The best way to find out what size HVAC system you need for your home is to get a professional “Manual J calculation,” that considers factors like square footage, natural shade or sunlight and how many heat-generating appliances you have.
However, you can get a rough estimate by finding the square footage of your floor space and multiplying it by 25 to get the base BTU (British thermal unit) measurement — that’s the unit used to measure the energy used for heating and cooling. Once you have the BTUs, you can use that to determine what size HVAC unit you need.