About CentExact

CentExact is a small, independent set of free personal finance calculators covering borrowing, saving, pay, tax and selling online. The goal is simple: give you the number you came for, and show enough of the working that you can check it.

How the calculators are built

Every calculator starts with the published formula: the amortisation equation a lender uses, the tax bands a revenue authority publishes, the CPI series a statistics office releases. That formula is implemented once, as a pure function, and pinned with automated tests against reference values: spreadsheet results, worked examples from official guidance, and figures from the institutions themselves.

Where a calculation depends on data that changes (tax thresholds, fee schedules, price indices), the data is stored separately from the code with its source, the date it took effect and the date we retrieved it. That provenance is shown on the page under Data sources, together with a freshness indicator, so you can see at a glance whether a table is current.

Reference data is never paraphrased into code. A tax ruleset is a data file holding every rate, threshold and formula coefficient the calculation needs, validated against a schema when the site builds, so a rate can be corrected without touching the logic that uses it. Where a country's payroll tax is defined as an algorithm rather than a table, as Germany's is, we implement the official algorithm itself and pin it to the tax authority's own published check figures.

Rounding follows the rule the institution uses, not whichever one is convenient. German wage tax is floored to whole euros and the notional insurance allowance is rounded up, because that is what the official program flow chart specifies; a schedule's interest is rounded to the cent each period.

Schedules are built the way a bank builds them: the payment and each period's interest are rounded to the cent, and the final payment settles the exact balance. That is why our totals can differ from a spreadsheet formula by a few cents, and why we show both figures.

What CentExact is not

CentExact is not a financial adviser, lender, tax agent or medical professional, and nothing here is advice. A calculator can tell you what a formula produces from the numbers you enter; it cannot know your circumstances. Check any decision that matters with the institution involved or a qualified professional.

Who publishes the guides

CentExact is the publisher named on these pages. That attribution identifies the site responsible for the content; it does not claim that a named accountant, financial adviser or independent reviewer has approved it. Linked authorities supply rules and data, but do not endorse CentExact.

Automated calculation tests can catch an incorrect formula or a changed result. They cannot establish that a guide covers every exception, that a market assumption fits your situation, or that a cited rule is still current. Source dates and the limits listed beside a result matter as much as the number itself.

Software, including AI tools, can assist with editing and checking explanations. The publisher remains responsible for the wording and calculations. We do not treat an AI-written explanation or a passing automated test as independent professional verification.

Funding

CentExact is free to use. When advertising is enabled, ads are labelled and placed outside the calculator's inputs and results. We do not run sponsored calculators. See the privacy policy for details of the advertising services, information collected and choices available.

Corrections

If you find an error (a wrong figure, an out-of-date threshold, a formula that does not match your lender's statement or your payslip), please tell us. Reports are checked against the source, fixed, and the review date on the page is updated.