tax calculator

Sales Tax Calculator

The Sales Tax Calculator models the arithmetic relationship between a before-tax price, an entered sales tax rate, the tax amount, and the after-tax price. It can add tax, remove tax from a tax-inclusive total, or estimate an effective entered rate from before-tax and after-tax prices.

Result

Result summary: Sales tax is $6.50 on a $100.00 purchase, making the after-tax price $106.50.
Before tax price: $100.00
Sales tax rate: 6.50%
Sales tax amount: $6.50
After tax price: $106.50

Scenario nameNotes

Save this result, then use saved scenarios to switch between assumptions.

Before you calculate

Use this calculator when you already have a rate assumption and want to check checkout arithmetic, read a receipt, or compare a tax-exclusive price with a tax-inclusive total. Use Discount before Sales Tax when a markdown changes the taxable starting price, VAT when the question is a VAT-style net or gross price, Payment or Loan Payment when the tax-inclusive amount feeds a financing estimate, and official state, local, or tax authority tools when you need a current rate or rule.

Formula summary

For after-tax price, convert the entered rate to a decimal with r = sales tax rate / 100, calculate tax amount = before-tax price * r, and after-tax price = before-tax price + tax amount. For before-tax reverse solve, before-tax price = after-tax price / (1 + r), then tax amount is max(0, after-tax price - before-tax price). For rate solve, tax amount is max(0, after-tax price - before-tax price), and sales tax rate = tax amount / before-tax price * 100 when before-tax price is greater than 0; if before-tax price is 0, the app returns 0 percent instead of dividing by zero.

Worked example

Example: add tax, solve rate, remove tax, and handle zero rate

With the default inputs, a $100.00 before-tax price and a 6.5 percent entered rate produce $6.50 of sales tax and a $106.50 after-tax price. A receipt-style rate solve with $80.00 before tax and $86.00 after tax reports $6.00 of tax and a 7.50 percent effective rate. A reverse before-tax example with a $107.50 after-tax total and a 7.5 percent entered rate solves back to $100.00 before tax and $7.50 of tax. A zero-rate example with $49.99 before tax stays $49.99 after tax and reports $0.00 of tax.

The calculator explains the entered-rate math. It does not decide which rate applies, whether an item is taxable, whether shipping or discounts are taxable, or how a tax deduction should be claimed.

Assumptions

The sales tax rate is an entered assumption. The calculator does not look up state, county, city, district, address-level, product, exemption, tax-holiday, shipping, delivery, marketplace, use-tax, seller-registration, filing, or deduction rules.
Currency values and percentages display to two decimals after full-precision math. Rate-solve mode floors a negative tax difference at 0, and the public input model accepts rates from 0 percent through 100 percent.
Scoped independent comparator checks pass 4 of 4 scoped fixtures for after-tax price, effective-rate solve, before-tax reverse solve, and zero-rate handling. That validation covers the same generic three-value calculator, not current jurisdiction-specific rate tables or tax law. Results are educational estimates, not financial, tax, legal, accounting, filing, consumer-rights, or personalised purchasing advice.

Common mistakes to avoid

Using an old, rounded, or statewide-only rate when the real transaction depends on state, county, city, district, address, product type, seller rules, or current tax authority guidance.
Mixing tax-inclusive and tax-exclusive prices. Label whether a number is before tax or after tax before using it in a receipt-style reverse solve.
Treating the output as a tax bill, final receipt answer, itemized-deduction answer, exemption decision, shipping-taxability answer, marketplace-rule answer, or legal conclusion. Official tax authority, retailer, marketplace, and legal sources can change the real result.

Methodology and sources

This calculator is an original implementation based on documented formulas, app-specific assumptions, deterministic fixtures, edge cases, rounding policy tests, and internal validation. It is not copied from a single source.

Outputs are checked with deterministic fixtures, edge cases, rounding policy tests, and internal independent comparator checks where overlapping outputs are available. The result remains an educational estimate, not a quote, approval, tax answer, or personalised advice.

Formula version 2026.05.22-generic-sales-tax. The version marks the calculation logic and validation fixture set used for this estimate.

State, local, or tax authority rate pages should be used when a real sales tax rate is entered.
Official examples include state revenue department guidance and address-level lookup tools such as California CDTFA sales and use tax rate resources or Washington Department of Revenue sales and use tax rate resources.
IRS sources are needed only if a separate page discusses federal itemized sales-tax deduction treatment; this generic calculator does not determine deductions.

Estimate only

The result is educational and is not financial, tax, legal, lending, investment, or regulated advice. Real provider terms, fees, rates, taxes, and personal circumstances can change the final answer.

Formula and help: read the full sales tax calculator methodology notes.