Sales tax is added at the register as a percentage of the pre-tax price. Rates vary widely by country, state, and even city, so the same item can cost different amounts depending on where you buy it.
How it's calculated
Multiply the pre-tax amount by the tax rate to get the tax, then add it back to get the total. This tool shows all three so you can check a receipt or budget for a purchase.
A worked example
On a $100 purchase with an 8.25% rate, the tax is $8.25 and the total is $108.25. On a $1,000 laptop at the same rate, the tax is $82.50 — the same percentage, a much larger dollar amount. That's why the rate difference between neighboring towns barely matters on a coffee but adds up on big-ticket items like appliances or furniture.
Where rates come from and what's taxable
In the US, sales tax is set at the state level and often topped up by county and city rates, so a single address can carry several stacked rates that combine into the number on your receipt. Five states have no statewide sales tax at all.
What's taxable also varies. Many states exempt groceries and prescription drugs, some tax prepared food differently from raw ingredients, and rules for digital goods and services differ widely. For a firm figure, use your exact local rate rather than a state average.
Working backward from a total
To find the pre-tax price from a tax-included total, divide by one plus the rate: a $108.25 total at 8.25% came from a $100 item ($108.25 ÷ 1.0825). That's useful for expense reports or receipts that only print the total, and for splitting a tax-inclusive price into its base and tax parts.