- Find the rules in force in the official source (Spain's Official Gazette, tax and social security authorities, the Bank of Spain). We keep the link and the date consulted next to every rate and table.
- Define the calculation: inputs, formula, rounding and what is left out.
- Code it separately from the page, so it can be tested automatically.
- Test it with at least 8 cases, including edge cases.
- Compare it with the official simulator where one exists, within €1.
- Publish with the review date and tax year shown.
We review every calculator each January and check for regulatory changes every month. Results are always a guide: a calculator cannot know your contract or personal circumstances. Report errors to [email protected].