this post might fit well to your question.
also check this thread. there is so much info here on SO.
googled a bit and here is the solution (found at android git)
File path = Environment.getDataDirectory();
StatFs stat = new StatFs(path.getPath());
long blockSize = stat.getBlockSize();
long availableBlocks = stat.getAvailableBlocks();
return Formatter.formatFileSize(this, availableBlocks * blockSize);