How we check our calculators

Reckonly's calculators work out UK tax and employment figures: employer National Insurance, s455 on directors' loans, statutory redundancy pay, Stamp Duty Land Tax and more. A wrong figure can cost a business money, so this page sets out where our numbers come from and how we test them.

Where the rates come from

• Every rate, threshold and allowance comes from GOV.UK: HMRC's published rates pages, legislation and guidance manuals. Each calculator page lists the exact GOV.UK pages it relies on in its "Sources" section.
• The rates live in one shared file, pinned by tests that cite the GOV.UK pages they came from. Changing a rate without updating its test fails the build.
• All calculators share one tax engine. The figures on the website and the figures returned by our paid API come from the same code.

How the calculations are tested

The calculation engine is covered by 400+ automated tests. They run on every change to the code, and a change that breaks one cannot be released. The expected answer in each test comes from outside our own code:

• GOV.UK worked examples: where HMRC publishes a worked calculation, we test against it and cite the page.
• HMRC rules worked by hand: where there is no published example, the arithmetic is written out step by step in the test.
• PolicyEngine UK: an independent, open-source model of the UK tax system. We use its figures to cross-check income tax, National Insurance, student loan repayments and dividend tax.

A test never copies its expected figure from the calculator it is testing. Otherwise a bug would end up "confirming" itself.

When rates change

• Most UK tax rates change on 6 April. Each April we update the rates file and its GOV.UK citations together, refresh the PolicyEngine UK figures for the new tax year, and fix any calculator the tests show is wrong.
• Mid-year changes, such as a new official rate of interest or a Budget measure with an immediate start date, are applied when GOV.UK publishes them.
• We last re-checked every calculator against GOV.UK on 19 October 2026, and corrected the ones that disagreed. Changes are listed in the changelog.

What the calculators don't do

• They model the common case. Where a result depends on facts a calculator can't ask about, such as a relief claim or an anti-avoidance rule, the page says so.
• They are not tax, legal or financial advice. For decisions with real money at stake, confirm the figure with a qualified accountant or adviser.

Corrections

If you think a figure is wrong, tell us which calculator, the inputs you used and the figure you expected. We check it against GOV.UK, add a test that covers your case, and record the fix in the changelog.

Who runs Reckonly

Reckonly is built and run by MBridge Technologies Limited, a UK limited company (company number 16708904) registered with the Information Commissioner's Office (ZC013807). Calculator inputs are processed in your browser and are not stored unless you choose to save a calculation. See About and Security.