What compression actually trades away
JPEG compression works by discarding the detail your eyes are least likely to miss. It leans on two quirks of human vision: we are far more sensitive to brightness than to colour, so colour information is stored at a lower resolution, and we struggle to see very fine, low contrast texture, so that gets simplified too. At sensible settings the trade is a bargain, a fraction of the bytes for a picture that looks the same at arm's length.
Push the slider down far enough, though, and the losses become visible: smooth gradients break into bands, sharp edges grow faint halos, and flat areas turn blocky. The slider on this tool re-encodes your image live, so you can watch the file size fall and judge with your own eyes exactly where the picture stops looking right.
Worked examples
Choosing a quality setting
The default of 80 suits most photos going on the web or into an email: substantial savings, invisible losses at normal viewing sizes. Drop to 60 or 70 for thumbnails and images that will only ever be seen small. Go up to 90 or so for anything you plan to print or edit further, and accept the bigger file. Settings above 90 inflate the file rapidly for improvements that are essentially imperceptible, which is why the slider stops at 95.
Two caveats worth knowing. Compression is lossy, so the detail removed can never be recovered; keep your original and compress a copy. And re-encoding an already compressed image at a higher quality than it was saved at can actually produce a larger file, because you are paying full price for detail that is no longer there. If the saving row says the file got no smaller, lower the slider or leave the original alone. For screenshots, logos and other flat colour graphics, PNG is usually the better format anyway.
Private by design
Your photo never leaves your device. The file is read locally, redrawn on a canvas in your browser, and re-encoded by your own machine's JPEG encoder; there is no upload, no server-side processing, no queue and no storage. That makes it safe for photos of your family, your documents or your clients, and it is also why the slider responds instantly: nothing has to make a round trip over the internet.
One side effect of the re-encode is worth noting: the output is always a JPEG, whatever went in, and transparency is not part of the JPEG format, so transparent regions come out white. If dimensions rather than quality are your problem, the image resizer will shrink the pixel count instead, and combining the two (resize first, then compress) gives the biggest savings of all.
Frequently asked questions
Is my image uploaded anywhere?
No. The compression runs as JavaScript on your own device using the browser's built-in JPEG encoder. Your photo never travels over the internet, nothing is logged or stored, and closing the tab removes everything.
What quality setting should I use?
For photos on the web, 75 to 85 is the sweet spot: big savings with differences you will struggle to spot. Below about 60 you start to see blockiness and smudged detail. Above 90 the file grows quickly for gains most eyes cannot detect.
Why is the compressed file sometimes bigger than the original?
If the original was already heavily compressed, re-encoding it at a high quality setting can produce more bytes than it started with. Graphics with flat colours and sharp edges can also grow, because they suit PNG better than JPEG. In either case, lower the slider or keep the original.
Can I get the quality back afterwards?
No. JPEG compression is lossy, meaning the discarded detail is gone for good, and recompressing an already compressed image loses a little more each time. Always keep your original file and compress copies of it.
Does it keep transparency?
No. The output is JPEG, which has no transparency, so any transparent areas are filled with white. If you need transparency to survive, keep the image as a PNG and use the image resizer to reduce its dimensions instead.