It is a measurement of capacity that equals the cooling accomplished by melting one ton of ice per day, and is a holdover from the era when refrigeration was transitioning from stored natural ice to mechanical refrigeration. But the ton is still used today in North America as a shorthand way of expressing the size of an air conditioning system, similar to the use of horsepower, another archaic term, to rate car engines. 

    The rest of the world uses the BTU (British Thermal Unit) as the standard measurement, and a ton is equal to 12,000 BTU. The advantage of the BTU is that it’s more accurate and universally used, but the ton persists. 

   Most residential air conditioning systems are between 1.5 and 5 tons, measured in half-ton increments; except that there is no 4.5 ton size available. So a 2-ton system is 24,000 BTU, for example, and a 3.5 ton is 42,000 BTU. A system rated at 2.5 tons may a little over or under 30,000 BTU and still be considered “two-and-a-half-tons.”

   The first two digits of the BTU rating is typically embedded in the model number on the manufacturer’s data plate of the condenser (outdoor unit} of the system. Look for a number divisible by 12 or 6. Here’s a few examples from different companies, with the BTU number circled. They are 3, 3-1/2, and 4 tons.

    Also see our blog post How can I find out the age of my air conditioner or furnace?

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  To learn more about heating and air conditioning systems, see these other blog posts:

• How can I find out the SEER of my air conditioner? 

• My air conditioner won't turn on. What's wrong? 

• How can I find out the size of my air conditioner? 

• How can I tell whether the condenser (outdoor unit) is an air conditioner or heat pump? 

• Where is the air filter for my central air conditioner and furnace? I can’t find it? 

• Does an old air conditioner use more electricity as it ages? 

• How did homes stay cool in Florida before air conditioning? 

• What is wrong with an air conditioner when the air flow out of the vents is low?

• Why has the thermostat screen gone blank? 

• Why does it take so long to cool a house when an air conditioner has been off for a while? 

• Why is my air conditioner not cooling enough? 

• What are the most common problems with wall/window air conditioners?  

• Will closing doors reduce my heating and cooling costs? 

   Visit our HEATING AND AIR CONDITIONING page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.