Ratings & Efficiency
What Is Tonnage (Nominal Capacity)?
Tonnage is the nominal cooling capacity of a system, where one ton equals 12,000 BTU/h. A three-ton air conditioner is rated to remove about 36,000 BTU of heat per hour. The term comes from the cooling once produced by melting a ton of ice in a day, not from the weight of the equipment.
Also called: tons · nominal capacity · ton of cooling
Why it matters to you
Oversizing is the common error on the coast, and it backfires: a system with too much capacity satisfies the thermostat quickly, shuts off, and never runs long enough to pull moisture out of the air. In a humid coastal summer that leaves a house that is cold and clammy rather than comfortable, and it shortens compressor life through short cycling.
What to do about it
Capacity should come from a Manual J load calculation on your actual house, not from the size of the old unit or a rule of thumb per square foot. If a quote names a tonnage without a load calculation behind it, ask how the number was reached.
By the numbers
| Tons | BTU/h |
|---|---|
| 1.5 | 18,000 |
| 2.0 | 24,000 |
| 2.5 | 30,000 |
| 3.0 | 36,000 |
| 4.0 | 48,000 |
| 5.0 | 60,000 |
Nominal tonnage to BTU/h
Common questions
- Is a bigger system better?
- No. Correct sizing beats extra capacity. An oversized system cools the air to setpoint faster than it removes humidity, so it short-cycles and leaves the house damp. In a climate where latent load is a large share of the total, matching capacity to the real load is what delivers comfort.
- Can I just replace like for like?
- Not always safely. The previous unit may itself have been mis-sized, and the house may have changed — new windows, added insulation, an enclosed porch, a different duct layout. A load calculation on the house as it stands today is the only way to know whether the old tonnage is still right.
- How does tonnage relate to BTU?
- Directly: one ton is 12,000 BTU/h. So a 2.5-ton system is 30,000 BTU/h, and a 4-ton system is 48,000 BTU/h. Equipment is often listed either way, and the conversion is exact.
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