Long float-number output shows letters

That is Scientific Notation.

AND you are getting 388 instead of 311 because you are dividing by 1000000 instead of 1048576 (1024 * 1024)

EDIT: 311 is not achieved even with 1048576, that way you get 370… so the error is probably in your calc 😉

As described here , you just have to convert your Scientific Notation to a Decimal Notation through a Formatter.

DecimalFormat df = new DecimalFormat("#.########");
return df.format(fileLengthMegabytes);

Running Example: http://ideone.com/2lkKv7

import java.util.*;
import java.lang.*;
import java.text.*;

class Main
{
    public static void main (String[] args) throws java.lang.Exception
    {
                DecimalFormat df = new DecimalFormat("#.##########");

        float fileLengthMegabytes1 = (float) 388 / 1000000;
        float fileLengthMegabytes2 = (float) 388 / 1048576;
        System.out.println("MB1 in Scientific Notation: " + 
                            fileLengthMegabytes1);        
        System.out.println("MB1 in Decimal Notation: " + 
                            df.format(fileLengthMegabytes1));
        System.out.println("MB2 in Scientific Notation: " + 
                            fileLengthMegabytes2);        
        System.out.println("MB2 in Decimal Notation: " + 
                            df.format(fileLengthMegabytes2));
        }
}

Output:

MB1 in Scientific Notation: 3.88E-4

MB1 in Decimal Notation: 0.000388

MB2 in Scientific Notation: 3.7002563E-4

MB2 in Decimal Notation: 0.0003700256