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Why saving a PNG again barely helps
PNG is a lossless format: it keeps every pixel exactly as it is. That is why it has no quality setting to lower. When a program saves a PNG again, all it can do is rearrange the same information, and the gain is usually small. The browser's PNG encoder, which is what this page uses, does not even offer that control.
To make a PNG genuinely light, the format has to change. That is what this tool does: it turns the PNG into a lossy WebP, at the quality you pick. Google publishes that this kind of WebP, keeping the transparency, typically comes out at a third of the PNG's size. Even lossless WebP is already 26% smaller.
Transparent background: pick WebP, not JPEG
A logo, a cut-out product, an icon: many PNGs exist because of the transparent background. JPEG has no alpha channel, which means it cannot store transparency. When you pick JPEG, the tool fills the background with white so it does not come out black. On a white page nobody notices; over a colour or a photo, a white box shows up around the image.
WebP keeps transparency, even when lossy, which is why it is selected by default. Leave JPEG for a PNG that has nothing transparent and has to go somewhere that refuses WebP.
One warning: if your browser cannot save WebP, the option shows as unavailable and the result comes out as JPEG, with a white background. To keep the transparency, open this page in another browser.
The most expensive mistake: a photo saved as PNG
PNG was made for flat-colour images: logos, icons, diagrams, screenshots. A photograph saved as PNG carries every colour variation of every pixel with no loss at all, and it weighs far more than it would as JPEG or WebP, with no difference anyone can see on screen.
That is where this tool helps most: product photos exported as PNG, banners downloaded as PNG from an image editor, screenshots used as featured images. Drop the file in, compare before and after in the list itself, and decide.
When keeping the PNG is right
Small text and sharp edges are where lossy compression is weakest: it smudges the outline of letters. For screenshots with text, diagrams and logos with few colours, raise the quality to 90 or 95 and check the result zoomed in before switching. If the letters still look dirty, keep the PNG and only reduce its dimensions.
Keep it as well when the file is a working master that will be edited again, or when whoever receives it requires PNG. Publishing on a web page is almost never one of those cases.
Screenshots: width first
A screenshot taken on a high-density screen comes out with twice the pixels you saw: a 1,440-point window becomes a PNG 2,880 pixels wide. If the article shows it at 800 pixels, most of the file's pixels are never drawn at all.
Use the compressor's max width, 1,200 or 800 px, or the image resizer if you need an exact size. Halving the width leaves only a quarter of the pixels.
On a WordPress site, without converting the library
Converting each PNG by hand fixes today's file. NimboCDN fixes it at delivery: the original PNG stays in your library, and each visitor gets the image at the size their screen draws, in WebP on the free plan or AVIF on the Pro plan, with the transparency preserved. Nothing in your library is rewritten.
To see what the images on your pages weigh today, the image size checker weighs each one and estimates what they would weigh afterwards.
Questions
Does the transparent background stay transparent?
Yes, if you save as WebP, which is selected by default. Not in JPEG: the format cannot store transparency, and the tool paints the background white.
Why did the result come out heavier than the PNG?
It happens with small PNGs with few colours that were already well compressed. The list marks them «already small», and the right move is to keep the original.
Can a PNG be compressed without losing anything?
Not with this tool: it swaps the PNG for a lossy WebP, at the quality you choose. Lossless PNG optimisers exist, which rearrange the compression without changing a single pixel, but they gain much less.
What about an animated PNG?
Only the first frame survives, so this is not the right tool for animations.
Are my files uploaded anywhere?
No. The conversion runs in your browser, on your device, and you can check it in the network tab of the developer tools: no request goes out.