On One Hand: Rule of ThumbFor a rough estimate of how big your new click here HVAC system should be, some contractors recommend 1 ton (12000 BTUs) for every 800 square feet you wish to heat or cool. This would then be adjusted up or down, depending on local climate.
Source:Energy Savers: Sizing Heating and Cooling Systems
Furnace Compare: HVAC hvac website Sizing Rules of Thumb
Furnace Compare: HVAC Sizing Calculations