In HVAC, a ton is a unit of cooling capacity equal to 12,000 BTU per hour. The term originates from the amount of heat required to melt one ton (2,000 lbs) of ice in 24 hours — a measurement historically used to compare cooling systems before mechanical refrigeration was widespread.
Residential air conditioners typically range from 1.5 to 5 tons. A 3-ton system produces 36,000 BTU/hr of cooling. As a rough rule of thumb, plan for approximately 1 ton per 400-600 square feet of living space, though climate, insulation, and other factors significantly affect the actual requirement.
Always size HVAC equipment using a proper Manual J load calculation rather than the square footage rule of thumb alone.