Own product · Web & software
Rechnerzentrale
212 calculators for tax, benefits and everyday life – every formula tested against official reference values before it goes live. Our own product.
- calculators for tax, benefits, finance and everyday life
212
calculators for tax, benefits, finance and everyday life
rechnerzentrale.com/llms.txt, as of 11 Oct 2026
- until content is visible (LCP) on desktop, 2.82 s on a phone
0.63 s
until content is visible (LCP) on desktop, 2.82 s on a phone
Lighthouse desktop/mobile, median of 3, 30 Sep 2026
- inputs leave the device – calculations run in the browser
0
inputs leave the device – calculations run in the browser
01Starting point
Online calculators are often slow, full of ads and rarely verifiable. The goal: fast calculators whose results can be backed up – even as laws and values change every year.
02Approach
- 01Calculation logic separate from the interface, every formula with a source.
- 02Golden master tests and a value register against official reference values – they block any release that deviates.
- 03Calculation in the browser: no input leaves the device.
- 04Delivered statically for very short load times, as a PWA and as an Android app on Google Play.
- 05A calculation line on the home page: “19 % von 250” computes instantly, “4000 brutto” opens the matching calculator pre-filled – no server involved.
- 06The same engine also powers client calculators – such as the severance calculator for a transfer company.
03In detail
Tests against official reference calculations
Calculators that depend on statutory values are checked by golden master tests: fixed inputs with expected results taken from official calculators and worked examples, such as the Federal Ministry of Finance’s programme flowcharts for wage tax – 160 cases for 73 calculators (as of 24 Sep 2026). A unit test checks whether the code does what was intended; a test against official values checks whether the law was understood correctly.
Values with a source, in one place
Basic tax allowance, contribution ceilings, church tax rates: the key statutory values live in a register with their official source. When new values apply in January, the register changes, the tests show which calculators are affected – and nothing goes online until all of them pass.
Checks in the build
Check scripts detect outdated annual values and verify the language purity of the English pages and the structured data. If one fails, there is no deployment.
Bilingual, static, offline
Every calculator has its own page per language with its own metadata and hreflang links. All pages are generated at build time and the server delivers finished HTML; installed as a PWA, the portal also works offline after the first load.
Analytics without cookies
We want to know which calculators are used, not who uses them: analytics are self-hosted, run without cookies and record no input values.
04Outcome
A portal with 212 calculators that loads in just over half a second on desktop, with every formula verified before it goes online.
- Next.js
- TypeScript
- Vitest
- Golden-Master-Tests
- PWA
- Android-App (TWA)
A similar task on your desk?
In the intro call we tell you how we would approach it – and how quickly you would see results.