THE NUMORIX GUIDE
How to use the Sales tax calculator
Last reviewed September 14, 2026
What this calculator does
The route rearranges the relationship between before-tax price, tax rate, and after-tax price.
Formula and method
The route rearranges the relationship between before-tax price, tax rate, and after-tax price. After-tax mode uses after = before x (1 + rate/100). Before-tax mode divides the entered after-tax amount by that factor. Tax-rate mode solves rate = (after / before - 1) x 100, then rounds the displayed values.
Variables and inputs
Choose the field to solve for through the page mode, then provide the other two values: before-tax price, sales-tax rate, and after-tax price. Prices are currency and the rate is a percentage. The tax amount is after-tax price minus before-tax price.
Worked example
For a $250 before-tax price and an 8.25% rate, after-tax price is 250 x (1 + 8.25 / 100) = 250 x 1.0825 = $270.625, displayed as $270.63. The tax amount before display rounding is 270.625 - 250 = $20.625.
How to interpret the result
The result shows the arithmetic relationship among the three values. It does not decide which items are taxable or whether shipping, discounts, services, or local surtaxes belong in the taxable base. A receipt can therefore differ from a purely mathematical estimate.
Common mistakes to avoid
Use 8.25 for 8.25%, not 0.0825 in the percent field. When solving a rate, divide after-tax by before-tax before subtracting one. Do not calculate a tax amount from a rounded display if the unrounded price matters to the transaction.
Assumptions and limitations
The calculator applies one rate to one taxable base and does not contain jurisdiction rules, exemptions, caps, use-tax treatment, or taxability of shipping and services. Validate the tax base and current rate with the relevant state and local authority.