How we calculate

Manual arithmetic, not an official tax lookup

This local version 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. All 20 country configurations await official-source review.

Brazil’s transitional and multi-component rules, US address-based rates, Canada’s multiple tax components, and India’s GST classifications are not modeled by this manual calculator.

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.

Sources and publication

No official rate presets are provided in this version. Before any are published, each rule must identify an appropriate official document, jurisdiction, transaction scope, effective date, review date, exceptions, and reviewer approval. A future rate or a rule awaiting review must never be presented as a verified preset.

The official reference register records source discovery for all 20 countries, including retrieval gaps and outstanding validation. It is not full legal review or approval of executable rules.

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.