Compress PNG

How do I compress a PNG file?

1. Drop your PNG files onto the drop zone, or click to browse.
2. Choose a Method: Lossy (reduce colours, much smaller) or Lossless (re-encode only).
3. In lossy mode, set Colours. 256 is usually indistinguishable from the original; try 64 for flat graphics.
4. Click Compress PNG.
5. Download each file, or Download all as ZIP for a batch.

PNG is a lossless format, which is why a 1920 by 1080 screenshot can weigh 2 to 4 MB while the same picture as a JPG is a few hundred kilobytes. Compress PNG attacks that from two directions. Lossy mode runs the pixels through UPNG.js colour quantisation, which builds a palette of up to 256 colours and maps every pixel to its nearest match. The result is an 8 bit indexed PNG instead of a 32 bit RGBA one, so each pixel takes one byte rather than four before the deflate stage even starts.

For screenshots, UI mockups, logos, charts and anything with flat areas of colour, that quantisation typically cuts the file by 60 to 80% with no visible change at the default 256 colours. Dropping Colours to 128 or 64 saves a little more and still looks clean on graphics with a limited palette. Photographs and soft gradients are the exception: with only 256 colours you may see faint banding, so keep the count at 256 or use Compress JPG for photos.

Lossless mode skips quantisation and simply re-encodes the image. The gains are modest, often 5 to 20%, because most PNGs are already reasonably well compressed, but the pixels are bit for bit identical to the original. Choose it when you need exact colours, for example brand assets that must match a hex value, or images that will be edited again.

Alpha transparency survives both modes. In lossy mode the palette includes transparency values, so semi-transparent edges and drop shadows are preserved rather than flattened onto a background. The tool does not resize; pair it with Resize Image if your PNG is also far larger in pixels than it needs to be.

Every step runs on your device. Decoding uses createImageBitmap in a Web Worker, quantisation and encoding happen in that same worker, and the finished file is handed straight back to your browser as a download. Nothing is uploaded, there is no queue and no file count limit.

How it works: this tool uses your browser's Canvas and WebCodecs APIs, plus libheif for HEIC. No server is involved, so Kilnfile never sees your file. Last reviewed 20 September 2026.

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