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

formatFilesize.cs

public enum FileSizeSystem { Decimal, Binary }
public static string FormatFilesize (int Size) { return FormatFilesize(Size, 1, FileSizeSystem.Decimal); }
public static string FormatFilesize (int Size, int Decimals) { return FormatFilesize(Size, Decimals, FileSizeSystem.Decimal); }
public static string FormatFilesize (int Size, FileSizeSystem System) { return FormatFilesize(Size, 1, System); }
public static string FormatFilesize (int Size, int Decimals, FileSizeSystem SizeSystem) {
	float OutputSize = (float)Size;
	string[] Units;
	switch (SizeSystem) {
		case FileSizeSystem.Binary:
			Units = new string[]{ "Byte", "kiB", "MiB", "GiB", "TiB", "PiB", "EiB", "ZiB", "YiB" };
			break;
		case FileSizeSystem.Decimal:
		default:
			Units = new string[]{ "Byte", "kB", "MB", "GB", "TB", "PB", "EB", "ZB", "YB" };
			break;
	}
	int n;
	for (n = 0; OutputSize > (SizeSystem == FileSizeSystem.Decimal ? 1000 : 1024); n++)
		OutputSize /= (SizeSystem == FileSizeSystem.Decimal ? 1000 : 1024);
	return OutputSize.ToString("N"+Decimals) + " " + Units.GetValue(n);
}

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

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