Windows Calculator.
You aren't looking to 'convert' a number, you are looking to change the representation of a value stored in a computer.
10 (base 10, decimal) is equal to 0A (base 16, hexadecimal) is equal to 12 (base 8, octal) is equal to 1010 (base 2, binary).
For that matter, it's also equal to 101 (base 3, trinary?) or 022 (base 4, quarnary?).
I bought a book the other day - "1001 Interesting facts about Binary" I took it back as there was only 9 things in it.
There are 10 types of people in the world, those that understand binary, and those that don't.