If you’re unsure what size air conditioning system you’ll need for your home, here is a simple formula that can help you to find out a rough idea of what the required tonnage will need to be: ( ( House square footage times 30 then divided by 12,000 ) – 1.0 ). If you’re still unsure of what size AC you will need, contact a professional HVAC technician. Your HVAC contractor will be able to give you the most accurate idea of how big your new air conditioning system will need to be for your home.