What this site calculates
Most money questions are arithmetic somebody has not done yet. What does this mortgage cost once the deposit, the rate and the term are settled? Which debt should the spare money go at first, and what is that order actually worth? What will a monthly deposit be worth in twenty years, and will the pot last to ninety? What does an offer in another country pay once its tax office is done? Each of those is a calculator here, grouped by the kind of question rather than by the kind of maths.
Borrowing is the largest group, because it is where the biggest numbers are. Any loan's monthly payment and its full amortisation, month by month. A house at its real monthly cost rather than at its loan payment alone, and how much of one your income supports. Refinancing, extra payments, equity already in a home, cars bought and cars leased.
Saving, investing and retirement is the other direction of the same arithmetic. What a starting amount plus a regular deposit grows into, what compounding adds on top, the return a stream of cash flows really earns, what a retirement account reaches by a chosen age with an employer match in it, and what a plan can be drawn down at before it runs out.
Salary and tax is the group with the strictest sourcing, and it is where this site started. Enter a gross salary for a country or a US state and you get the payslip breakdown: income tax, social insurance or payroll contributions, the net figure for the year and for the month, and what the employer pays on top. It matters to the rest because every figure above has to be paid for out of net pay.
Two kinds of calculator, two kinds of evidence
The tools here split in two, and they are held to different standards because the honest standard is different.
Statutory tools apply rules somebody published: income tax bands, social insurance ceilings, student loan thresholds. Every figure in one of those traces to an official document, and the page names it.
Formula tools apply arithmetic that is nobody's rule: a loan payment, compound growth, an internal rate of return, a margin. There is no authority to cite for those, so the page owes you the other thing instead — the formula it used, the convention it picked where more than one is defensible, and what it deliberately leaves out. A payment computed monthly in arrears is not the same number as one computed in advance, and a calculator that will not say which it did is asking to be trusted rather than checked.
Where the numbers come from
Every tax parameter on this site is read out of an official document: the revenue service of a country, or the department of revenue of a US state. Each data file records the document it came from and the date it was last checked, and both are shown on the page that uses it. Nothing is estimated, and nothing is written from memory.
Several of the borrowing and saving pages also arrive with a figure already in the field — a mortgage rate, a card rate, a contribution ceiling — and each of those says underneath it what the figure is, who published it and when. Where a published figure has gone stale rather than being merely old, the page drops it and says so instead of showing it anyway.
The same rules are implemented twice, once on the server and once in the browser, and both are tested against fixtures taken from official tables. If the two ever disagreed the tests would fail before the page shipped. That is the reason a calculator here can be fast without being loose about the numbers.
Keep rate, and why it is shown first
Most calculators lead with an effective tax rate. The number people actually want is the other side of it: the share of the salary that stays with them. This site calls that the keep rate and shows it alongside the standard effective rate, never instead of it, so the figure is easy to quote without being misleading.
What is not here
There is no sign-up, no account and no saved history. The salary you type is used to produce the page you asked for and is kept out of the server access log. Results are estimates for comparing options, not tax advice or a substitute for a payslip, and they assume the standard withholding case for the jurisdiction unless you change the options on the page.