How we check the math
MathBehind’s promise is that you can see and check every number. This page explains how the calculators are built, where their figures come from, how they are tested, and what happens when a rate or price changes.
The working you see is the working we use
Each calculator has one function that turns your inputs into results. The step-by-step “math behind it” panel is produced from that same function’s inputs and outputs, so the explanation and the answer cannot disagree.
The worked examples in our guides are generated the same way: an article asks the calculator for its working instead of typing numbers in by hand. If a formula or rate changes, every article that uses it updates with it.
Testing
Before each release, an automated check runs every calculator with its default inputs and confirms each result is a valid number. The formulas that matter most, such as self-employment tax, are also checked against answers worked out by hand from the official rules, including cases above and below thresholds like the US Social Security wage base. When we find a mistake, we add a test for it so it can’t come back.
Where the numbers come from
Tax rates, thresholds and allowances come from the tax authority that sets them: the IRS, HMRC, FBR, the Income Tax Department of India, ATO, Inland Revenue (NZ), CRA, BIR, the Nigeria Revenue Service, and the UAE and Saudi ministries. Prices for AI models, cloud storage, payment processors and software plans come from each provider’s own pricing page.
Tools that depend on outside figures show a “Figures checked” date and link to their sources next to the calculator. If a tool has no date, it uses only math you supply, such as a margin or a percentage change.
Keeping tax tools current
Tax rules change on each country’s own calendar. We review each country’s tools around these dates and update them from the official source, with a note in News when a change affects results.
| Country | Tax year starts | We review after |
|---|---|---|
| United States | 1 January | IRS inflation adjustments each autumn |
| United Kingdom | 6 April | Autumn Budget and Spring Statement |
| Pakistan | 1 July | Federal budget in June |
| India | 1 April | Union Budget on 1 February |
| Australia | 1 July | Federal budget in May |
| New Zealand | 1 April | Budget in May |
| Canada | 1 January | Federal and provincial budgets; GST/HST changes |
| Nigeria | 1 January | Guidance under the Nigeria Tax Act 2025 |
| Philippines | 1 January | BIR revenue regulations |
| UAE and Saudi Arabia | Varies | Ministry of Finance, FTA and MHRSD announcements |
Rounding
Calculations run at full precision and are rounded only when shown. Money is usually shown to the nearest whole unit or cent, depending on the tool, so a total can differ from the sum of its rounded parts by a cent or a unit.
What the tools don’t do
Results are estimates for planning. Each tool says what it includes and leaves out; for example, the freelance rate calculator covers self-employment tax but not income tax. They are not financial, tax or legal advice, and for decisions with real consequences you should confirm the figures with a qualified professional.
Your numbers stay with you
Calculations run in your browser. The values you type are saved only in your own browser so they are there when you come back, and they are not sent to us. See the Privacy Policy for details.
Found a mistake?
Tell us at contact@mathbehind.com or through the contact page. Every report is read by Muhammad Ahmad, who runs the site. Confirmed errors are fixed, tested and, where they affected results, noted in News.