How we calculate and check our numbers

Our principles

• One question per calculator. Each calculator answers one concrete money question for one country, with that country’s currency, number format and rules.
• Official sources only. Tax rates, thresholds and allowances come from the authority that sets them – tax authorities such as HMRC, government departments publishing on GOV.UK, the Bank of England and the Office for National Statistics. We do not copy rates from other calculator sites.
• Every value is documented. Each rate we use is stored with its source link, the date it takes effect, the date it ends (if known) and the date we last checked it. The “Sources” section and the “Last verified” date on every calculator page come straight from that record.
• No guessing. If we cannot confirm a value from an official source, the calculator that needs it is not published. It stays hidden until the value is confirmed.

From the rule to the calculation

The rules and the arithmetic are kept apart. The rules for each country and tax year live in their own data file. The arithmetic lives in small calculation modules that contain no page layout at all. The calculator you see on the page runs exactly the same code in your browser, so nothing you enter is sent to a server.

Where an authority publishes its own algorithm, we use it directly instead of rewriting it. For example, our German payroll-tax calculator runs the official program of the German Federal Ministry of Finance, translated automatically into our code.

How we test

Before a calculator goes online it has to pass automated tests:

• Mathematical checks – results are compared with the closed-form formula where one exists. For compound interest, a single deposit must give exactly P × (1 + r/n)^(n·t), and regular deposits must match the annuity formula.
• Official examples – where an authority publishes worked examples or a reference calculator, our results must match them to the cent.
• Edge cases – zero values, very large values and the exact boundaries between tax brackets.
• Rounding – amounts are rounded the way payslips and bank statements round them.

The tests run again before every update of the site. If one fails, the update is not published.

Keeping calculators up to date

Tax rules change, usually once a year and sometimes in between. When an authority publishes new values, we check them, record the new source and date, update the tests and publish the change. Every calculator shows when its figures were last verified.

What our calculators do not do

Results are estimates based on the figures you enter. They are not financial, tax or legal advice. Each calculator lists what it does not include, for example fees, special allowances or personal circumstances, so you can see where your real result may differ.

Compound interest: our method

Our compound interest calculator converts the nominal annual rate into the equivalent monthly rate, (1 + r/n)^(n/12) − 1, and then follows the balance month by month, adding your contribution at the start or end of each month. This gives exactly the same yearly growth as the compounding frequency you choose. Thirteen automated tests check it against the closed-form formulas.