In the old days, cooling was done by saving big blocks of ice. When cooling machines were first used, their capacity was rated by the equivalent amount of ice that would melt in a day, which is where the term “ton” came from in sizing air conditioning.

Today, a ton of cooling is defined as delivering 12,000 BTU/hour of cooling. BTU is short for British Thermal Unit. The BTU is a unit of heating – or in this case, cooling – energy. A window air conditioner is usually less than one ton. A small home central air conditioner would be about two tons and a large one about five tons.