A 10-ton air conditioning unit is a cooling system capable of removing 120,000 British thermal units of heat per hour from the air in the building to which it is attached. In this system of measurement, 1 ton is equal to 12,000 BTUs of heat extraction per hour.
Prior to the invention of modern air conditioning, total cooling capacity was measured in tonnage, referring to the amount of ice that was used. The amount of heat that is necessary to melt 1 ton of ice in 24 hours is 286,000 BTUs, or about 12,000 BTUs per hour. This is the basis for the measurement of modern cooling capacity.