Methodology

How we build our calculators

Every calculator is built from an identifiable basis: a mathematical formula, a piece of regulation or a recognised technical reference. Before publishing a tool we test it against known cases and worked examples to check that the results match, and we review periodically those whose parameters can change over time (tax brackets, contribution rates, legal limits). Each page explains how the result is obtained so you can understand the number, not just read it.

Reference sources by area

Depending on the calculator, we work from sources such as:

• Finance — standard financial formulas plus, where a calculation depends on Spanish regulation, parameters from the Spanish Tax Agency (AEAT), the Bank of Spain, Social Security and the Official State Gazette (BOE).
• Work — Social Security, the State Employment Service (SEPE), the Ministry of Labour and the BOE for unemployment benefit, severance, self-employed contributions and working-time rules.
• Health — the World Health Organisation's BMI classification, the Mifflin-St Jeor equation (1990) for energy expenditure and the Tanaka formula for maximum heart rate, among other peer-reviewed references. Health results are estimates and never replace medical advice.
• Education & physics — the International System of Units (SI/BIPM) and standard academic references.

How we choose a source

When a result depends on regulation, we take the figure from the body that sets it, not from a secondary summary: the BOE for the wording of a rule, the Tax Agency for income tax and VAT, Social Security for contribution bases and pensions, the SEPE for unemployment benefit, and bodies such as the WHO or EFSA for health references. Where a page relies on a published formula rather than a rule — Mifflin-St Jeor for basal metabolism, Tanaka for maximum heart rate — we cite the original paper, so you can check what the formula was derived from and on which population.

We avoid press articles and blog posts as primary sources. They are useful for spotting that something has changed, but not for stating what it changed to. When an official source is ambiguous or a rule is being phased in, we say so on the page rather than picking the reading that gives the tidier number.

How we verify a figure before publishing it

Every parameter that comes from regulation is checked against its official source before it goes live, and the check is recorded as a review date next to the figure. We also run the calculator against worked examples whose result is known, so that a wrong constant shows up as a wrong total. When a figure cannot be verified, we do not publish a review date for it: an unverified date is worse than none, because it claims a check that never happened.

A single figure is rarely enough on its own. A contribution base means little without the rate applied to it, and a percentage means little without the cap that limits it, so we check the whole chain that produces the number rather than the headline value alone.

Keeping data up to date

Calculators whose parameters depend on regulation are reviewed once a year, when the figures for the new tax and contribution period are published, and additionally whenever a rule changes in the meantime. That is the cadence we commit to, and we prefer to state a modest one we keep rather than an ambitious one we do not: review dates are visible on each page, so a missed cadence would be visible too.

Even so, results remain indicative and some parameters may depend on your collective agreement, contract or personal situation.

What happens when the rules change

When a reform alters a parameter, we update the calculation and the text that explains it in the same pass, because a correct number under an outdated explanation is still misleading. If the change applies from a given date or is being phased in gradually, the page says so instead of presenting the new figure as if it had always applied. The Spanish and English versions are updated together, so the two never disagree.

How we correct an error

If we get something wrong, we fix it and we record it. Corrections are not made silently: the change is logged with its date in the change history, and if it affected a result rather than the wording, we say so plainly. We would rather show that a figure was wrong for a while than quietly overwrite it.

If you spot a figure that looks wrong or a formula that seems out of date, check our corrections policy and let us know through the contact page. Every revision we release is logged, with its date and what it covered, in the change history.