29/7/2017 · ((House square footage times 25, divided by 12,000 BTU) – 0.5) = required tons. So, as an example, a 1,500-square foot home would look something like this: 1. 1,500 ft2 x 25 = 37,500 ft2 2. 37,500 ft2 / 12,000 BTU = 3.1 tons 3. 3.1 – 0.5 = 2.6 tons, so you