Lossy vs Lossless Compression, Explained
One kind of compression is reversible and one isn't. Knowing which you're using decides whether you can edit a file again later without watching it fall apart.
Lossy compression makes a file smaller by permanently throwing away part of the picture; lossless compression makes it smaller while keeping every original pixel, so it can be restored exactly. Both shrink the file. Only one of them is reversible, and that single difference decides which you should reach for.
The short version: if a file is heading into more editing, keep it lossless. If it just needs to be small enough to send, upload, or load on a page, lossy is almost always the right trade.
Lossy vs lossless, side by side
| Lossless | Lossy | |
|---|---|---|
| What it does | Records the same image more compactly | Discards detail the encoder judges you won't miss |
| Reversible? | Yes — every pixel comes back exactly | No — the discarded detail is gone for good |
| Best for | Screenshots, logos, line art, anything you'll edit again | Photographs, and anything with a hard size limit |
| Typical formats | PNG, GIF, lossless WebP | JPG, WebP, AVIF |
| File size | Larger — nothing is thrown away | Much smaller — that's the whole point |
Which one do you actually want?
Answer one question first: will this file be edited and saved again?
A screenshot you'll annotate, a logo you'll drop into three different designs, a diagram you'll relabel next week — those want lossless, because you'll open and re-save them repeatedly and lossless survives that untouched. A holiday photo you're emailing, or a picture that has to fit under a form's upload limit, wants lossy: the file is finished, and the size saving is worth the detail you'll never notice is missing.
Photographs are the case people get wrong. A photo saved as a lossless PNG is often several times larger than the same photo as a good JPG, and looks identical. A photograph has almost no exact repetition for lossless to shorten, so the format barely earns its weight — the usual surprise for anyone who tries shrinking a PNG photo and watches the size hardly move. For photos, lossy is the honest answer.
How to tell which one you're already using
Look at the format. JPG is always lossy. PNG is always lossless. WebP does both, and that catches people out: PNG to WebP has a single quality slider that behaves as lossless at its top setting and lossy everywhere below, so the same control can hand you either. Which mode you actually get, and why the file sizes diverge so much, is the subject of the deeper guide on lossless compression.
The trap: lossy can't be turned back into lossless
Converting a JPG to a PNG does not undo the compression the JPG already applied. You get a lossless copy of a lossy image — a perfect record of what was already thrown away, usually in a bigger file. JPG to PNG is still worth doing before you start editing, because it stops any further loss from here on, but it can't restore detail that's gone. Re-saving a JPG over and over makes this worse each time, an effect called generation loss.
The working habit that falls out of all this: do your edits on a lossless file and hand out a lossy one at the end. The master you keep editing stays PNG; the copy you send or upload is the JPG. And when that final copy has to come in under a specific size, that's inherently a lossy job — Compress Image lets you set a target and does the work in your browser, nothing uploaded.
FAQ
Is lossless always better quality than lossy?
At maximum settings, yes — lossless keeps everything. But for a photograph, a good lossy JPG looks identical to the eye at a fraction of the size, so "better quality" you can't see isn't worth several times the file size.
Can I compress something without losing any quality?
Yes, with a lossless format like PNG or lossless WebP — but only where the image is repetitive enough to shrink much, which mostly means graphics rather than photos. For photos, real size savings come from lossy compression.
Is WebP lossy or lossless?
Either. WebP supports both modes. The quality control decides: at 1 it's lossless, below 1 it's lossy, from the same slider.
Does converting a lossy file to a lossless format restore its quality?
No. It preserves the image from that point on, but the detail an earlier lossy save discarded is already gone and can't be recovered.