What does lossless compression mean?
“Shrinking without quality loss” can mean two different things. With lossless methods the image data is not changed; the same pixels are written more efficiently or redundant data is dropped, so the result looks identical to the original. With lossy methods (such as setting JPG quality to 80) some detail is deliberately sacrificed; the savings are much larger but the image is no longer identical.
What happens to each format, and what to expect
| Format | Method | Typical saving |
|---|---|---|
| PNG | Filters and deflate are re-optimised (oxipng); unnecessary embedded chunks are removed. | 10–50% |
| JPG | The image is not re-encoded; only the Huffman tables are optimised and EXIF/GPS, comments and thumbnails are removed. Orientation and colour profile are kept. | 5–12% |
| WebP | Lossless WebP is re-encoded with more effort and checked pixel by pixel; for lossy WebP only EXIF/XMP chunks are removed. | 5–15% for lossless, very little for lossy |
| AVIF | Only lossless (identity-matrix) AVIF files are re-encoded and verified; lossy AVIF is left untouched. | Mostly none |
| SVG | Comments, editor metadata (Inkscape etc.), whitespace and unused namespaces are removed; the drawing is unchanged. | 20–60% |
Why WebP and AVIF often cannot be shrunk
These two formats usually arrive already lossy. Rewriting a lossy file “losslessly” does not shrink it, it normally makes it bigger: the lost detail does not come back, yet the encoder now tries to store every resulting pixel exactly. In that case this tool leaves your file alone and tells you so. If you need a smaller file, pick the “Original” format and lower the quality, or use compress to a target KB; those methods accept a small quality loss.
Safety: is the result really identical?
Every re-encoded file (JPG, WebP, AVIF) is opened in your browser and compared pixel by pixel with the original. If anything differs, or if no saving is possible, the original file is returned unchanged. So the output is never larger than the input.
Good to know
- EXIF (location, camera) is removed from JPGs; photo orientation and the colour profile (ICC) are kept.
- Data appended after the end of a JPEG, such as a phone’s HDR “gain map”, is removed; the standard view does not change.
- Comments in SVG files are removed; keep that in mind for files whose licence or attribution lives in a comment.
- GIF, BMP, ICO and HEIC have no lossless mode here and are left as they are. Converting BMP to PNG saves far more.
- In lossless mode the size, quality, rotate and watermark settings do not apply.