Airflow rule
CFM per Ton
Air conditioning needs 400 CFM of airflow per ton as the standard design point — 350 in humid climates, 450 in dry ones. Convert tons to CFM below (at 400 CFM/ton).
3 tons = 1,200 CFM
Airflow by tonnage and climate
| Tons | Humid · 350 | Standard · 400 | Dry · 450 |
|---|---|---|---|
| 1 | 350 | 400 | 450 |
| 1.5 | 525 | 600 | 675 |
| 2 | 700 | 800 | 900 |
| 2.5 | 875 | 1,000 | 1,125 |
| 3 | 1,050 | 1,200 | 1,350 |
| 3.5 | 1,225 | 1,400 | 1,575 |
| 4 | 1,400 | 1,600 | 1,800 |
| 5 | 1,750 | 2,000 | 2,250 |
Turn the CFM into duct sizes with the ductulator, or get the tonnage first from the tonnage calculator.
FAQ
- How many CFM per ton of air conditioning?
- The standard design airflow is 400 CFM per ton. Use about 350 CFM per ton in humid climates, where slower air improves moisture removal, and up to 450 CFM per ton in dry climates, where sensible cooling dominates.
- How many CFM does a 3-ton AC need?
- About 1,200 CFM at the standard 400 CFM per ton — roughly 1,050 CFM in a humid climate and 1,350 CFM in a dry one. The duct system must actually deliver that airflow, which is where duct sizing comes in.
- What happens if airflow is too low?
- Low airflow makes the evaporator coil run too cold: capacity drops, the coil can freeze, and compressor life suffers. Chronically low CFM usually points to undersized or restrictive ducts — check the sizing with a ductulator.