AC Tonnage Calculator

A ton is a unit of cooling capacity, not the weight of the equipment. One ton equals 12,000 BTU per hour, the amount of heat needed to melt one ton of ice over 24 hours. So a 3-ton AC removes about 36,000 BTU of heat per hour from your home.

Start with a base cooling load of about 20 BTU per square foot for a moderate climate, then adjust for conditions. This calculator multiplies your living area by a climate factor (14 to 25 BTU per square foot), scales it for ceiling height above 8 feet, adjusts for sun exposure, and adds 600 BTU per occupant. Dividing the total BTU by 12,000 gives the tonnage.

Cooling load depends on the volume of air your system has to condition, not just the floor area. The base BTU per square foot figures assume an 8-foot ceiling, so a 10-foot ceiling raises the load by a factor of 10 divided by 8, or 1.25. Taller rooms hold more air, which means more heat to remove.

An oversized unit short-cycles, turning on and off too quickly to pull humidity out of the air, which leaves your home cool but clammy and wears out the compressor. An undersized unit runs constantly and still struggles to keep up on hot days. Right-sizing keeps both comfort and efficiency in balance, which is why the result rounds to the nearest standard half-ton unit.

No. This is a planning estimate to help you understand the ballpark size before you talk to a contractor. A full ACCA Manual-J calculation accounts for window area and orientation, insulation R-values, air leakage, ductwork, and local design temperatures. Always have a licensed HVAC professional run a Manual-J before you buy or install a system.