AC Tonnage Calculator: Tons & BTU/hr Needed

Air-conditioner capacity is measured in tons, where one ton equals 12,000 BTU/hr — the cooling a ton of ice gives as it melts over a day. To size a whole house quickly, estimate its cooling load and divide by 12,000.

The load itself comes from floor area times a factor that depends on how leaky the house is and how hard the climate pushes. A tight, well-insulated home in a mild climate needs only about 20 BTU/hr per square foot, which works out to the familiar 600 square-feet-per-ton rule. An average home lands near 24 BTU/hr per square foot, or 500 sq ft per ton, and a poorly insulated or very sunny home closer to 30, or 400 sq ft per ton. A 1,500 square-foot average home therefore needs about 36,000 BTU/hr, which is exactly 3.0 tons.

Because condensers are only sold in half-ton steps — 1.5, 2, 2.5, 3, 3.5, 4, and 5 tons — the calculator rounds your load up to the next stocked size. Resist the urge to round up two sizes "to be safe": an oversized system short-cycles, cools unevenly, and leaves humidity behind. A Manual J load calculation is the definitive way to pick between two sizes before you buy.