What a JPG to PNG conversion can and cannot do
This is the one conversion on the site where people most often expect the wrong thing. PNG is lossless, so it is tempting to hope that converting a JPG into it will restore sharpness. It will not. The JPG's compression artefacts, the blocks around edges and the smeared colour in shadows, were baked in when the JPG was saved. Converting to PNG copies those pixels exactly as they are, artefacts included. What you gain is that nothing further is lost from this point on. What you pay is size: a lossless copy of a photograph is large.
There is no quality slider on this page because there is nothing to trade off. The worker decodes the JPG, and the WebAssembly PNG encoder writes every pixel back out as 8-bit RGBA.
Good reasons to convert anyway
- An app or form that only accepts PNG. Some upload fields, presentation tools and CMS image slots are hard-coded to PNG.
- You are about to edit the image in several rounds. Each time a JPG is saved it loses a little more. Convert once, do all your edits on the PNG, and export a single final JPG at the end.
- You want to add transparency. A JPG cannot hold an alpha channel. Convert to PNG, then cut the subject out with Remove background.
- You need a clean master for a screenshot or diagram that was unfortunately delivered as a JPG and will be reused in documentation.
What changes inside the file
The decode runs through a browser canvas, and that has consequences beyond the codec. EXIF data, including camera model, capture date and GPS coordinates, is not copied. Modern browsers apply the EXIF orientation while decoding, so a phone photo that was stored sideways and rotated by a tag comes out physically upright in the PNG, which is generally what you want. Any embedded ICC profile is applied and then discarded; a Display P3 photo from a recent phone is mapped into sRGB, which is a slight narrowing of the most saturated colours. Progressive encoding, a JPG-only concept, no longer applies. The output is fully opaque with the original pixel dimensions.
If the reason you wanted a new file was to strip location data rather than change the format, Remove EXIF does that while keeping the JPG small.
Expect the file to grow a lot
Typical figures, not promises:
- A 4032x3024 iPhone photo of 2.8 MB as JPG usually becomes 15 to 22 MB as PNG.
- A 1920x1080 web image of 350 KB usually becomes 2.5 to 4 MB.
- A 800x600 screenshot of a mostly flat interface that someone saved as a 90 KB JPG may come out at 300 to 600 KB, because the JPG noise defeats PNG's pattern compression.
The encoder here writes a straightforward 32-bit RGBA PNG. It does not reduce the image to a palette or run the slow brute-force filters that dedicated optimisers use, so a tool built specifically for shrinking PNGs may get 10 to 30 percent off the result. For most uses that step is unnecessary.
When not to do this
Do not convert photos to PNG for use on a website; the page will load several times slower for no visible benefit, and JPG to WebP is the right direction there. Do not convert to send a smaller attachment, since the file only grows. And do not convert in the hope of repairing a heavily compressed image. The article PNG vs JPG explains the two formats side by side if you are unsure which one a given task needs.
Very large JPGs, say 50 megapixels and up, can exceed what a phone browser will allocate for a canvas. The conversion then fails with an error rather than producing a corrupt file, and nothing has been sent anywhere in the meantime.