How we calculate

Manual and country-specific calculations

The global calculator applies one percentage entered by you. Selecting a country changes the currency and context; it does not select an official rate, identify taxable products, determine exemptions or combine regional taxes.

The dedicated UK VAT and Australian GST calculators provide reference treatments with their own rounding policies. Independent professional tax review is not yet complete. The manual calculator does not model Brazil's transitional taxes, US address-based rates, Canada's multiple tax components or India's GST classifications.

Adding and removing a percentage

For adding tax: tax = amount before tax × rate ÷ 100. The total is the original amount plus the calculated tax.

For extracting included tax: amount before tax = total ÷ (1 + rate ÷ 100). The included tax is the total minus the rounded amount before tax. These formulas apply only to a simple proportional tax.

Decimal arithmetic and rounding

The engine uses decimal.js with 40 significant digits of internal precision. For this manual example, monetary outputs use two decimal places and half-up rounding. In add mode, the tax is rounded before being added to the original amount. In remove mode, the net amount is rounded and the tax is the remaining difference, so the displayed parts reconcile to the entered total. Country-specific legal rounding policies have not been verified.

Amounts accept up to two fractional digits and a maximum of 1,000,000,000,000. Manual percentages accept up to six fractional digits, from zero through 1,000%. These are input limits, not statements about legal tax rates. Calculation engine version: manual-proportional-v1.

Official sources and applicability

Country calculators link to official guidance and explain the supported scope, source dates and exclusions. No treatment is chosen automatically. A rate can depend on the transaction date, location, product or service and any applicable exceptions.

Browse official references for all 20 countries. These references support your research, but do not replace a transaction-specific tax decision or professional advice.

Your data and error reports

Calculations run on your device. This version has no analytics, advertising, tax API, account service, or reporting backend. Only your interface-language preference is stored locally. Calculation links include inputs only after you explicitly choose to copy one. Shared inputs appear in the URL and may be retained by your browser or server access logs.

Error reports download as JSON files and are not sent automatically. Financial inputs are excluded unless you check the inclusion box. Downloading a report does not create a submitted support ticket. The interface fonts and icons are served locally.

Corrections and scope

Report a suspected calculation or display issue from the calculator. A confirmed formula defect must be corrected and retested; switching to a manual percentage does not resolve a formula defect. This version offers informational calculations, not personalized tax advice or legally certified invoices.