Accurate square footage of a home?

Can one of you expert realtors please tell me how to accurately measure the total square footage of living space in my home? I'd like to claim my home on Zillow and update/edit the facts, which are near non-existent on Zillow. There is nothing on public record that I can find or track down anywhere or in my own official paperwork on my house that indicates the accurate square footage of my home since it was remodeled in 1975 (two previous owners before me) and since we bought it in 1991. Apparently in 1975, a 2-car garage was converted to 2 additional bedrooms, a den, and full bath, and a 3-car attached garage was added. My home is still listed on public record as what is was when it was originally built in 1967: a 3-Br, 2-Ba home with a 2-car garage and 2,352 sq. ft. When it was appraised in 1996, the appraiser accurately identified all the rooms and indicated that it was a 5-bedroom, 3-bath, 3-car garage home, but listed the square footage as 2,220, LESS than the original square footage when it was built and obviously less than it should be after  the remodel of 2 additional bedrooms, a den, and bath. (She did not count the foyer or laundry room, which I understand.) One MLS snapshot record listed my house as 2250-2500 sq. ft.!!! Well, what IS it? Square footage matters in the estimated value and pricing of a home, and for doing a comparative analysis. This is nuts. Which rooms do I measure to get an accurate square footage of living space? Thanks so much! (I understand that for whatever reason, some public records may not have been updated when someone did a remodel or addition, and I don't wish to pursue correcting the facts about my house anywhere but on zillow.)