Percentage calculator

Four percentage questions, four boxes of results, all on one page. Type numbers into any card and the answer updates as you go, so you can work out a tip, a discount, a test score, or a price rise without opening a spreadsheet.

The first card answers “what is X% of Y”, which covers tips, VAT, and a percentage off a price. The second answers “X is what percent of Y”, the one you need for scores and shares of a total. The third gives the percentage change between two numbers, with the direction and the plain difference underneath. The fourth works backwards: if 45 is 30% of something, the original number was 150.

Is my data uploaded anywhere?

No. The arithmetic happens in your browser as you type. Nothing is sent to a server, nothing is stored, and there is no account. You can use it for a salary figure or a private invoice total without those numbers leaving your device.

Why does a card sometimes say there is no answer?

Because some percentage questions genuinely have none. “5 is what percent of 0” cannot be answered: dividing by zero has no result. The same goes for a change that starts at zero, since going from nothing to something is not a finite percentage increase. “Y is 0% of what number” fails for the same reason. Leaving a box empty also blanks that card’s answer rather than treating the empty box as a zero, which would give you a wrong number that looks right.

The other case is numbers so large that the answer runs past what a computer can represent. Rather than printing “Infinity”, the card says it has no answer.

How are the results rounded?

Computers store decimals in binary, so 0.1 × 3 comes out as 0.30000000000000004 in raw arithmetic, an artefact of the IEEE 754 format. Each answer is trimmed to twelve significant digits, which removes that noise and still keeps far more precision than any everyday calculation needs. Display then follows your language settings through Intl.NumberFormat, so a thousands separator and a decimal mark appear the way you expect to read them.

Common uses: restaurant tips, sale prices, tax on an invoice, exam marks, commission, a rent increase, a weight change, and the growth between last month’s figure and this month’s.