The short answer
At an 8.25% rate, a $100 item costs $108.25 with tax added. Working backward, a $108.25 total at 8.25% breaks down to $100.00 before tax and $8.25 in tax. And if you paid $108.25 for something priced at $100, the effective tax rate was 8.25%. All three modes describe the same relationship — you just have to know which two numbers you already have.
Key takeaways
- Adding tax multiplies: total = price × (1 + rate). Extracting tax divides: pre-tax price = total ÷ (1 + rate) — dividing by the rate itself, rather than the (1 + rate) factor, is the most common mistake people make.
- Sales tax rates in the US stack: a state rate plus city and county add-ons combine into one "combined" or "effective" rate, which is what this calculator's rate field expects.
- Not every state charges sales tax — Delaware, Montana, New Hampshire, and Oregon have no statewide sales tax, so a 0% rate is a legitimate input in those states.
- US sales tax applies once, at the final retail sale — unlike VAT, which is charged at each production stage but ultimately borne by the end consumer either way.
The three formulas, side by side
Add tax: total = $100 × (1 + 0.0825) = $108.25
Extract tax: before-tax = $108.25 ÷ (1 + 0.0825) = $100.00
Find rate: rate = ($108.25 − $100.00) ÷ $100.00 = 8.25%
All three modes work off the same relationship between price, tax, and total — they just start from different known values. Use "Add Tax" when you know the sticker price and the local rate and want the final total. Use "Extract Tax" when a receipt or invoice only shows a tax-included total and you need to know how much was pre-tax versus tax (common for expense reports or accounting). Use "Find Rate" when you have both the before-tax price and the total and want to confirm or discover the rate that was actually applied — useful for checking a receipt against the rate your locality is supposed to charge.
The most common extraction mistake
A frequent error is subtracting the tax rate directly from the total instead of dividing by (1 + rate). On a $108.25 total at an 8.25% rate:
Wrong: $108.25 × (1 − 0.0825) = $99.32 (understates the true pre-tax price)
Right: $108.25 ÷ (1 + 0.0825) = $100.00 (correct pre-tax price)
The tax rate applies to the pre-tax price, not the tax-included total, so simply knocking a percentage off the total overcorrects and understates the real pre-tax amount. This is exactly the calculation this calculator's "Extract Tax" mode automates correctly, so it's a good way to double-check manual math on a receipt or invoice.
Related calculators
- VAT Calculator — for value-added tax used outside the US.
- Discount Calculator — combine a sale discount with tax to find your true final price.
- Margin Calculator — for retailers pricing goods before tax is even applied.