A British Thermal Unit (Btu) is a measurement of heat energy. One Btu is the amount of heat required to raise 1lb of water by 1°F. Btu per hour, or Btuh, is the capacity of a heating system such as a 40,000 Btuh furnace.
With air conditioning, you’ll often hear people express the cooling capacity in tons. The term, “tons,” or “tonnage,” is a Canada/USA refrigeration measurement. The ton of refrigeration is equivalent to using one ton of ice per day when using ice for refrigeration was the standard in the days before switching to mechanical refrigeration.
1 ton of air conditioning has the same cooling ability as melting 1 ton of ice in 24 hours.
288,000 / 24 hours = 12,000 Btuh
1 ton of air conditioning = 12,000 Btuh
Once you know the Btuh rating of a system, you simply divide it by 12,000.
Example: A 30,000 Btu air conditioning system / 12,000 = 2.5 tons of cooling