The smallest residential unit is 1.5 tons (18,000 BTU) and the largest is 5 tons. On average you need 1 ton of cooling for every 1000 square feet of home.
AVERAGE TON PER SQUARE FOOT OF HOME
1000 sq.ft = 1 ton
2000 sq.ft = 2 ton
3000 sq.ft = 3 ton
4000 sq.ft = 4 ton
5000 sq.ft = 5 ton
HISTORY OF AIR CONDITIONING IN TONS
Before refrigeration air conditioning was invented, cooling was done by saving big blocks of ice. When cooling machines first started to be used, they rated their capacity by the equivalent amount of ice melted in a day. This is where the term “ton” came from in sizing an air conditioner.