Calculators
Eleven tools for the sums people actually do — money over time, body measures, dates and durations, and a draw that has to be fair. The interesting part of each is the assumption sitting under the number.
11 tools · nothing uploaded · no sign-up
- Compound Interest Calculator Watch regular saving grow — and find the year the interest out-earns you.
- Loan & Mortgage Calculator Monthly payment, full amortisation, and what overpaying actually saves.
- Retirement Calculator Both directions: what your saving buys, or what your target costs.
- Calorie & TDEE Calculator Daily calories, plus a weight projection that bends the way real ones do.
- Percentage Calculator Percent of a number, percent change, and "X is what % of Y".
- Discount Calculator Sale price, what you save — and stacked discounts done right.
- Tip Calculator Tip, total and the per-person split.
- BMI Calculator Body mass index, metric or imperial — with the caveats.
- Age Calculator Exact age in years, months and days — and the next birthday.
- Date Difference Days between two dates — including working days.
- Random Picker Spin a wheel, roll dice or draw names — without the same one coming up twice.
Which convention the answer follows
Compound growth has two defensible conventions and both are in use here. A savings projection converts the quoted rate to an effective annual one — six per cent compounded quarterly is worth 6.14 — then grows the balance by the twelfth root of that each month, so the compounding setting moves the answer. A repayment schedule divides the annual rate by twelve and stops there, because that is the lender’s arithmetic and the payment has to match the offer document.
Both pages say which convention they use. Stacked reductions, likewise, multiply rather than add: a fifth off and then a tenth off is 28 per cent, not 30, and the discount page prints that beside the answer.
A calendar is not a number line
The date fields hand over a string like 2026-08-25, and the browser’s date constructor reads that as midnight UTC — the day before, west of Greenwich. Both date tools build a local date from the three numbers instead, which decides whether an age is right for half the planet.
Whole months are counted the way people say them, not as days over 30.4; a 29 February birthday falls on 1 March in a common year; working days mean Monday to Friday. Public holidays differ by country, so none are deducted.
The edges are written down
Every model here stops somewhere. One fixed return applied to every year of a long run describes no real portfolio: when the bad years land changes the outcome even when the average does not. The 7,700 kcal-per-kilogram constant behind a weight projection is an approximation too, and it overstates long-run loss.
The four chart calculators carry the habit into their number fields. A hard limit comes from a record rather than a round number — 122 years, the oldest verified human age — and a refused number stays on screen beside that fact, so you can disagree. The loan amount is capped nowhere a borrower would meet it: a ¥50,000,000 mortgage is ordinary in a currency the picker offers.
Even the draw states its edge. A 32-bit value taken modulo the number of entries is biased: 2³² divides evenly into almost no count, so the low entries come up slightly too often. On six sides that is a part in 700 million; across a 200-name raffle drawn all afternoon it is real, so draws are rejection-sampled.