Does quality 80 mean the file will be 80 percent of its original size?
No. The quality number sets how much detail the encoder keeps, and it has no direct link to the output size. Quality 80 can shrink one image by 87% and another by only about a third, and it can make a third one larger. That is why the tool shows the measured size and percentage change for each file instead of predicting them.
So what does the number mean?
Picture the encoder cutting the image into small blocks and deciding, block by block, how much fine detail is worth keeping. The quality number is how generous it is: at 100 it keeps as much as it can, and as you go lower it drops finer and finer detail. The output size depends on two things the number knows nothing about: how much detail is in the picture, and how it was stored before it reached you. A clear sky and a patterned rug at the same quality will never come out the same size.
Four images, one quality, four results
All compressed in Chrome as JPEG at quality 80:
A 640×480 PNG of 271.3 KB became 35.2 KB, 87% smaller.
A 1600×1200 JPEG of 689.5 KB became 250.9 KB, 63.6% smaller.
An 800×600 JPEG of 95.4 KB became 58.4 KB, 38.8% smaller.
A 400×300 JPEG of 5.74 KB, already saved at low quality, became 7.82 KB, 36.3% larger.
If quality 80 meant 80% of the size, all four would have landed on the same ratio. The spread comes from the source: the PNG keeps everything, so it had lots to drop, while the tiny pre-compressed JPEG had nothing left to give.
How the percentage in the table is worked out
The Change column in the Image compressor divides the difference between the two sizes by the original size. For the second image: the original is 706,043 bytes, the output is 256,970 bytes, and the difference divided by the original is 63.6%. When the output is bigger, the same calculation is shown as an increase with a warning, never as a confusing negative number.
Putting this to use
- Do not assume quality 50 will give you half the size. Try it and read the number.
- When you have a size target, move the slider while watching the size, not the quality number.
- If the saving is small at 80, the photo was already well compressed, and going lower will hurt the look for little gain.
- If the saving is huge, as with photographic PNGs, you can often raise the quality a little and still get an excellent reduction.
Common questions
Does the same quality number give the same result in every program?
No. Each program and browser has a slightly different encoder, so quality 80 in one is not necessarily quality 80 in another.
Is 80 a good place to start?
Yes, it is a sound starting point for JPEG store photos. Then judge the preview with your own eyes.
Does the size change if I switch format but keep the same number?
Yes. Quality 80 as WebP gives a different size from quality 80 as JPEG.