How the compression works

Drop an image onto the panel or click it to browse. The file is decoded, drawn onto a canvas at the size you asked for, and re-encoded in the browser. Three things are measured while that happens: the size of the file you picked, the size of the result, and the percentage saved.

Nothing is uploaded. The original stays on your machine, and the compressed copy is created from it locally, which is why a photo that arrived as a 6 MB file can come out at 400 KB without going near a server.

The three settings

  • Quality runs from 10 to 100 and starts at 80. It applies to JPEG and WebP output.
  • Max width resizes the image. Leave it at 0 to keep the original dimensions; set a number and the height follows, scaled proportionally. The image is never enlarged, so a width larger than the original changes nothing.
  • Output format keeps the format you uploaded, or converts to JPEG, PNG or WebP.

How to use it

  1. Add the image. The preview and the size comparison appear as soon as it is processed.
  2. Set a maximum width if the file is too large because of its dimensions rather than its compression — a 4000-pixel photo for a 1200-pixel slot is mostly wasted pixels.
  3. Move the quality slider and watch the "saved" figure and the preview. The preview updates on every change, so you can see the artefacts before you decide.
  4. Download the result. It is saved next to the original name with a -compressed suffix, so your source file is not overwritten.
  5. To send a set of images as one document, the Images to PDF tool turns them into a single PDF.

Choosing a quality setting

For a photograph on a web page, 75 to 85 is the range where the difference stops being visible. Below 60 you will start to see blocking around edges and faces, and below 40 the file looks recompressed. For flat graphics, screenshots and logos, quality matters much less — but PNG output is lossless, so the slider has no effect there at all.

Good to know

  • Converting to JPEG fills transparent areas with white first, because JPEG has no transparency channel. A PNG logo that relies on transparency will lose it unless you keep the PNG output.
  • With PNG output the file is re-encoded losslessly, so the savings come from resizing rather than from the quality slider.
  • Very large images can exceed the canvas limits of some browsers, particularly on phones. If the download never enables, resize the image first and try again.
  • Compression reduces the file size; it does not sharpen the picture. Repeatedly compressing an already compressed image is the fastest way to make it look bad.
  • The tool handles one image at a time. For a batch, work through them one by one — or, if the goal is one document rather than separate files, the Images to PDF converter takes a whole set at once.