Air conditioners need roughly 400 CFM of airflow per ton. Tons × the CFM/ton target gives the required system airflow — the basis for duct and blower sizing.
The example
4 tons × 400 CFM/ton.
| A | B | |
|---|---|---|
| 1 | Item | Value |
| 2 | 4 × 400 | — |
| 3 | Airflow | → 1,600 CFM |
The formula
The formula:
How it works
How it works:
- Use the CFM/ton target — ~400 for cooling, ~350 in humid climates.
- Multiply by tons for total system CFM.
- Per-room CFM = room load ÷ total load × system CFM for balancing.
- Verify with a measured airflow (TrueFlow/static pressure) — ducts often underperform.
Low airflow is the hidden cause of comfort and efficiency complaints. A correctly sized AC with undersized or leaky ducts can’t move its rated CFM, so it freezes coils, short-cycles, and never cools evenly. Compute the target (tons × 400), then measure actual airflow — the gap is the real problem on many service calls. Per-room CFM splits the total by each room’s share of the load for balancing.
Try it: interactive demo
Tons and CFM per ton.
Variations
Per-room CFM
Balance by load:
From BTU
If you have BTU:
Tons from airflow
Back out:
Pitfalls & errors
~400 CFM/ton. Use 350 in humid climates for more dehumidification.
Measure airflow. Target is design; ducts often deliver less.
Per-room by load. Split CFM by each room’s share of the load.
Practice workbook
Frequently asked questions
How do I calculate required airflow per ton in Excel?
How do I split CFM per room?
What's the CFM-per-ton target?
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