Where oversized PNG files come from
PNG is a lossless format. Every pixel you put in comes back out exactly, which is why screenshot tools, design apps, scanners and game capture software all default to it. The price is size. A full-screen 1920x1080 screenshot lands at 1 to 2 MB, a 4000x3000 scan of a photo print can pass 20 MB, and a photo exported as PNG from an editor is often ten times bigger than the same picture as a JPG. PNG compression works well on flat colour and repeated patterns, and poorly on the subtle noise and gradients that make up a photograph or a scan.
JPG is the opposite trade. It throws away detail your eye is unlikely to notice, and in exchange it makes photographic content dramatically smaller. That is what this page does: it decodes the PNG into raw pixels inside a Web Worker and re-encodes them with the mozjpeg codec compiled to WebAssembly, on your device, without an upload.
What changes in the conversion
Three things are worth knowing before you click Convert all.
- Transparency is dropped. JPG has no alpha channel. The encoder simply ignores it, and because a fully transparent pixel carries black colour values underneath, transparent regions typically come out black, not white. If your PNG has a transparent background, flatten it onto a background colour in an editor first, or use PNG to WebP which keeps the alpha.
- The compression is lossy and permanent. Sharp edges, thin lines and small text pick up faint ringing and blocks. This is not a defect of the tool, it is what JPG does. Keep the PNG if you might need to edit the image again.
- Metadata is removed. The pixels pass through a canvas, so text chunks, colour profiles and any EXIF the PNG carried are gone. The output is a progressive JPG in standard sRGB with the same pixel dimensions as the input.
Choosing a quality setting
The slider runs from 20 to 95 and starts at 80. For photographs and scans, 75 to 82 is the sweet spot: the file is small and artefacts are hard to find at normal viewing size. For screenshots that contain text, push it to 85 or 90. Text is where JPG struggles most, and a few extra kilobytes buy clean letter edges. Above 90 the file grows quickly while the visible gain is small. Below 60 you will start to see mottled colour around edges on most content.
Some typical outcomes, not guarantees:
- A 1920x1080 desktop screenshot of 1.4 MB as PNG usually becomes 180 to 320 KB at quality 80, and 350 to 500 KB at quality 90.
- A 4000x3000 scanned photo of 22 MB as PNG usually lands between 1.5 and 3 MB at quality 80.
- A 1200x800 flat illustration with few colours may only shrink from 90 KB to 60 KB, and will look worse. PNG was already the right format for it.
When to keep the PNG
Do not convert logos, icons, diagrams, UI mockups or anything with a transparent background. They compress well as PNG already and JPG will visibly damage them. Do not convert an image you intend to keep editing, because every re-save as JPG loses a little more. And if the PNG is itself a saved copy of a JPG, converting it back gains nothing except a second round of loss.
If your real problem is a file size limit rather than the format, Compress image lets you set a target and also resize, which often matters more than the format switch. For a longer comparison of the two formats, see PNG vs JPG.
Practical limits
Because everything runs in your browser, the ceiling is your device memory. A 12000x9000 PNG decodes to over 400 MB of raw pixels and may fail on a phone or in Safari, which caps canvas dimensions. Files with 16-bit channels are reduced to 8 bits, and CMYK is not supported at all, since the browser decoder only produces RGB. Batch conversion is supported, but each file is processed one after another and downloaded individually.