2,000 square feet x 20 BTU = 40,000 BTU. Since 1 ton equals 12,000 BTU, you'll divide 40,000 by 12,000 with a math equation that looks like this: 40,000 BTU / 12,000 BTU = 3.33333 tons. Round that up, and you're looking at a 3.5-ton AC unit.