What is used to calculate JPEG compression?

I ran a test and got this on one of my JPEGs:

FAILED - (39.7 KB, compressed = 25.2 KB - savings of 14.5 KB)

But I checked a couple of sites and didn’t get anywhere near that much compression. I checked smush.it and jpegmini which I saw mentioned in another thread here.

Where does the 25.2 KB come from?

At the bottom of the “Performance Review” Page you can read the following about image compression metric:
What is checked GIF - All pass
PNG - Must be 8 bit or lower (no 24-bit PNGs will pass)
JPEG - Within 10% of a photoshop quality 50 will pass, up to 50% larger will warn and anything larger than that will fail.
The overall score is the percentage of image bytes that can be saved by re-compressing the images.

Not sure why jpegmini didn’t show benefits but smush.it does not do lossy recompressions which is why it wouldn’t help. The 25KB comes from re-compressing the JPEG image at jpeglib quality level 85 (roughly equivalent to photoshop save-for-web 50).

Ah, okay. Didn’t realize it was talking about lossy compression. I chose the compression I had because that’s what looked best at a reasonable file size. I find that anything under 90% starts to look quite bad.

Makes sense. Is there any text in the image (that’s usually the main driver for needing higher qualities). Like with most of the rules, they are generally guidelines but there are always cases where they won’t be applicable.