Math and finance.

There are 20 tools: calculator, age calculator, currency converter, invoice generator, number base converter, time zone converter, unit converter, loan calculator, compound interest calculator, discount calculator, tip calculator, percentage calculator, date difference calculator, add days to a date, time duration calculator, random number generator, dice roller, coin flip, roman numeral converter, data size converter.

No. The arithmetic runs in your browser and the pages make no request with the numbers, dates or amounts you enter.

The invoice generator saves its draft in your browser's localStorage so closing the tab loses nothing; it stays on that device. The other tools keep nothing once you close the page.

After the page has loaded, none of them needs the network: the currency converter uses a fixed table and fetches nothing, and the rest are plain arithmetic.

The amount times the monthly rate (the APR divided by 12), divided by 1 − (1 + monthly rate)^−payments, or the amount divided by the payments at 0%. The loan calculator rounds interest and payments to the cent each month, and the last payment takes what is left.

No, they compound: each discount is taken from what the one before left, so 20% off and then 10% off is 28% off, not 30%. In the discount calculator, sales tax comes after the discounts and each step is rounded to the cent.

In the US the custom is to tip on the bill before tax, and the tip calculator can take the tax off first. It splits the total so the shares add up to the cent, and can round the total or each share up to a whole unit.

The APR is the nominal yearly rate; the APY is what it earns once compounding is counted, (1 + r ÷ n)^n − 1 for n compoundings a year. 5% compounded monthly is an APY of 5.116%.

A fixed table of reference rates against the US dollar for 30 currencies, dated 2026-10-23 and shown in the status line. The page never fetches newer ones.

The tools make no network request with what you enter, so there is no rate feed behind the page. A dated table that you can edit shows plainly what the numbers are.

Change any rate in the table under the converter. It is units of that currency per US dollar, and Reset restores the table.

Every amount goes through the dollar: divided by the source's rate, multiplied by the target's. The cross rate under the result is that ratio for one unit.

8 categories: length, mass, temperature, area, volume, speed, data, time. Each result is also listed in every unit of its category.

They are the international definitions, such as an inch being exactly 25.4 mm. Results show ten significant digits with float noise removed.

Type digits in any of the four fields and the other three follow. It uses BigInt, so large numbers convert exactly, and reads the prefixes 0x, 0b and 0o.

Basic and scientific mode, with degrees or radians. The expression is read by a small parser and is never passed to eval.

On the calendar: whole months first, then the days left over. Someone born on 31 January is one month and one day old on 1 March.

Their birthday is treated as 1 March in years without a 29 February, which is how most countries treat it.

It uses the zone rules built into your browser, worked out for the instant you enter, so a meeting in March is placed under the rule in force in March.

A time that doesn't exist, on the night clocks spring forward, is moved an hour ahead. A time that happens twice takes the first of the two.

Fill in the parties, dates and lines, then press Download PDF. The totals update as you type and the A4 PDF is made in your browser.

Lines add up to the subtotal, the discount comes off, tax applies to what is left, and the total is that plus the tax. Every figure is rounded to the cent so the lines add up to the totals.

10 currencies: USD, EUR, GBP, CAD, AUD, CHF, JPY, INR, SGD, BRL.

In this browser's localStorage, on this device, as you type. It isn't sent to a server and so isn't on your other devices. New invoice clears it.