Methodology
How a calculator is built
Each tool starts from the standard financial formula for its problem — compound interest, annuity repayment, present value — implemented in plain JavaScript that runs in your browser. The mathematics is not proprietary and can be checked against any finance textbook.
Tax and regulatory parameters are held separately from the calculation logic, in one data file per country. This separation is deliberate: it means a change in the law is a change to a single value, not a rewrite, and it makes every rate we apply auditable in one place.
Where the figures come from
Rates, allowances and thresholds are taken from primary legislation and the publications of national tax authorities, not from secondary summaries. Each page lists the legal provisions it relies on and carries a dated update stamp.
Where a country's treatment depends on the type of account rather than a single rate — the French envelope system, the Italian split between ordinary and government-bond taxation — the calculator asks you to choose, instead of assuming the most common case and presenting it as universal.
Verification
Every figure printed in an article is produced by running the scenario through the live calculator, never computed by hand and typed in. This rule exists because hand-entered numbers drift from the tool over time, and a page whose text contradicts its own calculator is worse than no page.
Each tool is checked against reference scenarios with known results before publication, and those same scenarios are re-run after any change to shared code.
What the calculators do not include
No result here is adjusted for inflation. All figures are nominal.
Tax is generally applied once, to the total gain, rather than year by year. For a taxable account where interest is taxed as it accrues, this slightly overstates the net result; for tax-free wrappers it is exact.
Country-specific charges outside the scope of a given tool are named on the page rather than silently ignored — the Italian stamp duty on financial products and the German treatment of residual debt insurance are two examples where we state the omission and suggest how to account for it.
Credit decisions, individual rate offers and the effect of personal circumstances on a tax bill cannot be modelled here and are not attempted.
Review and updates
Tax parameters are reviewed at the start of each calendar year and whenever a change is announced during the year. Pages carry the tax year they apply to, so an out-of-date page is visible as such rather than silently wrong.
Corrections reported by readers are checked against the legislation and applied where warranted. We would rather publish a correction than defend a figure.