How Momentix checks its calculators and guides
Last updated 13 November 2026 · Maintained by Ahad Noor
Every number on Momentix comes from a stated formula or an official source, and the guides use the same code as the calculators. This page explains where each rate comes from, how the work is tested, when it is updated and how to report a mistake.
Where do the rates and rules come from?
Tax rates, thresholds and rules are copied from official pages and stored in one configuration file per topic, together with the tax year they are valid for and the date they were last checked. When a source changes, one file is updated and every calculator and guide that uses it follows.
| Source | Used for | Last checked |
|---|---|---|
| Income Tax rates and Personal Allowances | UK tax tables (2026/27) | 29 September 2026 |
| Rates and thresholds for employers 2026 to 2027 | UK tax tables (2026/27) | 29 September 2026 |
| Repaying your student loan | UK tax tables (2026/27) | 29 September 2026 |
| GOV.UK: VAT rates | VAT calculator rates | 29 September 2026 |
| GOV.UK: VAT registration thresholds | VAT registration threshold | 29 September 2026 |
The UK take-home pay calculator models England, Wales and Northern Ireland. Scottish Income Tax bands are not modelled, and the pages say so.
How are calculations tested?
- Each formula is written as a separate function and covered by automated tests that include edge cases such as leap days, midnight shifts, zero values and 100% discounts.
- Worked examples in guides are computed by the calculators' own functions, and a test fails if an article's figure no longer matches the tool.
- Where a rule is unclear, it is left out and the page says what is not included, instead of guessing.
How often is it updated?
UK tax figures are re-checked each April for the new tax year and again when a Budget changes a rate. Every calculator and guide shows a last-updated date, and the sitemap only changes a page's date when its content really changed.
What is each calculator's method and source?
| Calculator | Method and source |
|---|---|
| Age Calculator | Gregorian calendar rules; whole calendar months are added first, then leftover days. |
| Business Days Calculator | ISO 8601 weekday numbering; weekends and holidays are user-defined. |
| Date Difference Calculator | Gregorian calendar arithmetic; the same month rule used by the age calculator. |
| Time Duration Calculator | 24-hour clock arithmetic with a midnight rollover for overnight shifts. |
| Percentage Change Calculator | Standard percentage-change formula (new − old) ÷ |old| × 100. |
| Reverse Percentage Calculator | Inverse of the multiplier method: final ÷ (1 ± p ÷ 100). |
| Profit Margin Calculator | Gross margin = profit ÷ revenue; markup = profit ÷ cost. |
| Break-Even Calculator | Cost-volume-profit analysis: fixed costs ÷ contribution margin per unit. |
| Freelance Rate Calculator | Revenue-required method: income grossed up for tax, plus expenses, over billable hours. |
| Savings Goal Calculator | Future value of an annuity with monthly compounding. |
| Compound Interest Calculator | Compound-growth formula with periodic contributions (end of month). |
| Loan Repayment Calculator | Standard fixed-rate amortisation formula. |
| Unit Price Calculator | Unit conversion to grams, millilitres or items; SI and US customary factors. |
| Weighted Grade Calculator | Weighted arithmetic mean; required score solves the weighted average for the final. |
| Sun, Moon & Rising Calculator | Jean Meeus, Astronomical Algorithms (Sun ch. 25, Moon ch. 47); ascendant from local sidereal time. |
| UK Take-Home Pay Calculator | GOV.UK: Income Tax rates and Personal Allowances; Rates and thresholds for employers 2026 to 2027 (England, Wales and Northern Ireland). |
| VAT Calculator | GOV.UK: VAT rates (standard 20%, reduced 5%, zero 0%) and VAT thresholds. |
| Mortgage Calculator | Standard fixed-rate repayment (amortisation) formula. |
| Hourly to Salary Calculator | Standard pay conversion using your own hours and paid weeks. |
| Overtime Pay Calculator | Standard premium-pay method; US federal rule is 1.5× over 40 hours a week for most non-exempt employees. |
| Discount Calculator | Successive percentage-change method (multiplying the remaining fractions). |
| Tip and Bill Split Calculator | Simple percentage and equal-division arithmetic. |
| BMI Calculator | Adult BMI categories used by the NHS and World Health Organization. |
| Math & Science Quiz | Math questions are generated with random numbers and checked by tests; science questions are drawn from a fixed, reviewed bank. |
How do I report a mistake?
Send the page, what you entered and what you expected through the contact page or to momentix.online@gmail.com. A confirmed error is corrected in the calculator or guide, the last-updated date changes, and the fix is covered by a new test.
What are the limits?
Results are estimates for planning and education. They are not financial, tax, legal or medical advice, and your own situation may differ. See the disclaimer. The site shows advertising (see the privacy policy); advertising does not change any calculation.
More about who runs the site is on the About page and the author profile.