How our calculators work

Every RightSums calculator works out its answer with a fixed formula written from the official rules, and each one is checked against hand-worked examples before it goes live. This page explains how a calculation is built, how it is tested, what it assumes and how its figures stay current. Who checks the figures, and how to report a mistake, is covered in our editorial policy.

One calculation, used everywhere

Each calculation lives in a single module of code. The tool you use and every page built from it, such as the take-home pay page for a salary or the Council Tax page for a council, read their figures from that same module. A take-home figure on a salary page therefore matches the calculator to the penny, and a rate change is made once and flows to every page that uses it.

Shared rules are written once and reused. The UK Income Tax and National Insurance rules behind the take-home calculator also drive the pay rise, bonus, second job and self-employed calculators, so a new tax year is updated in one place.

Where each figure comes from

The top of every calculation module lists the official pages it uses and the date each was read, for example GOV.UK for UK tax bands, the IRS for US federal brackets, the Canada Revenue Agency for CPP and EI, or a state's own labour department for its paid-leave deduction. Each tool page shows these sources in its "Privacy and accuracy" box, taken straight from the code, so the list on the page is the list the calculation uses.

When no official figure exists, the tool says so and asks you for the number instead of guessing. The fuel cost calculator, for example, leaves the public charging price for you to enter, because there is no official average.

How calculations are tested

• Hand-worked examples: before a tool is published, its results are compared with examples worked out by hand from the rules, and with the worked examples the source publishes where there are any, such as HMRC's or a state court's guideline examples.
• Edge cases: every calculation is tested with zero, negative, very large and invalid inputs, so a typing mistake gives a clear message rather than a wrong number.
• Checks when rules change: when a calculation gains a new option, the old results are compared before and after. When tax codes and salary sacrifice were added to the UK take-home calculator in September 2026, its results were compared across 347,760 combinations of salary, tax year, pension and student loan, and none changed.
• Pages and tools agree: automated tests check that every generated page loads, has unique titles and descriptions, and appears in the sitemap.
• Full audit: in September 2026 every tool on the site was traced to the code that does the work, run, and checked against an official figure. The problems found were fixed, such as notice periods in months being counted as 30 days.

What the calculators assume

Each tool states its assumptions next to the result. The common ones are:

• UK pay: the current tax year, tax code 1257L unless you enter your own, and Scottish bands only when you tick Scotland.
• US pay: the 2026 federal brackets and standard deduction, a single filer unless you choose otherwise, and state income tax and state payroll insurance only when you pick a state.
• Canada: the 2026 federal and provincial rates, and CPP and EI at the 2026 rates and maximums.
• Estimates, not bills: a calculator works out what the published rules give for the figures you enter. Your employer's payroll, a court, a council or a lender may use details the tool doesn't have.

How figures stay current

Figures that change, such as tax bands, the Ofgem price cap, benefit rates, pump prices and immigration points, are listed on a refresh calendar with their source and the date they next need checking. The calendar currently tracks 49 sets of figures. A weekly check flags anything due, and each page shows the date its figures were last checked. Prices that change weekly or monthly are dated wherever they appear, for example "DESNZ, week of 21 September 2026".

Files and privacy

PDF tools run in your browser: the file is read and changed on your device and is not uploaded, with one exception, PDF Invoice to E-Invoice, which says so on its page. Document checkers that need our server read the file in memory and discard it when the request ends. Photos and scanned PDFs are read with text recognition in your browser first, unless our Google Gemini reading is switched on. The privacy policy sets out what happens in each case.