QUICKTOOLS GUIDE
How Image Compression Works
Learn the difference between image dimensions, quality, and file size before compressing a photo.
In this guidePixel dimensions describe width and height; compression describes how the image data is stored.JPEG is usually effective for photographs.PNG is useful for lossless graphics and transparency.WebP can be efficient for web delivery when supported by the destination.
The key ideas
- Pixel dimensions describe width and height; compression describes how the image data is stored.
- JPEG is usually effective for photographs.
- PNG is useful for lossless graphics and transparency.
- WebP can be efficient for web delivery when supported by the destination.
How the calculation or process works
Reducing dimensions can remove pixels, while lossy compression keeps the dimensions but approximates some image information. For many photos, combining a sensible output size with a moderate JPEG quality setting produces a useful balance.
Common mistakes
- Expecting compression to make an already tiny file dramatically smaller.
- Converting a transparent PNG to JPEG without noticing that transparency is lost.
- Over-compressing an image that will later be edited again.
Use the related QuickTools utility
Once you understand the method, the QuickTools How Image Compression Works page can handle the repetitive part of the task.
Important: A general guide explains a method; it does not know the rules, assumptions, or requirements of a particular business, contract, institution, or software system. Check those requirements when they matter.