AC Tonnage Calculator

How it works

The total cooling capacity is the floor area multiplied by the load factor in BTU per hour per square foot. Dividing that capacity by 12,000 converts it to tons, because a ton of refrigeration is defined as 12,000 BTU per hour. The kilowatt figure divides the same capacity by 3,412, the number of BTU per hour in a kilowatt. The nearest standard size rounds the exact tonnage up to the next half ton, matching the sizes air conditioners are actually sold in, so you are never left specifying a unit smaller than the room needs. Because the calculation rests on a single rule of thumb factor, the honest way to use it is to adjust that factor for the room in front of you rather than trusting one number for every space.

Worked example

Take a 600 square foot open plan room with average sun and a normal ceiling, using the default 20 BTU per hour per square foot. The total capacity is 600 multiplied by 20, which is 12,000 BTU per hour. Dividing by 12,000 gives exactly 1.0 ton, and dividing 12,000 by 3,412 gives about 3.52 kW. Since 1.0 is already a standard size, the nearest standard size is also 1.0 ton. Double the room to 1,200 square feet and the capacity rises to 24,000 BTU per hour, which is 2.0 tons, while a sunnier room at 25 BTU per square foot over the same 1,200 square feet would need 30,000 BTU per hour, or 2.5 tons.

Related calculators

• Air Conditioner BTU Calculator: metric room sizing with climate zones.
• BTU to Tons Converter: convert a known BTU/hr to tons.
• AC Wattage Calculator: power draw and running cost.
• Electricity Cost Calculator: cost of running the unit.
• Heat Pump Savings Calculator: heating cost comparison.