Converting KB to MB, GB, TB dynamically

You are performing integer division. So the result of division is also integer. And fractional part is truncated.

so, 1245 / 1024 = 1

Change your division to floating point division: -

double m = size/1024.0;
double g = size/1048576.0;
double t = size/1073741824.0;

Also, your comparison is faulty. You should do the comparison with 1.

if (m > 1), if (t > 1), if (g > 1)

Ideally I would change your comparison to: -

    if (t > 1) {
        hrSize = dec.format(t).concat("TB");
    } else if (g > 1) {
        hrSize = dec.format(g).concat("GB");
    } else if (m > 1) {
        hrSize = dec.format(m).concat("MB");
    } else {
        hrSize = dec.format(size).concat("KB");
    }

You need to compare with the higher unit first, and then move to the lower one.

The problem is that you're using integer division. Change your code to:

double m = size/1024.0;
double g = size/1048576.0;
double t = size/1073741824.0;

In your original code, double m = size/1024 would divide the integer size by 1024, truncate the result to an integer, and only then convert it to double. This is why the fractional part was getting lost.

A modified version. Only calls format once. Includes "Bytes".

public static String formatFileSize(long size) {
    String hrSize = null;

    double b = size;
    double k = size/1024.0;
    double m = ((size/1024.0)/1024.0);
    double g = (((size/1024.0)/1024.0)/1024.0);
    double t = ((((size/1024.0)/1024.0)/1024.0)/1024.0);

    DecimalFormat dec = new DecimalFormat("0.00");

    if ( t>1 ) {
        hrSize = dec.format(t).concat(" TB");
    } else if ( g>1 ) {
        hrSize = dec.format(g).concat(" GB");
    } else if ( m>1 ) {
        hrSize = dec.format(m).concat(" MB");
    } else if ( k>1 ) {
        hrSize = dec.format(k).concat(" KB");
    } else {
        hrSize = dec.format(b).concat(" Bytes");
    }

    return hrSize;
}

I love this:

public static String getDynamicSpace(long diskSpaceUsed)
{
    if (diskSpaceUsed <= 0) {
        return "0";
    }

    final String[] units = new String[] { "B", "KiB", "MiB", "GiB", "TiB" };
    int digitGroups = (int) (Math.log10(diskSpaceUsed) / Math.log10(1024));
    return new DecimalFormat("#,##0.#").format(diskSpaceUsed / Math.pow(1024, digitGroups))
            + " " + units[digitGroups];
}