When I save a JPG file with GIMP, I can adjust the quality I save it at, from 0-100 (I use 89). It seems like I've used an app to see what this number was on saved file but if I did I can't for the life of me figure out what it was. Any suggestions as to what to use?


6 Answers 6


Once saved, you cannot tell the quality anymore.

(Setting the quality while saving just tells the software how much loss you find acceptable, but once saved: what's lost is lost. You'd need a human to say if something looks nice.)

Hmmm, I guess I was wrong. I still think the above is correct, but ImageMagick's identify proves me wrong?

identify -verbose myimage.jpg

Image: myimage.jpg
  Format: JPEG (Joint Photographic Experts Group JFIF format)
  Class: DirectClass
  Geometry: 358x240+0+0
  Resolution: 300x300
  Compression: JPEG
  Quality: 90
  Orientation: Undefined

I don't know how the image in my test was saved, but it does not have any EXIF data. Could the quality still be stored in the image?

  • Can't you experiment converting to different qualities? I find it hard to believe, unless ImageMagick stores some private data in the jpg (so this might not work with other packages).
    – harrymc
    Oct 29, 2009 at 20:37
  • Interesting. I'll wait to see how this pans out.
    – Nathaniel
    Oct 29, 2009 at 21:10
  • 1
    +1 Yes imagemagick works. I can repeatedly change the jpeg quality and use identify to see the change. This works if I use convert (another imagemagick untility) or another tool like MS Photo Editor. Oct 29, 2009 at 22:20
  • 3
    ImageMagick is doing something different. It gives an estimate, rather than reading what your original software did. Your original, now-crossed-out, answer is really more correct. See @sleske's answer.
    – mattdm
    Mar 24, 2017 at 13:14
  • ImageMagick returns a quality assessment number, but that doesn't mean it's correct. See faqs.org/faqs/jpeg-faq/part1/section-5.html from the jpeg creators themselves, and photo.stackexchange.com/questions/88167/… Feb 18, 2021 at 23:11

To add to Arjan's answer:

ImageMagick's identify appears to actually look inside the JPEG image to guess the quality setting used to encode it.

ImageMagick's source code (cheer for free software :-)) contains the lines:

  Determine the JPEG compression quality from the quantization tables.
for (i=0; i < NUM_QUANT_TBLS; i++)
  if (jpeg_info.quant_tbl_ptrs[i] != NULL)
    for (j=0; j < DCTSIZE2; j++)

(coders/jpeg.c, line 843ff. in my recent version of ImageMagick's source code).

I don't know enough about JPEG to really understand, but it appears to do something like described in this article:

Determine the JPEG quality factor by using Visual C# .NET (link dead as of Januar 2018; copy on archive.org from 2015)

So yes, identify can actually determine the quality setting of a JPEG just from the compressed file alone (though the result may not always be completely accurate).

  • 2
    Whoa. Very nice of you to check the source code. Cool.
    – Nathaniel
    Jan 4, 2010 at 20:17
  • @Nathaniel, can you please select this answer as being the accepted one, instead of mine? Thanks! (I cannot delete mine as long as it's accepted.)
    – Arjan
    Mar 24, 2017 at 14:22
  • It is searching for the JPEG quantization table that best accounts for how the compressed bitstream looks.
    – jbarlow
    Apr 19, 2018 at 4:05
  • Constants (jpeglib.h): NUM_QUANT_TBLS = 4. DCTSIZE2 = 64. quantval[i] just gets the number at given position.
    – user136036
    Feb 25, 2020 at 23:58

As Arjan metioned identify -verbose myimage.jpg will do it. As imagemagick is a CLI tool, it may be useful for scripting. The approach identify -verbose myimage.jpg | grep ... is preety slow. I recommend using IM like this

identify -format '%Q' myimage.jpg

It is massively faster.


JPEGsnoop is a nice alternative to ImageMagick's identify. The download is quite small and is available in portable format.

After processing a jpg, you will find the "Approx quality factor" under the DQT marker.


Picasa 3 has the properties pane which shows the jpeg quality but it is an abandonware at the moment. Picasa 3 Dick Masterson


With ImageMagick++ library it's easy:

Image magick_image( pathname );
size_t compressionFactor = magick_image.quality(); // 0..100

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.