JPG to WebP

At matched visual quality a WebP is typically 25–35% smaller than the JPEG it came from. On an image-heavy page that is the difference between a site that feels immediate and one that does not.

The cost is that you are performing a second lossy encode. Your JPEG has already discarded detail; the WebP encoder now has to spend bytes faithfully reproducing what remains, including the JPEG's own artefacts. The result is still smaller, but you get a noticeably better outcome converting from an original that has not been through a lossy encoder yet.

Where the source JPEG is all you have — which is usually the case — converting is still worth it. Just do not run the file back and forth between formats.

Do not carry your JPEG number across. WebP holds together better at the low end, so WebP at 75 looks roughly like JPEG at 82 while being meaningfully smaller. Starting from your usual JPEG value produces files larger than they need to be, which is how people conclude — wrongly — that WebP did not help.

For photographs converted from JPEG, 75 to 80 is the useful range. The default here is 78.

Every time a JPEG is decoded, edited and saved again, the encoder re-runs its quantisation step on data that has already been quantised once. Errors from the previous pass are treated as real detail and get their own new errors layered on top. This is called generation loss, and it accumulates in one direction only.

Saving an already-compressed JPEG at quality 90 does not restore anything — it just spends more bytes preserving the artefacts of the earlier pass. Saving repeatedly at a low quality is what produces the smeared, blotchy look of an image that has been through a dozen chat apps.

Two habits avoid it. First, always compress from the highest-quality original you have, not from a copy that has already been through a compressor. Second, decide on your output size once and do a single pass, rather than nudging the quality down repeatedly and re-saving each time. If you need to try several settings, run each of them against the original file.

The quality number in a JPEG or WebP encoder is not a percentage of anything. It is an index into a set of quantisation tables that control how aggressively detail is discarded. The relationship to file size is steep and non-linear.

For a typical 12-megapixel phone photograph the curve looks roughly like this:

• Quality 95–100 — near-original appearance, but the file is often only 20–35% smaller. Rarely worth it for the web.
• Quality 80–90 — the sweet spot for photography you care about. Typically 50–70% smaller with no artefact you would notice at normal viewing distance.
• Quality 70–80 — the sweet spot for ordinary web images. Usually 70–85% smaller. Artefacts appear only if you zoom in on flat sky or skin.
• Quality 50–70 — visible softening and blocking in smooth areas, though thumbnails and background images survive it well.
• Below 50 — obvious blocking, ringing and colour banding. Useful only for placeholders.

Diminishing returns set in fast at the top of the range. Going from quality 100 to quality 90 usually halves the file for no perceptible change; going from 90 to 80 saves far less and costs slightly more. If you only remember one number, use 80.