Number Base Converter

How it works

Every value you type is parsed into a JavaScript BigInt, an arbitrary-precision integer type built into the browser. Parsing walks your input digit by digit, multiplying the running total by the base and adding each digit's value; converting back out repeatedly divides by the target base and collects remainders. Digits above 9 use letters, so base 36 runs through 0-9 then a-z. Negative numbers work too: a leading minus sign is peeled off before parsing and stuck back on at the end.

The BigInt part is the detail worth knowing. Converters built on plain JavaScript numbers silently corrupt anything above 9,007,199,254,740,991 (2^53 - 1), because that's where floating point stops representing every integer exactly. Paste a 16-digit hex value like ffffffffffffffff into one of those and you get a rounded, wrong answer with no warning. Here the math is exact at any length: a 100-digit decimal converts to binary correctly, it just takes more digits.

Each field validates against its own base as you type, so entering a 9 in the octal box or a g in hex shows an error instead of a garbage result, and the other three fields update live from whichever one you edited. All of it happens in your tab: check the Network panel in DevTools and you'll see the page makes no requests while you type, so the numbers you convert never leave your machine.