Images · guide

PNG to JPG: what the conversion costs you

By Alberto Gulotta · Updated 5 September 2026 · 8 min read

Converting png to jpg takes a few seconds in any of a hundred free tools, and the tools are not the interesting part. The interesting part is that the two formats are built for different jobs, so the conversion always trades something away — and depending on the picture, that trade is either free or ruinous.

Read those two lists side by side and the decision usually makes itself. A holiday photograph as a PNG is a large file for no benefit. A logo as a JPG is a small file that has lost the one property that made it usable.

Mozilla is not the only place this is written down, and the second place is the format’s own specification. The W3C’s PNG specification — a Recommendation of 24 June 2025 — opens by describing “an extensible file format for the lossless, portable, well-compressed storage of static and animated raster images”, and lists what the format carries: “Indexed-color, greyscale, and truecolor images are supported, plus an optional alpha channel.” The transparency a conversion destroys is not a feature some PNG files happen to have: it is one of the things the specification defines a PNG to be, and there “an alpha value of zero represents full transparency”.

The other cost is quieter. JPEG is lossy, so every save discards some detail permanently. Convert once and you will probably never see it. Convert, edit, save, convert again, and the damage accumulates — most visibly around sharp edges and lettering, which is exactly the content the documentation warns about: “applying lossy compression to content requiring sharpness, like diagrams or charts, can produce unsatisfactory results.”

This is also why the reverse search exists. Jpg to png is people trying to undo the trade, and it does not work that way: turning a JPG into a PNG produces a lossless copy of an image that has already lost detail, and gives it a transparent-capable background that is still opaque. The file gets bigger and nothing comes back. Image converter and transparent background are the same question arriving from other directions.

So the honest version of this guide is not “here is the fastest converter”. It is: work out which of the two formats your picture wants, convert once, and keep the original.

One practical habit follows from all of this, and it is worth stating before the guides. Treat the file you started with as the master copy and never overwrite it. Convert into a new file, check that the result still looks right at the size it will actually be seen at, and leave the original where you can find it. It costs a few megabytes on a disk and it means that the next time something needs a different format — a printer wanting one thing, a website wanting another, a form refusing both — you are converting from the good copy rather than from a copy of a copy. Almost every ruined image people bring to this problem has been through that second route without anyone noticing.

Not covered here. It will not send you to a converter site without saying what that site publishes about your file. For most of these jobs your own computer or phone can do it, offline.

It will not pretend the conversion is free. Transparency does not survive it, and lossy detail does not come back.

And it will not rank the converter sites yet. That needs the same set of awkward images — a logo with a soft shadow, a screenshot full of small text, a photograph — through each of them, with the results shown side by side. What holds instead is simple: the format facts on this page are quoted from Mozilla’s developer documentation and the W3C’s PNG specification.