Compress PNG

Why a PNG is so big, and what actually makes it smaller

A PNG stores every pixel exactly as it was given: three bytes of colour and, if it has transparency, a fourth byte of alpha. That is why it is the right format for a cutout and the wrong one for a photograph. A 12-megapixel photo saved as PNG is tens of megabytes, and the same picture as JPG is two.

The PNG format also has a second, quieter mode: instead of a colour per pixel it can store a table of up to 256 colours and then one number per pixel saying which of them to use. For a photograph that sounds ruinous, and for a cutout it usually is not: put the two side by side and the difference is hard to find, while the file is between a half and a fifth of the size. That is what this page does, and it is what every PNG compressor worth using does.

Transparency survives it

The thing most palette compressors get wrong is the alpha. A cutout's edge is not opaque or transparent, it is a ramp through every value in between, which is what makes hair look like hair. A compressor that treats transparency as an afterthought spends its whole table on the opaque middle of the subject and leaves that ramp in three or four steps, which shows as a hard, stepped outline.

Here transparency is part of the colour from the start, so the table is spent where the picture needs it. A cutout keeps its soft edge, and a picture with no transparency at all is not given any.

Many files at once

Drop a folder's worth. Files are compressed one after another rather than all at once, on purpose: quantising is the heaviest thing this site does on the page's own thread, and a queue that finishes a file and shows you what it saved is better company than a frozen tab. Each row carries its own before and after, and the Save all button puts every compressed file into one ZIP.

There is no limit on the number of files and no limit on their size, because there is no server to pay for: the work happens on your machine. The tools that upload have to cap both, which is why they do. The practical limits here are your own: the tab's memory on a very large batch, and the few seconds each big picture takes. It runs in any modern browser on Windows, macOS, Linux, Android and iOS, with nothing to install.

A size to fit inside

Most of the time the question is not "how many colours" but "how do I get this under 200 KB", because an upload form, an email limit or a visa portal gave somebody a hard number. Set a size limit above and each file is searched for the largest palette whose PNG fits under it, which is a handful of encodes and a few seconds. The picture is never resized to reach the number, and when even the smallest palette will not fit, the row tells you what it managed rather than quietly handing back something the wrong shape.

What it will not do

It will not make a file bigger. Artwork with few colours, and a PNG somebody has already compressed, can come back larger after a round trip; when that happens the original is handed back untouched and the row says so, rather than quietly growing your file.

It will not resize your picture either. The dimensions that went in are the dimensions that come out, because a compressor that silently shrinks a photo is solving a different problem from the one you asked about. If a smaller picture is what you want, the PNG to WebP converter will usually beat any PNG for a website, and the background remover has canvas sizes for social media.

And it cannot tell you which pictures on your website are the heavy ones, because it only sees the file you drop on it. For that, getReport's image size checker loads a page you have published and lists every picture on it by weight, by the size it is displayed at against the size it was saved at, and by whether it has alt text. Compress the ones it names and you have spent the effort where it counts.