Number base converter
Binary, octal, hexadecimal and every base in between, on numbers of any size. Most converters go through a JavaScript number and quietly round anything past 9,007,199,254,740,991; this one reads your digits one at a time, so it does not. Nothing you type here leaves your browser — it is worked out on your own device, so there is nothing to send and nothing to log.
How a base works
A base is a way of writing a number, not a property of the number itself. In base b, each digit is worth b times the one to its right, so the digits are the coefficients of a sum of powers. That is the whole idea, and the working below shows that sum with your own digits in it.
Base 36 is as far as this goes because that is the digits 0–9 and the letters a–z, and there is no agreed thirty-seventh character. Base 64 is a different thing entirely — it encodes bytes rather than numbers, and it has its own pages.