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Methodology and sources: understand and check our calculations

A result is useful when you can understand how it was obtained. This page explains our calculators’ conventions, the checks we carry out and the references used in our guides. To check tax information, use the table of contents to find sources by topic.

How amounts are calculated

The calculator uses the amount and rate you enter. It calculates a consistent amount excluding VAT, VAT amount and total including VAT: net amount + VAT = gross amount. It does not select the rate that applies to your transaction.

Starting with a price excluding VAT, the tax is calculated and rounded; the gross amount is the sum of the net amount and that tax. Starting with a price including VAT, the net amount is found by dividing by the VAT multiplier and rounding; the tax is the difference. Starting with the tax amount alone, the net amount is calculated from the known rate; this is not possible at a rate of 0%.

Which rounding convention is used

Amounts are rounded to the nearest cent. Values exactly halfway between two cents are rounded away from zero: +€0.015 becomes +€0.02 and −€0.015 becomes −€0.02. Converting back from a rounded value may lose information beyond the second decimal place.

The examples below let you reproduce the expected result. They describe the tool’s convention; on their own, they do not determine the method required for a particular document or transaction.

Examples for checking a result
InputExcluding VATVATIncluding VAT
€100 excluding VAT at 21%€100.00€21.00€121.00
€100 including VAT at 21%€82.64€17.36€100.00
€99.99 excluding VAT at 21%€99.99€21.00€120.99

Precision and input limits

Calculation engine version: 1.1.0.

Entered amounts can have up to two decimal places and are converted into whole cents. Rates can have up to two decimal places in percentage terms. The calculation engine uses BigInt integers for intermediate multiplications and divisions, then rounds at the designated step. VAT is not calculated by multiplying monetary amounts using floating-point arithmetic.

The main calculator accepts positive and negative amounts from −€999,999,999.99 to €999,999,999.99. A dot or comma can be used as the decimal separator, and spaces can group thousands. Ambiguous formats, values outside the limits and expressions are rejected. An error clears the previous results so that they cannot be mistaken for a response to the new input.

The engine version identifies the convention used by the calculators. Project tests check its amounts, limits and reference results. This technical version does not certify the tax treatment of a transaction.

Multiple lines and simulation exports

The multiple-line table calculates VAT after multiplying each line’s whole-number quantity by its unit price excluding VAT. Each tax amount is rounded before the amounts are added together. A method that groups taxable amounts by rate before rounding may give a different result. Compare the same data using the same convention before concluding that there is an error.

The table accepts up to 100 lines, with whole-number quantities from 1 to 10,000 and non-negative unit prices. For a simple credit note calculation, the main calculator accepts a negative amount.

CSV files are simulations, not invoices. Their description fields are made safe to prevent spreadsheet software from interpreting them as formulas. The laboratory lets you compare methods; it does not identify a convention that is legally required in every case.

How the guides are prepared and checked

Texts and translations are prepared with artificial intelligence assistance. Numerical examples are checked using the calculation engine and project tests. Statements about tax rules are checked against the cited references, taking account of their conditions and access dates.

Automated tests cover cents, positive and negative signs, limits, invalid inputs, consistent totals and browser interactions. They check a mathematical implementation and its interface; they do not constitute approval by a tax authority.

We do not claim that the content has been reviewed by a qualified accountant or tax adviser. The choice of rate, entitlement to deduct VAT and required invoice details must still be checked separately. For a decision concerning your circumstances, consult the applicable legislation and a professional.

Official sources and references by topic

Open the topic that matches your query. Each reference retains its title, link and access date. The guides also cite sources alongside the relevant explanations.

Rates, taxable amounts and deductions

Belgian and European references for understanding a VAT rate, a taxable amount or a deduction.

VAT schemes, small business exemption and returns

VAT schemes, filing obligations and references on the small business exemption. Announcements of reforms must be distinguished from the applicable legal texts.

Renovation, construction and heating

Catering, accommodation, publications and art

Payment rounding

References on rounding payments, which is separate from rounding VAT in the calculator.

Invoices with multiple VAT rates and bookkeeping

VAT details on invoices, the classification of single supplies and references from the Belgian chart of accounts.

Electronic invoicing and technical references

Explanations from the federal portal and the Peppol BIS technical specification. Preparatory reform documents are identified as such in their titles.

EU transactions and checking VAT numbers

References for cross-border transactions, recapitulative statements, the One Stop Shop and VIES checks.

How references are selected

We prioritise officially published legislation and regulations, followed by explanations from the relevant public authorities and institutions. An announcement of a reform is not treated as its final legal text; Peppol specifications are technical references, not sources of tax law. A background article or a competing website is not enough to establish a rule.

What an access date means

The access date records a past editorial check. It is separate from the date a rule takes effect and does not guarantee that information will remain up to date. It does not change when the website’s design is updated. Check the conditions and exceptions in the reference before applying it to your situation.

Follow a change or report an error

The rules we track are used to guide the rate assistant. Their conditions, exceptions and known dates remain essential: when information is missing to establish the nature of a transaction, the assistant indicates what needs to be checked. It does not guarantee a default rate.

To report a problem, identify the page, result or statement concerned and, if possible, provide a reproducible example or a more recent reference. We distinguish between arithmetic corrections, wording clarifications and changes to a rule. Corrections that change useful information are recorded in the update history.