In heating and cooling, a ton has nothing to do with weight. One ton is a measure of cooling power, equal to 12,000 BTU (British Thermal Units) of heat removed per hour. So a 3-ton air conditioner pulls 36,000 BTU of heat out of your home every hour.
The ton rating tells you how much cooling a system delivers. Most central systems sold for homes fall between 1 and 5 tons, and getting this size right is what keeps both your comfort and your power bill in check. A unit that is too large cools the air quickly but shuts off before it removes humidity, leaving rooms cold and clammy.
The unit comes from the days before mechanical refrigeration, when buildings were cooled with blocks of ice. Melting one ton of ice absorbs about 286,000 BTU of heat, and spreading that over 24 hours works out to roughly 12,000 BTU per hour. Engineers rounded to that figure, and the word ton stuck as the standard label for cooling capacity.
When you collect quotes, every contractor should land on a similar tonnage for your home. A bid that is a full ton larger than the others is a warning sign of lazy sizing. The right way to set tonnage is a Manual J load calculation — an industry-standard worksheet from the Air Conditioning Contractors of America that accounts for your insulation, windows, and climate, not just square footage. Ask to see that calculation before you sign.