The three CFM formulas
- From tonnage: CFM = tons × CFM per ton. 400 is the standard-air baseline; manufacturers publish a 350–450 band. This is the target, what the system should move.
- From temperature rise: CFM = heat output ÷ (1.08 × ΔT). This is what the blower is actually moving, measured with two thermometers and the nameplate. The 1.08 is 60 × 0.075 lb/ft³ × 0.24 BTU/lb·°F, so it only holds for standard air near sea level.
- From air changes: CFM = volume × ACH ÷ 60. Exhaust fans and ventilation, where the question is how fast the room's air should turn over.
Airflow comes first. On a cooling call, a starved coil reads high superheat and looks like an undercharge; on a furnace, low airflow shows up as a temperature rise above the nameplate range. Confirm the CFM, then read the gauges.
Target airflow by system size
| System | 350 CFM/ton (humid) | 400 CFM/ton (baseline) | 450 CFM/ton (dry) |
|---|---|---|---|
| 1.5 ton | 525 | 600 | 675 |
| 2 ton | 700 | 800 | 900 |
| 2.5 ton | 875 | 1,000 | 1,125 |
| 3 ton | 1,050 | 1,200 | 1,350 |
| 3.5 ton | 1,225 | 1,400 | 1,575 |
| 4 ton | 1,400 | 1,600 | 1,800 |
| 5 ton | 1,750 | 2,000 | 2,250 |
Got the target? Size the duct that carries it.
Quick answers
How many CFM per ton of air conditioning?
400 CFM per ton is the standard-air baseline, so a 3-ton system needs about 1,200 CFM across the indoor coil. Run closer to 350 CFM/ton in hot-humid climates for more moisture removal, and up to 450 CFM/ton in hot-dry climates for more sensible cooling. The blower table in the install manual is the real spec.
How do I calculate CFM from temperature rise?
CFM = heat output (BTU/hr) ÷ (1.08 × temperature rise °F). For a gas furnace, output is input × steady-state efficiency; for electric heat, kW × 3,412. Let it run 10 minutes, keep the supply probe out of sight of the heat exchanger, and treat the result as a ballpark, not a flow-hood number.
What is the CFM formula for a room?
CFM = room volume in cubic feet × air changes per hour ÷ 60. A 10 × 8 ft bathroom with an 8 ft ceiling at 8 air changes per hour needs about 85 CFM of exhaust.
Why does the sensible heat formula use 1.08?
1.08 = 60 min/hr × 0.075 lb/ft³ (density of standard air) × 0.24 BTU/lb·°F (specific heat of air). It is only right for standard air near sea level; at altitude the air is lighter and the factor drops.
What happens if airflow is too low?
On cooling, low airflow drops the coil temperature: high superheat readings look like an undercharge, the coil can ice, and capacity falls. On heating, the furnace runs a high temperature rise and trips its limit. Set airflow before you touch the refrigerant charge.
Sources & standards
Read the method
CFM Per Ton: The 400 Rule
400 CFM/ton standard air — when to run 350 in humid climates and 450 in dry ones.
Read the guide →What Size Duct for a 3-Ton Unit?
Trunk and return sizes at 1,200 CFM, round-to-rectangular equivalents, and velocity limits.
Read the guide →How Many BTU Per Square Foot?
The 20 BTU/sq ft cooling baseline, climate-zone heating table, and square-footage-to-tons chart.
Read the guide →