Sales tax is often expressed as a percentage of a taxable price. The arithmetic is straightforward, but the taxable base and combined local rate can make the real total different from a quick mental estimate.
The basic formula
Tax equals price multiplied by the tax rate as a decimal. Total price equals price plus tax, or price multiplied by 1 plus the decimal rate. A 7% rate on $100 produces $7 tax and a $107 total.
Check what is taxable
Shipping, services, discounts, fees, food, and digital goods may receive different treatment depending on jurisdiction. Use the taxable base shown on the receipt or guidance for the location.
Reverse the calculation
If you know the total and rate, divide the total by 1 plus the rate to estimate the pre-tax amount. Rounding at the register can make the final cents differ, so keep full precision until the last step.
A worked example
Worked example: a $75 taxable item at 8% creates $6 tax and a $81 total. To reverse a tax-inclusive $81 total, divide by 1.08 rather than subtracting 8% from $81.
The arithmetic cannot decide whether a fee, delivery charge, or product is taxable. Confirm the local rule and use the taxable base shown by the merchant or authority.
For a real purchase, record the jurisdiction and the date because tax rules can change. If several items have different exemptions, calculate each taxable group separately before adding the totals.
A correct sales-tax estimate requires both the formula and the right jurisdiction-specific taxable base.
COMMON QUESTIONS
Frequently asked questions
Should I add the tax rate as 7 or 0.07?
Enter the rate in the format the calculator requests. In a formula, 7% is represented as 0.07.
Is sales tax the same everywhere?
No. Rates, exemptions, and taxable items can vary by country, state, city, and transaction type.