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.php

<?php
function format_filesize($size, $decimals=1, $system='decimal') {
	$size = intval($size);
	$fileSizes = array(
		'decimal' => array('Byte', 'kB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB'),
		'binary' => array('Byte', 'KiB', 'MiB', 'GiB', 'TiB', 'PiB', 'EiB', 'ZiB', 'YiB')
	);
	for ($n=0; $size>($system=='decimal'?1000:1024); $n++)
		$size = $size/($system=='decimal'?1000:1024);
	return round($size, $decimals).' '.$fileSizes[$system][$n];
};

JavaScript: https://gist.github.com/EarMaster/6974018

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