Process.pdf ↩
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B. Johnson, "Btu Buddy 159: Heat Pump Air Distribution Problem," ACHR News, June 20, 2016 (trade press). https://achrnews.com/articles/132765-btu-buddy-159-heat-pump-air-distribution-problem ↩ ↩2
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ENERGY STAR Single-Family New Homes, National HVAC Commissioning Checklist, Version 3.1 / 3.2 / 3.3 (Rev. 14), Sections 3 and 4, revised Jan. 15, 2025. https://www.energystar.gov/sites/default/files/2025-01/National%20HVAC%20Commissioning%20Checklist_Rev%2014.pdf ↩
