2 times the sq feet and add a zero. If you have a 3000 sq ft area times it by 2 which is 6000 and add a zero. 60,000 btu's is needed for cooling or heating. Of course windows, two levels, and insulation play a role in how fast and how well the area will cool or heat. However I would recommend always upsizing to the next ton level depending how your math formula plays out. 12,000 btu's is one ton so if your formula says you need 16,000 btu's I would go with a 18,000 btu unit, because units are usually sold by the ton or the ton and a half. 6000 btu = half ton (300 sq ft) 12000 btu = 1ton (600 sq ft) 18000 btu =1 and a half tons ( 900 sq ft) 24,000 btu = 2 tons( 1200 sq ft) 30,000 btu =2 and a half tons(1500 sq ft) and so on and so on.