public static string FileSizeToString(long fileSize)
{
string[] units = { “bytes”, “KB”, “MB”, “GB”, “TB”, “PB”, “EB” };
string[] formats = { “{0:f0} {1}”, “{0:f1} {1}”, “{0:f2} {1}” };

double value = fileSize;

int thousand = 1000;
int kilobyte = 1024;

int index = 0;
int decimals = 0;

if (value >= kilobyte)
{
while (value >= thousand)
{
value = Math.Floor(value) / kilobyte;
index++;
}

if (value < 10)
{
value = Math.Floor(value * 100) / 100;
decimals = 2;
}
else if (value < 100)
{
value = Math.Floor(value * 10) / 10;
decimals = 1;
}
else
{
value = Math.Floor(value * 1) / 1;
// precision = 0; // already zero
}
}

return String.Format(formats[decimals], value, units[index]);
}