How compression works
The compressor re-encodes your image as a JPEG at the quality you choose. JPEG is a lossy format, so lowering the quality discards fine detail the eye is least likely to notice — which is what shrinks the file. The live stats show the original size, the new size, and how much you've saved so you can find the sweet spot between size and appearance.
Frequently asked questions
What format is the output?
Always JPG. JPEG compression is what makes photos dramatically smaller, so the tool standardises on it for the download.
How much smaller will my image be?
It depends on the image and the quality you pick — the “Saved” figure updates instantly as you move the slider so you can see the trade-off in real time.
Will the image look worse?
At high quality the difference is usually hard to spot. As you lower the slider you'll eventually see softness or blocky patches, so stop just before quality noticeably drops.
Are my images uploaded?
No. Compression happens entirely in your browser. Your files never leave your device.