Formats a filesize into a human readable format. First parameter is filesize in bytes, second parameter is the precision of the resulting float and the last parameter is the system (choose between 'decimal' aka SI-based or 'binary' based calculation).

formatFilesize.js

var formatFilesize = function (size, decimals, system) {
	size = parseInt(size, 10)
	if (!decimals && decimals!==0) decimals = 1
	if (!system) system = 'decimal'
	var fileSizes = {
		decimal: ['Byte', 'kB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB'],
		binary: ['Byte', 'KiB', 'MiB', 'GiB', 'TiB', 'PiB', 'EiB', 'ZiB', 'YiB'],
	}
	var base = system=='decimal' ? 1000 : 1024
	var steps = Math.floor(Math.log(size) / Math.log(base))
	return parseFloat((size / Math.pow(base, steps)).toFixed(decimals)) + ' ' + fileSizes[system][steps]
};

PHP: https://gist.github.com/EarMaster/da9b2360867f6df12868

C#: https://gist.github.com/EarMaster/cde681fa6b7d6b307e8e7ed0d3847662