What this calculator does
This is the classic conversion pair behind every residential HVAC quote: cooling tons from a heat load, and airflow from tons. It is a sizing rule of thumb, not a load calculation — start from a proper block load and use this to convert the result.
How to use it
- Enter the cooling load you calculated, in Btu/h.
- Choose the cfm-per-ton figure appropriate to your system type — 350 suits low-temperature systems with economisers, 400 is the general default, 450 suits high latent loads.
- The tons and matching airflow come straight out.
Governing formula
tons = Btu/h ÷ 12,000 ; CFM = tons × cfm/ton Where:
-
12,000= Btu/h in one ton of cooling [Btu/h]
Worked example — 60,000 Btu/h load at 400 cfm per ton
The figures below were produced by running this page's own engine with the example inputs (Cooling load (Btu/h), Airflow per ton (CFM/ton)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Cooling capacity required | 5 tons |
|---|---|
| Matching airflow | 2000 CFM |
| Load (for comparison) | 17580 W |
| Basis | Rule-of-thumb: 1 cooling ton = 12,000 Btu/h; 400 cfm/ton is a common default for comfort cooling. |
- 60,000 / 12,000 = 5 tons.
- 5 tons × 400 = 2,000 CFM, which is the airflow a 5-ton system is designed around.
Assumptions & limitations
Screening-level estimate. Rule-of-thumb relationship: 1 ton = 12,000 Btu/h, and airflow is selected as cfm-per-ton. Use a full block-load calculation for final equipment selection.
- Assumes a single cooling system serving a conditioned space — zoned systems and 100 % outdoor air need different airflows.
- No latent-load, economiser or ventilation-air component.
- Equipment selection must satisfy the manufacturer minimum sensible capacity ratio, which rules out undersized coils.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- AHRI Standard 550/590 — rating and testing for unitary air conditioners.
- ACCA Manual J — load calculation.