Our methodology

Our calculators deal with money, tax and major life decisions, so accuracy matters. This page explains exactly where our figures come from, how we test each tool, and what our calculators can and can’t do. If you ever find a number that looks wrong, please tell us.

The official sources we use

Every calculator is built from the rates, thresholds and formulas published by the relevant national authority for that market — not third-party estimates.

🇺🇸 United States

• IRS — federal income tax brackets, qualified-dividend rates, estate-tax exemption, FICA
• Federal Reserve — interest rates, the H.15 and G.19 releases
• Bureau of Labor Statistics — CPI/inflation and earnings data
• CFPB and SSA — consumer-credit guidance and Social Security

🇬🇧 United Kingdom

• GOV.UK & HMRC — Income Tax, NI, dividend allowance, SDLT, Inheritance Tax
• Bank of England — base rate and lending data
• ONS — inflation and earnings; MoneyHelper & the FCA for consumer guidance

🇦🇺 Australia

• ATO — income tax, PAYG, HECS/HELP, CGT, super
• RBA and ABS — cash rate, inflation and wages
• ASIC MoneySmart and state revenue offices — consumer guidance and stamp duty

How we build and test

• Built from primary sources. Each calculator implements the official formula and the current-year figures for its market.
• Automated checks on every change. Before anything ships, an automated suite boots every calculator and confirms the core results are finite and sensible, validates each page’s structured data and canonical tags, and checks every internal link resolves.
• Worked-example verification. We sanity-check results against the official authority’s own worked examples (for instance, a known estate or dividend scenario) so the numbers line up.
• Privacy by design. Calculators run entirely in your browser — GlobalCalc does not store your figures as calculator records, and the calculator code does not submit them to obtain a result.

How automation and AI are used

Most calculators on GlobalCalc are produced from a written definition: the inputs, the formula, worked examples, questions and sources for one calculator, which a shared template turns into the page you see. That is automation, and we would rather say so than have you wonder.

• Where AI is used. AI tools help write the code, draft calculator definitions and draft explanatory text. They are directed by a person, Adam Formosa, who decides what is published.
• What is tested by machine. A calculator built from a definition, other than a unit converter, has its formula run against awkward inputs (zeros, negatives, very large and very small numbers) before it ships, and against its worked examples where an expected answer has been recorded, which is not yet every example. Unit converters use fixed, published conversion factors and do not yet have that input testing. The published page of every definition-built calculator, unit converters included, is checked to match its definition exactly. Older calculators that were written by hand are not covered by those checks; they are loaded and exercised by a separate automated test.
• What a person checks. Figures that come from law or official guidance, such as tax rates, thresholds and health guidelines, need a person to compare them with the official source named on the page. When a person has done that for a group of calculators, each page in the group says who did it and when.
• What that means for you. A calculator without a named check has passed the automated tests but not an individual human review. Treat every result as an estimate and confirm anything important with the official source or a qualified professional.

Update schedule

• Each tax year we update brackets, allowances and thresholds (e.g. the dividend allowance, nil-rate bands, the estate-tax exemption, FICA and personal-allowance figures).
• Rate-driven tools (mortgage, currency) use indicative or mid-market rates and clearly tell you to confirm a live rate before you transact.
• Each calculator shows the year it’s built for, and major pages carry a “last updated” date.

Limitations

Our calculators are for general information, not personal financial, tax or legal advice. They use standard assumptions and may not cover every edge case — complex estates, unusual income mixes, devolved-nation differences, or state-specific rules. For decisions that matter, confirm the result with the relevant authority above or a qualified professional.

Corrections & feedback

We take accuracy seriously. If you spot an error or an out-of-date figure, contact us and we’ll review and fix verified issues quickly. See also our about page and free embeddable calculators.