How PNG compression works
PNG compression is not a quality dial, and that is the first thing worth understanding. There is no setting that trades detail for size, because PNG is lossless by definition — every pixel that goes in comes out exactly.
What can be improved is the *encoding*. PNG compresses in two stages: a per-row prediction filter that makes the data more repetitive, then DEFLATE over the result. Most software picks a filter quickly and moves on, which leaves real savings on the table. This tool runs oxipng, which tries the filter combinations properly, strips chunks that carry nothing useful, and reduces the colour type when the image does not need what it declares — a greyscale photo stored as full RGB, for instance, or an image with an alpha channel where nothing is actually transparent.
None of that touches the pixels. Open the before and after in any editor and difference them, and you get a black frame.
The typical result is 10–25% smaller. On files that came out of a well-configured export pipeline it can be nothing at all, and this tool will tell you so rather than pretending otherwise — a file that is already optimal is reported as such and left alone.
What to expect
Screenshots and UI exports compress best, often 20–30%, because they contain large flat areas and few colours and are usually written by tools that do not optimise. Photographs saved as PNG barely move — perhaps 5% — because photographic data is close to incompressible losslessly. If you have a photographic PNG and you want it substantially smaller, the honest answer is that you need a different format, not better PNG compression.
Files exported from image editors with an "optimise" or "export for web" option are frequently already close to optimal, and you should expect single-digit savings or none.
When compressing PNG is the wrong move
Do not use this if you need a dramatic size reduction on a photograph. Lossless compression cannot deliver it. Convert to WebP or AVIF, which will be five to ten times smaller at quality settings where the difference is invisible.
Do not use it as a substitute for resizing. A 4000-pixel-wide PNG displayed in a 400-pixel column is wasting 99% of its data, and no encoder can fix that — resize first, then compress.
Do not expect it to help with an already-optimised file. If a previous tool did the same work, there is nothing left to find, and a second pass changes nothing.
Frequently asked questions
Does this reduce image quality?
No. PNG is a lossless format and this is a lossless optimisation — the output contains exactly the same pixels as the input. The savings come from encoding the same data more efficiently, not from discarding any of it.
Why did my file barely get smaller?
Either it is a photograph, which does not compress losslessly no matter what you do, or it was already optimised by whatever exported it. The tool reports "already optimal" rather than producing a slightly different file of the same size, because there would be no point.
Why is my PNG so large in the first place?
Almost always because it is a photograph. PNG stores every pixel exactly, including sensor noise, and photographic data is close to random from a compressor's point of view. The same photo as JPEG or WebP will be five to ten times smaller with no visible difference.
Does compression remove transparency?
No. The alpha channel is preserved. If the image declares an alpha channel but nothing in it is actually transparent, the optimiser may drop the channel — which is a real saving and changes nothing about how the image looks.
Is there a file size limit?
Only your device's memory. Compression runs in your browser, so it costs us nothing and there is no reason to cap it, meter it, or ask you to sign up. Very large files simply take longer.