CFM vs ACH: Which Airflow Calculation Do You Need?
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Use ACH when the goal is replacing or cleaning a room's air a certain number of times per hour: CFM = volume × ACH ÷ 60. Use the sensible heat equation when the goal is removing a cooling load: CFM = sensible BTU/h ÷ (1.08 × ΔT). They are not substitutes.
CFM (cubic feet per minute) is an airflow rate. ACH (air changes per hour) expresses that rate relative to a room's volume. Both are useful, but for different jobs.
Run the numbers with the Room-by-Room CFM Planner.
The two formulas
| Purpose | Formula | Inputs you need |
|---|---|---|
| Air changes (ventilation, air cleaning) | CFM = ft³ × ACH ÷ 60 | Room volume, chosen ACH target |
| Cooling supply air | CFM = BTU/h ÷ (1.08 × ΔT) | Room sensible load, supply ΔT |
| Convert CFM back to ACH | ACH = CFM × 60 ÷ ft³ | Measured or rated CFM, volume |
Worked example
A 12 × 15 × 8 ft bedroom holds 1,440 ft³. An air cleaner rated at 120 CFM gives 120 × 60 ÷ 1,440 = 5 ACH. If the same room's Manual J sensible cooling load is 3,500 BTU/h and the system supplies air 20 °F below room temperature, supply airflow is 3,500 ÷ 21.6 ≈ 162 CFM. The 5 ACH figure tells you nothing about whether 162 CFM of cool air is delivered.
Where 1.08 comes from
1.08 ≈ 0.075 lb/ft³ (standard air density) × 60 min/h × 0.24 BTU/lb·°F (specific heat of air). At high altitude air is less dense and the constant is lower.
Common mistakes
- Picking an ACH number and calling it the HVAC airflow
- Using total (sensible + latent) load in the 1.08 equation
- Converting CFM straight to a duct diameter without a Manual D friction-rate design
Assumptions and limitations
ACH targets are chosen by purpose; ventilation requirements for dwellings are set by ASHRAE 62.2 and local codes, not by this formula. Duct sizing belongs to ACCA Manual D.