Every time you save an image, you are making a quiet choice between two approaches to compression. Lossy compression shrinks files dramatically by discarding information. Lossless compression shrinks files more modestly but keeps every pixel exactly. Understanding the difference helps you choose formats confidently, avoid gradual quality loss, and keep files no larger than they need to be.
What compression is, in everyday terms
Imagine describing a picture over the phone. For a white wall with a single black dot, you could say "two thousand pixels of white, one black, then more white" instead of listing every pixel. That is the idea behind lossless compression: describe repetition efficiently, so the receiver can rebuild the picture exactly.
Now imagine describing a photo of a forest. Almost every pixel is a slightly different green. There is very little exact repetition to exploit, so lossless compression does not shrink it much. Lossy compression takes a different approach: "this area is roughly this shade of green, with this kind of texture". The receiver gets something that looks the same to a human, but is not identical pixel for pixel.
Lossless formats
- PNG: the everyday lossless format for graphics and screenshots.
- Lossless WebP: a more efficient lossless option for websites.
- TIFF with LZW or Deflate compression: lossless, for printing and archives.
- BMP: usually uncompressed, so lossless but very large.
- GIF: lossless for the colours it can hold, but limited to 256 colours per frame, so converting a photo to GIF is effectively lossy.
Lossy formats
- JPG: the classic lossy photo format.
- Lossy WebP: smaller than JPG at similar quality.
- AVIF: smaller still, using modern techniques.
What lossy compression actually throws away
Lossy image compression is designed around human vision. It prioritises what we notice and sacrifices what we do not.
- Fine colour detail. Our eyes are much more sensitive to brightness than to colour. JPG stores colour at lower resolution than brightness, and most people never notice.
- Very fine texture. Tiny variations in areas like sky, skin or fabric are smoothed.
- High-frequency detail. Extremely fine patterns are simplified.
At high quality settings, these changes are invisible at normal size. At low settings, they become visible as blockiness, smeared colours and halos around sharp edges.
When lossy is the right choice
- Photos for sharing, emailing and websites. The savings are huge and the difference invisible.
- Large collections where storage costs matter.
- Slow connections and mobile data, where smaller files load faster.
When lossless is the right choice
- Screenshots, text, diagrams and logos, where lossy artefacts around sharp edges are noticeable.
- Images you will edit repeatedly.
- Masters and archives, where you want the best possible copy forever.
- Medical, scientific and legal images, where exact pixel values may matter.
The generation loss trap
Here is the single most important practical point. Every time you open a lossy image, edit it and save it again as lossy, it is compressed again, and a little more detail is lost. One save is invisible. Ten saves can be very visible. This is called generation loss.
The fix is simple:
- Keep your original.
- Do your editing in a lossless format such as PNG.
- Export a lossy copy only at the end, for sharing.
Our JPEG to PNG converter is a quick way to create a lossless working copy of a JPG before editing.
Converting between lossy and lossless
- Lossy to lossless (JPG to PNG): the image is preserved exactly as it is now, including any existing artefacts. Quality does not improve, but it stops degrading. The file usually gets bigger.
- Lossless to lossy (PNG to JPG): the file shrinks, sometimes dramatically, with a small, usually invisible loss. See PNG to JPEG.
- Lossy to lossy (JPG to WebP): the image is compressed a second time. Use sensible quality settings, around 80 to 85, to avoid visible loss. See JPG to WebP.
- Lossless to lossless (BMP to PNG): nothing is lost, and the file often shrinks dramatically. See BMP to PNG.
Quality settings explained
Lossy formats offer a quality setting, usually from 1 to 100. It is not a percentage of "how good" the image is; it controls how aggressively detail is discarded.
- 90 to 95: almost indistinguishable from the original; larger files.
- 80 to 85: the sweet spot for most photos.
- 70 to 75: fine for thumbnails and previews.
- Below 60: visible artefacts in many images.
Our converters default to sensible values, 85 for JPG and around 80 for WebP, so you can usually leave them alone.
How to test for yourself
Convert a favourite photo at quality 95, 85 and 70. Open them side by side at normal size. Most people cannot tell 95 from 85, and many cannot tell 85 from 70 for ordinary photos. Then zoom in on a sky or a face, where differences first appear. Trying this once gives you an intuition that lasts.
Compression beyond images
The same two ideas appear everywhere in computing. ZIP files are lossless: unzip them and you get exactly the original files back, which is essential for documents and programs. MP3 audio is lossy: it removes sounds people are unlikely to hear, which is why music files are small enough to stream. Video streaming is lossy too, carefully tuned to what eyes notice. Once you know the pattern, you will recognise it in every format you use, and you will know instinctively when exactness matters and when "looks the same" is good enough.
A simple personal rule
If you only remember one thing from this guide, make it this: keep one lossless or original copy of anything you care about, and make lossy copies for sharing. Storage is cheap; lost detail cannot be bought back.
Common questions
Is PNG always bigger than JPG? For photos, yes, by a lot. For simple graphics, PNG can be smaller and sharper.
Can lossy compression ever be reversed? No. Discarded detail is gone. That is why keeping originals matters.
Does lossless mean uncompressed? No. Lossless formats compress, often very effectively; they just do it without losing information.
In short
Lossy compression gives tiny files by discarding detail you rarely notice; lossless keeps every pixel. Use lossy for sharing photos, lossless for graphics, editing and archives, and avoid repeated lossy saves by editing in a lossless format and exporting once at the end. With that habit, your images will stay sharp for years.