Caution: The "Official" RFA Calculator has known errors on many properties and property types. If it passes you will have 30 days from their bill to appeal their charges and you will need to have information in which to challenge their assessment.
Go to Your Property data on the Thurston County Assessor's Website to find your total square footage by clicking here. https://tcproperty.co.thurston.wa.us/propsql/front.asp
Remember the RFA includes your living space PLUS garages, basements, finished attics, car ports, balconies, gazebos and sheds etc.
It is important to remember that what you think your s/f is probably not what the RFA will use to charge you. They count your garage, you covered porches, your attics and finished basements and any other structure on your property.
Note that the bigger the apartment building(s) on the parcel the lower the cost per square foot. So smaller apartment buildings owned by small landlords will pay a higher rate than a large corporate apartment complex.
Also, if your apartment has a commercial space in it, such as a restaurant or offices you will not be charged at the Apartment rate factor of 1.50. Instead, you are considered a "mixed-use" building and will be charged as a Commercial property which rate factor ranges from .6 to 5.0.