How the sales tax calculator works
Why the rate is never just the state's
US sales tax stacks: a state rate plus county, city, and special-district rates. That is why California's statewide 7.25% becomes 9.5–10.25% in Los Angeles, and why "state rate" searches mislead for anything local. The 45 states with sales tax run combined averages from ~4% to ~9.5%; five states (DE, MT, NH, OR) charge none.
What is taxable varies too
Groceries are exempt or reduced in most states (fully taxed in the South's grocery-tax states), clothing is exempt in a few (PA, MN, NJ on most items), and services are mostly untaxed outside a handful of states. Two identical purchases can carry different tax lines across a state line — or across a product category in the same store.
The online era
Post-2018 (South Dakota v. Wayfair), most large online retailers collect destination-based sales tax — your rate, not theirs. Marketplace purchases usually include it automatically; the calculator's forward mode tells you what that line should be, so the checkout math gets checked instead of trusted.