What is the quality parameter of pictures saved in JPG/JPEG format files?

Since JPG algorithm uses a lossy compression algorithm to reduce the file size, a percentage parameter, also called JPG quality, is introduced to control how picture quality should be maintained in the compression process.

For example, if JPG quality is set to 100%, then no compression should be applied to maintain the original quality (the best quality) of the picture. Of course, 100% quality will result the largest file size.

If JPG quality is set to 0%, then maximum compression should be applied which will result the worst quality loss, but the smallest file size.

Here is table of a single picture saved with different JPG qualities and their file sizes in bytes. The original picture size is 1233x1233 pixels:

Quality     Size   Saving
 10.00%    37692   94.74%
 20.00%    52510   92.68%
 30.00%    64510   91.00%
 40.00%    75039   89.53%
 50.00%    85690   88.05%
 60.00%    97051   86.46%
 70.00%   114526   84.03%
 80.00%   145561   79.70%
 85.00%   172326   75.97%
 90.00%   216516   69.80%
 95.00%   317914   55.66%
100.00%   717012    0.00%

As you can see from the above table, a slight loss of quality from 100% to 95% gives a large saving, 55.66%, in file size from 717012 bytes to 317914 bytes.