How we verify our data
A tax calculator is only useful if the result is right. At UtilXter we do not publish a figure we cannot back with an official source, and every calculation is checked against real cases before it goes live. This page explains how we work, what the tools cannot do and how to tell us about an error.
1. Official sources only
Data comes from the published rules or from the body that applies them, never from other calculators or blogs:
- Tax and payroll: Spain's Official State Gazette (BOE), the Spanish Tax Agency's income tax manual, the annual Social Security contributions order and the Ministry of Finance report on regional taxation for each region's scale.
- Electricity in Spain: CNMC network toll resolutions, the Ministry for the Ecological Transition's charges orders, Red Eléctrica's hourly PVPC data (ESIOS) and OMIE prices.
- Electricity in other countries: each country's regulator or utility (for example Enemalta, T&TEC or Edesur); when a decision has only been reported by the press, the page says so.
Every tool and guide links to its sources and shows the date of its last review.
2. Control cases
Before a calculation is published we compare it with worked examples from the official body or calculated by hand step by step. For example, the income tax calculator reproduces to the cent the case of a single employee earning €30,000 gross in Madrid (€4,598.02 of tax), using the method in the Tax Agency manual: the scale applied to the taxable base minus the scale applied to the personal allowance, for both the state and the regional portion. For electricity tariffs, sample bills match those published by each country's regulator or financial press.
3. Twin implementation and automated tests
Each calculator's logic exists twice: in Python, which builds the static tables and examples on the page, and in JavaScript, which runs the calculation in your browser. Every time a page is generated, an automated test runs both versions on dozens of cases (low salaries with reductions, salaries above the maximum contribution base, families, every region) and publishing stops if they disagree.
4. Reviews and updates
- The default Euribor in the mortgage calculator updates automatically every month, and reference electricity prices every quarter.
- Income tax scales, Social Security rates and tariffs are reviewed whenever the rules change and at least once a year with the new tax manual.
- If a figure cannot be confirmed in an official source, we do not use it in calculations: the page flags it as pending verification.
5. What the calculators do not do
Results are indicative estimates for common situations. They do not cover every regional deduction, full joint filing or the separate tax systems of Navarre and the Basque Country, and they do not replace the Tax Agency's Renta Web or a professional adviser. Each tool explains its own limits next to the result.
6. Corrections
If you find an error or outdated figure, email [email protected] or use the contact page. We fix it and record significant changes:
- October 2026: the net salary calculator now applies each region's scale, the MEI contribution (6.50% in total) and the official method for allowances; it previously used a general scale and subtracted the allowance from the base, which skewed results. Electricity tariffs for ten countries were reviewed against official sources.
Who makes UtilXter
UtilXter is a project by Tamara Sutil Moral, an engineer with a background in exact sciences, who builds and reviews every tool personally.