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Articles · HVAC Basics · Updated Sept 2026
What "tons" means on an air conditioner, and how to pick the right size without guessing.
Short Answer
A "ton" on an air conditioner has nothing to do with how heavy it is. It's a measure of how much heat the system can pull out of your house in an hour, and one ton is 12,000 BTU per hour.
The odd name is usually traced back to ice. Melting a 2,000 lb ton of ice takes about 288,000 BTU (roughly 144 BTU a pound). Spread that over 24 hours and you get 12,000 an hour, which is the figure NIST lists for a ton of refrigeration.
So a 3-ton central system removes roughly 36,000 BTU an hour at its rated conditions. A 12,000 BTU wall unit? That's a 1-ton system.
Conversion Table
| Tons | BTU per hour |
|---|---|
| 0.75 | 9,000 |
| 1 | 12,000 |
| 1.5 | 18,000 |
| 2 | 24,000 |
| 2.5 | 30,000 |
| 3 | 36,000 |
| 3.5 | 42,000 |
| 4 | 48,000 |
| 5 | 60,000 |
The math is simple. BTU ÷ 12,000 = tons, and tons × 12,000 = BTU.
You can often read the size right off the model number. On a Goodman condenser, for example, a "36" in the model number means 3 tons, or 36,000 BTU. Double-check the data plate or spec sheet, though. Every manufacturer names things a little differently.
Rules of Thumb
You'll hear shortcuts like one ton for every 400 to 600 square feet (ACCA's blog describes exactly that habit). They're easy to remember. They're also easy to get wrong, because two houses with the same floor area can need very different amounts of cooling.
Insulation matters. So do the size and direction of your windows, how leaky the house is, ceiling height, and where you live.
The U.S. Department of Energy is blunt about it. Rules of thumb "are too often used to size comfort systems, which results in excessively oversized systems." And bigger isn't safer. An oversized unit runs in short bursts, pulls less moisture out of the air, and costs more up front.
Single Rooms
If you're cooling one room, ENERGY STAR has a simple chart for room air conditioners. Treat it as a first estimate for a single space. It won't size a whole house.
| Room size (sq ft) | Capacity (BTU/hr) |
|---|---|
| 100–150 | 5,000 |
| 150–250 | 6,000 |
| 250–300 | 7,000 |
| 300–350 | 8,000 |
| 350–400 | 9,000 |
| 400–450 | 10,000 |
| 450–550 | 12,000 |
| 550–700 | 14,000 |
| 700–1,000 | 18,000 |
Then adjust for the room itself:
Here's how that plays out. A 500 sq ft living room starts at 12,000 BTU. It bakes in the afternoon sun, so add 10% to get 13,200. Four people usually hang out there, which adds another 1,200. You end up near 14,400 BTU, so you'd look at a 14,000 to 15,000 BTU unit instead of a 1-ton one.
Whole House
Sizing a whole home properly means a load calculation called ACCA Manual J, which can be run room by room. According to ACCA, national building codes and most state and local jurisdictions require a load calculation done to the Manual J procedure. A good contractor can show you that report. If all you get is a tonnage number, ask for more.
A few questions worth asking when you compare quotes:
Want your own number first? The sizing calculator on this site asks for room size, your ZIP code, ceiling height and windows, then does the math.
FAQ
30,000 BTU per hour. Multiply 2.5 by 12,000 and you're there.
It depends on the house. The common 400-to-600-square-feet-per-ton shortcut would put it around 1,200 to 1,800 sq ft, but insulation, windows and climate can push the real answer well outside that. A Manual J load calculation gives you the actual number.
Usually not. An oversized system cycles on and off more, leaves more humidity behind and runs less efficiently. Size it to the calculated load.
The name is usually traced back to cooling with ice. Melting one ton of ice over 24 hours takes about 288,000 BTU, which works out to 12,000 BTU per hour.