How We Calculate

No calculation on this site is sent anywhere. Every tool is JavaScript that executes on your own device, which means your salary, debts, property value and estate details never reach our servers — because there is no server-side calculation to send them to. We do not store, log or transmit anything you type into a calculator.

A practical consequence: results are instant and work offline once the page has loaded, and there is no account to create. It also means we cannot recover a calculation for you — sharing a result works by encoding your inputs into the page URL, which stays entirely on your side.

Every rate, threshold and allowance is taken from the responsible authority's own published figures, not from other calculator sites:

Tools that use a standard financial formula rather than a government-set rate — loan amortisation, compound interest, the 4% retirement rule — say so on the page instead of citing an authority that does not apply to them.

A recurring failure in calculator sites is that different pages quietly disagree: the rate table says one thing and the calculator does another. We avoid that structurally rather than by proofreading.

The UK and US rate tables are written from the same constants the calculators use, so a published table and the tool it links to cannot drift apart. The US tax engine — federal brackets, the standard deduction, FICA and state schedules — is a single shared module, which is why the paycheck calculator, the state comparison and mortgage affordability always return the same take-home figure for the same salary.

Calculations are checked against an independently written implementation of the published rules before being released, rather than against the code's own output. Where a result has a known correct answer — the UK 60% personal-allowance taper band, the point National Insurance drops to 2%, a full bracket schedule at several income levels — those are the cases used to verify it.

Stating limitations plainly is more useful than implying precision we do not have. Known simplifications:

• Filing status. US figures assume a single filer taking the standard deduction unless a tool explicitly offers married filing jointly. Itemised deductions are not modelled.
• US state coverage. State income tax covers California, New York, Illinois, Texas, Florida, Nevada and Washington. Other states are not yet included.
• Local taxes. City and county income taxes are excluded — most significantly New York City, whose residents pay a separate local income tax on top of the state figure shown.
• Scotland. UK Income Tax figures use the England, Wales and Northern Ireland bands. Scotland sets its own and is not separately modelled.
• Benefits and credits. Means-tested benefits, tax credits, child benefit and the High Income Child Benefit Charge are not included in take-home figures.
• Investment returns. Projections use a fixed annual rate you choose. Real markets do not deliver constant returns, and no figure on this site is adjusted for inflation unless the page says so.
• Interest rates. Default rates in loan and mortgage tools are illustrative starting points, not live market quotes. Always use a rate you have actually been offered.

None of these make a result useless — they make it an estimate, which is what it is labelled as. For anything with a real financial consequence, confirm with the relevant authority or a qualified professional.

Tax rates change on a predictable cycle: the IRS publishes inflation adjustments in the autumn for the following January, and the UK confirms rates at fiscal events for the tax year starting 6 April. We update the calculators when the responsible body confirms new figures, not when they are proposed or reported.

Each calculator shows a badge naming the body its figures come from and the date they were last verified, so you can judge how current a page is rather than assuming.

If you believe a figure is wrong, please tell us — a specific report (the page, the input you used and the result you expected) is genuinely useful and we would rather hear it than not. You can reach us via the contact page. Corrections to a calculation are made as soon as we can verify the error against the source.

JoinCalc is free and carries advertising. We do not charge for any calculator, there is no premium tier, and we do not sell leads, mailing lists or user data — none of which would be possible anyway, since calculations never leave your browser.

Advertising is served by Google AdSense and is not connected to our calculations: no result, ranking or recommendation on this site is influenced by an advertiser. Where a page suggests one option is cheaper than another, that conclusion comes from the arithmetic on the page and nothing else.