AC Tonnage Calculator

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.