How these calculators are built and checked

Every tool here produces a number that someone may act on. This page sets out exactly where those numbers come from, how they are verified, and what they leave out — so you can judge the result instead of taking it on faith.

1. Every figure traces to a primary source

Rates, brackets, wage bases, contribution limits and statutory thresholds are taken from the body that sets them — not from another calculator site. In practice that means the IRS for federal tax figures, the Social Security Administration and IRS Topic 751 for FICA, the Department of Labor for overtime rules, the Office of Personnel Management for federal retirement, the CFPB for lending disclosures, and published state rate tables for state-level tax. Health tools cite the original clinical literature or the issuing body.

Each tool lists its own sources at the bottom of its page, linked, so you can check the figure at the source rather than trusting the restatement. The full list currently in use across the 19 sourced finance and health tools appears at the end of this page.

2. The math is verified against worked examples before launch

A formula that looks right and computes wrong is the failure mode that matters. So each calculator is checked by hand against a known example before it goes live, and the expected value is recorded. A mortgage tool has to reproduce a documented amortization figure to the cent; a payroll tool has to land on the same withholding a published example produces; a body-composition tool has to match the value from the paper that defines the equation.

Tools also state the formula they use on the page itself. If the arithmetic is visible, an error is something you can spot rather than something you have to trust us about.

3. Tax year and jurisdiction are stated, not implied

US tax figures change annually and vary by state, so a number without a year and a jurisdiction is not a usable answer. Tools that depend on either name both on the page and in the disclaimer — the tax year the figures belong to, and the state whose rates are applied. Where a state figure is simplified, the disclaimer says which parts are approximated.

4. What these tools deliberately do not do

Every result is an estimate for general information. These calculators apply published rules to the numbers you type in; they know nothing about your credits, local and city taxes, pre-tax deductions, filing complications or medical history. For anything that turns on those, the right next step is a licensed professional — a CPA or enrolled agent for tax, a clinician for health. That limit is printed on the tools themselves, not buried here.

5. Named review where a licence is required

The tools are built and maintained by Christoph Ballmer, a developer whose background includes several years of digital work inside the Swiss banking sector. That covers building the tool, reading the source correctly and stating the limits — and it stops short of a US tax or medical licence.

So the standard here is specific: where a page carries a professional review, it names the professional, their credential and the date they reviewed it, with a link you can verify. A page without such a line has been built and sourced as described above, and makes no further claim. You can always tell which is which by looking.

6. Corrections

Errors get fixed and the fix ships. If a figure looks wrong, a rate is out of date, or a result contradicts an official source, a short note to info@ziffaro.com — with the tool and the number you expected — is enough. Reports that identify a genuine error are acted on rather than filed.

Sources currently in use

Generated from the tool registry, so this list reflects what the live tools actually cite.

All calculators on ziffaro are non-binding aids. Results are guideline values for general information, provided without warranty, and are not a substitute for professional tax, legal, financial or medical advice.