Sales tax calculator

Adding runs forward: tax is price times rate over 100, and the total is price plus tax. Extracting runs the same rate backward: a total of T at rate r holds T times r over (100 + r) as tax, and the price is whatever is left. The two directions are exact inverses, so nothing drifts when you flip a number from one mode to the other. What this does not do: jurisdiction rounding rules (tax to the nearest cent per line or per receipt), tax tiers within one receipt, discounts applied before or after tax, and any tax that is not a flat percentage. Rates change; check yours against the current published rate.