How To Convert Numeric String To Formatted String

I have a string value returned from a background tool that will range from 0 to possibly terabytes as a full number.  I want to format that number to use commas and to reduce the character count using an appropriate size modifier (KiB, MiB, GiB, etc).  I've tried converting the string number to a Double value using Double.parseDouble() and then performing the math based on the size of the value with this code:
 
Double dblConversionSize;
String stCinvertedSize;
dblConversionSize = Double.parseDouble(theValue);
if (dblConversionSize > (1024 * 1024 * 1024))
     stConvertedSize = String.format("%,.000d", dblConversionSize / 1024 / 1024 / 1024) + " TiB";
...
 
I've also tried using
 
     String.valueOf(dblConversionSize / 1024 / 1024 / 1024) + " TiB";
 
However, the formatting is failing and I'm either getting a format exception or the result is displayed as a number with no decimal component.