What is a PNG file?

PNG works in two stages. First it filters each row of pixels, replacing each value with its difference from a neighbouring pixel — so a smooth gradient becomes a long run of near-identical small numbers. Then it runs DEFLATE, the same algorithm a ZIP archive uses, over the result.

That design explains precisely which images PNG is good at. Flat colour, repeated patterns, sharp edges and text all produce long predictable runs after filtering, and DEFLATE collapses them dramatically.

Photographic noise produces nothing predictable at all, which is why a photograph in PNG is enormous. It is not a failure of the format — it is the format doing exactly what it promises, on content that offers it nothing.

PNG's alpha channel stores 256 levels of opacity per pixel, not a single transparent colour. That means genuinely soft edges: a drop shadow fading to nothing, an anti-aliased logo that sits cleanly on any background colour.

This is the main reason PNG remains indispensable. GIF offers only binary transparency, which produces jagged fringes; JPEG offers none at all.

WebP matches PNG here — full alpha, typically 25–35% smaller — which makes it the better choice for the web specifically.

PNG-24 stores full colour. PNG-8 stores a palette of at most 256 colours, which can be four times smaller.

Reducing an image to 256 colours is a lossy operation, performed before the lossless save. On a logo with six flat colours it is invisible and the saving is real. On a photograph or anything with a gradient it produces visible banding and dithering noise.

This matters when reading tool claims. A "lossless PNG compressor" advertising 70% savings is almost certainly quantising the palette. That may be fine for your image — but it is not lossless, and you should know which you are getting. The PNG tool on this site does not quantise, which is why its numbers are more modest and genuinely reversible.

Three usual causes, in order of frequency.

It is a photograph. Wrong container. Convert to JPEG or WebP and it will drop by 80–95%.

It is a screenshot from a high-density display. A screenshot of a window 1440 points wide on a 2× screen is really 2880 pixels wide. Halve the dimensions and the file drops by roughly three quarters.

It has a lot of noise or texture. Photographic grain, film effects and heavy gradients all defeat lossless compression. If the content is not flat, PNG cannot help you.