Methodology

Every rate, threshold and legal limit is taken from the body that sets it, in this order of preference:

• Legislation, read on its official publisher: legislation.gov.uk, gesetze-im-internet.de, the BOE, Légifrance, Normattiva, overheid.nl, ISAP and EUR-Lex.
• Guidance from tax authorities and financial regulators, such as HMRC and the Financial Conduct Authority.
• Official summaries from EU institutions, used alongside national law rather than instead of it.

Figures are never taken from banks' marketing, from news summaries or from other calculators. Each tool page lists the sources it was checked against.

All the figures a tool relies on live in a single data file, together with the date they were verified. The calculator, the worked examples, the tables and the numbers in the text are all generated from that file; none is typed into the prose. If the data and a page ever disagree, the site refuses to build.

Every formula is covered by automated tests that compare its output with reference values, for example the standard annuity result for a loan, and check that a schedule repays exactly what was borrowed. Amounts are rounded to the cent the way a lender or a tax authority would, and the tests run before every build, so a failing formula cannot be published.

Each tool page also lists cases with a known answer under Checked against known answers: the inputs, the figure its source fixes, and what the calculator returns. Where an official test table exists, as for the German wage tax programme of the Federal Ministry of Finance, the case comes from it; elsewhere it is worked by hand from the published rates. The same cases run on every build, so a page cannot show a case it fails.

The word tools and the daily word game work from open word lists that anyone may reuse. Each tool page names its lists, and the full attributions and licences are in the licence file published with the lists: CC BY-SA 4.0 for most languages and the Mozilla Public License 2.0 for French.

Words are normalised the way each language's own word games treat letters. English, French, Italian and Dutch fold accents, so café counts as CAFE. Spanish folds accents but keeps Ñ as a letter of its own. German keeps Ä, Ö and Ü and writes ß as SS. Polish keeps all nine of its own letters, from Ą to Ż. Dutch IJ counts as two letters, I and J.

Letter values come from the published Scrabble rules for each language. The matching behind Wordle-style guesses is tested against the game's own colouring rule, repeated letters included. Offensive words are kept out of suggestions and game answers, although a search can still find them.

Results can differ from the lexicons used in tournaments, such as Collins Scrabble Words in English, the ODS in French and the OSPS in Polish. Those lists are copyrighted and are not used here, so a word may be valid at a club and missing here, or the other way round.

Each language version is written for its own market: its law, its currency, its vocabulary and its worked examples. A German page explains the Sollzins and the effektiver Jahreszins, a French page the TAEG and the taux d'usure. Nothing is translated from English.

Each tool page shows the date its figures were last checked. The figures are checked again whenever a known change in the law takes effect, and the date moves only when that check has been done.

If you find an error, write to contact@abdullahawais.com with the page address and the source you rely on. Confirmed errors are corrected, and the page's verification date is updated.

The tools do not know your credit history, a provider's own fees or the terms of your contract, and they do not give advice. They show how the figures work, so that you can check an offer or a bill yourself.