PNG vs JPG: Which Image Format to Use

Does PNG lose quality?

No. Opening a PNG gives back exactly the pixels that were saved, however many times the file is opened and saved again.

PNG stores a picture without losing anything: the specification defines lossless as reconstructing the original data exactly, bit for bit. It carries an alpha channel, and since the Third Edition it can carry frame-based animation as well.

JPG, the usual file name for JPEG, works the other way, and the loss is permanent:

JPEG saves space by throwing detail away. The baseline process, which is what a .jpg file on the web uses, is lossy and cannot be undone, and the standard defines no transparency at all.

Every save as JPEG compresses the picture again. A JPEG that has been converted and saved once more carries the loss from both steps, and the second one cannot be undone by going back.

So the quality question has two halves. A PNG never loses anything it was given, and a JPG loses a little more each time it is saved as JPG.

PNG and JPG Side by Side

PNG keeps every pixel and can carry transparency. JPG, the baseline JPEG process a web .jpg is saved with, discards detail to make a smaller file and has no transparency at all.

Compression, transparency and bit depth are the rows that decide most choices. The size limits matter only for very large pictures, and there JPEG has the lower of the two; the animation row matters only to a PNG, since a JPG holds one picture.

Why use PNG over JPG?

Because PNG keeps three things JPG cannot: clear areas, exact pixels and more than eight bits per channel.

• Transparency. A logo or an icon on a clear background needs an alpha channel, and JPEG defines none. Saved as JPG, the clear areas are filled with a solid colour.
• Exact pixels. A screenshot, a diagram or a line of text in a graphic is only right when every pixel is where it was drawn, and only a lossless format promises that.
• Depth. A PNG can store up to 16 bits per sample. A JFIF file, which is what an ordinary .jpg is, stores 8 bits per channel.

The same reasons explain what a conversion to JPG takes away, which is why the converter on this site states it before it writes a file:

PNG to JPG makes the file much smaller and changes two things for good: the transparent areas are filled with a background colour, and JPEG compression discards detail according to the quality setting.

Are JPEGs smaller than PNGs?

For the same photograph the JPG is usually the smaller file, but how much smaller depends on the picture and on the quality setting, so this page gives no ratio.

JPG buys its size with the detail it discards, and PNG has no such option, because it stores every pixel. The JPEG quality setting decides how much detail is discarded, which is why one photograph can come out at very different sizes.

A percentage quoted without the picture it came from tells you nothing about yours. The converters on this site measure both files after they write one, so the size you see is the size of your own picture in each format.

Should you use PNG or JPG?

Decide by what the picture is and what happens to it next, not by habit. Two rules cover almost every case:

Choose PNG for a picture that needs transparent areas, that you will edit and save again, or whose exact pixels matter, such as a screenshot, a logo or a diagram.

Choose JPG for a finished photograph that has to be small and has no transparent areas.

A picture can take both routes. Keep the working copy as PNG while you edit it, then write a JPG at the end if the finished file has to be small and has nothing transparent in it.

Converting Between PNG and JPG

Converting writes a new file; it does not turn one kind of picture into the other. You can convert a PNG to JPG yourself, choosing the quality and the background colour, and the page tells you what changes before you convert.

JPG to PNG keeps every pixel the JPG already had and adds nothing. The JPEG compression that happened before you got the file is baked into those pixels, and the PNG has no transparency to give an opaque photograph.

Going the other way, you can store a JPG as a PNG, and nothing the JPG lost comes back: the PNG keeps the JPG exactly as it is, losses included.

Converting a picture back to the format it came from does not return the original file. Whatever a step discarded stays discarded, and the return trip writes a third file.

A BMP faces the same choice: you can keep every pixel of a BMP in a PNG, or shrink a BMP into a JPG. A GIF can go either way too: convert a GIF to PNG to keep its clear pixels, or convert a GIF to JPG for a flat, opaque picture.