Property tax is usually calculated as an assessed value multiplied by a local tax rate (a "mill rate" or percentage), set by the county/municipality and used to fund schools, infrastructure, and local services. Rates vary widely — not just state to state but county to county, and sometimes reassess upward after a sale at the new purchase price.
This calculator auto-fills an estimated monthly property tax based on the property's location using typical county/state rates, but always verify the actual current tax bill (and whether it will be reassessed after your purchase) with the county assessor or your title company before finalizing a deal.