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German tax and benefit calculators are easier to scrutinize when they show not only a result, but how they reached it. In a DEV Community post published October 3, 2026, hongtao fang describes building SteuerKlar around that idea: each displayed calculation is accompanied by intermediate values, formulas and legal references. The account is the builder’s description; the implementation, tests and legal rules discussed here have not been independently audited.
The problem: a result without an explanation
Fang says the project began while trying to estimate Elterngeld, Germany’s parental allowance. The calculators the author encountered returned a number without showing how it had been computed. Fang also recalls seeing calculators that still displayed an income limit the post associates with a change in April 2025. That is the author’s experience, not a market-wide survey or an independent confirmation of the rule or date.
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The resulting design goal was more specific than “make a calculator”: make each result inspectable. As Fang puts it, “If a number appears in the UI, it exists in the calculation trace.”
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Make the calculation trace part of the result
Pure functions separate rules from the page
Fang describes implementing German tax and benefit calculations as pure TypeScript functions, apart from the page that displays them. The example function, berechneElterngeld, derives assessment income, selects a percentage and applies a cap. It also returns a schritte array containing intermediate values, formulas and legal references; the interface renders those steps alongside the outcome.
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This arrangement makes the calculation path a product feature rather than a debugging artifact. A reader can see which stages produced a result, while developers can test the calculation logic separately from its presentation. It also raises the standard for the interface: any value shown to a user needs a trace entry if the project is to meet its stated design principle.
Legal references travel with individual steps
The post gives BEEG § 2c as an example of the basis cited for deductions in the parental-allowance calculation, and EStG § 32a as an example of a piecewise income-tax formula. It also names the severance Fünftelregelung under EStG § 34 among the rules covered by tests. These are Fang’s descriptions of the code and its legal basis, not an independent legal review or confirmation that the implementation reflects current law.
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Attaching a paragraph reference can make a calculation easier to inspect, but it is not by itself proof of legal correctness. Users still need to distinguish a cited provision from a reviewed interpretation of that provision, particularly when rules or thresholds change.
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Fang reports 68 tests covering examples including the Elterngeld minimum and cap, the income limit the post associates with April 2025, sibling, twin and premature-birth bonuses, and the severance Fünftelregelung. The article reports figures of €300 for the minimum benefit, €1,800 for the cap and €175,000 for the income limit it describes as applying from April 2025, identified there as zvE. These are figures stated in the 2026 post, not independently verified current legal thresholds.
The author also says the Minijob employer-cost model was checked against the official Minijob-Zentrale calculator “to the cent.” The post’s account does not provide an independently reproduced comparison, and the test suite itself was not inspected. The figure of 68 tests and the comparison should therefore be read as project claims, not as an external test report.
The tricky implementation detail: monthly deductions
Fang says translating deductions under § 2c BEEG into code took three attempts. The challenge, as described in the post, was that monthly treatment of Lohnsteuer, Solidaritätszuschlag (Soli) and Kirchensteuer differs in subtle ways from the annual calculation. The author says tests against official Prognoserechnung examples helped catch drift, although the post does not include those examples for independent review.
This is a useful illustration of why tax software needs more than a formula transcribed into code. A calculation may depend on which period is being modeled and on how related deductions are applied. Exposing the sequence can help a user or reviewer spot an unexpected step, but the result remains dependent on the accuracy and maintenance of the underlying rules.
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Reported stack and privacy approach
Fang reports building the site with Astro static output, TypeScript calculation modules shared between page JavaScript and the test suite, and Cloudflare Pages hosting. The post says calculations run entirely in the browser, with no cookies, tracking or signup. It also claims output below 200 KB and zero third-party requests. Those are the author’s reported project characteristics; they have not been independently measured or audited here.
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- Quick reference learning guide
- Definitions and glossary of terms
- Tax tips, and everything else you need to know about filing
- Common mistakes, understand audits
- Claiming a dependant and more
For this kind of tool, browser-side calculation and a small static deployment can be attractive design choices, but they should not be mistaken for an independent privacy or performance assessment. The article does not establish live-site network behavior, nor does it verify how the site handles every category of user data.
What this project demonstrates—and what it does not
- A transparent calculation can be designed into the interface. In Fang’s account, values, formulas and legal references accompany the result through a structured trace.
- Separating calculation logic from rendering supports focused tests. The post describes reusable TypeScript functions used by both page code and the test suite.
- A test count is not a legal audit. The reported 68 tests and Minijob-Zentrale comparison do not independently establish that every rule is complete or current.
- Legal references help users inspect a rule’s stated basis. They do not alone establish that a provision was interpreted correctly or that a threshold remains applicable.
This is a single-project build account, not a measured comparison with other calculators or frameworks. To assess a calculator for consequential decisions, useful questions include whether it exposes formulas and intermediate values, documents rule coverage and update practices, tests boundary cases, and clearly explains its data and network behavior.
Source: hongtao fang’s DEV Community post, October 3, 2026. The source supports the author’s account of the project, not an independent verification of its legal accuracy, current-year applicability, tests, privacy claims or performance.
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