The stack
A US combined rate has up to four layers: the state base, the county, the city, and special districts (transit, tourism, stadiums). Chicago combines to 10.25%; Long Island City, NY to ~8.875%; Portland, OR to 0%. The state's "sales tax rate" headline is one layer of the stack — the register runs the whole thing.
Finding your true rate
Your state's department of revenue publishes a rate lookup by address (search "your state + sales tax rate lookup"). Enter the combined figure into the custom field of any calculator here. For an online purchase, the rate follows the shipping address — destination-based since the Wayfair era.
The exemptions layer
Rates are half the system; the taxability matrix is the other half. Groceries: exempt in ~32 states, reduced in some, fully taxed elsewhere. Clothing: exempt in PA, MN, NJ, MA (with limits). Services: mostly untaxed, with a growing taxed list. Two neighbors can pay different tax on the same cart — legally.