Convert JPG to WebP

How it works

The JPG is decoded to pixels and re-encoded by the browser's WebP encoder at the quality you choose. Nothing else happens unless you set a maximum width, in which case the image is scaled first using progressive halving.

WebP's advantage over JPEG comes from its compression method. JPEG divides the image into eight-by-eight blocks and approximates each one independently, which is why heavy compression produces visible square edges. WebP uses prediction — each block is guessed from the pixels already decoded around it, and only the error in that guess is stored. Areas that continue a pattern cost almost nothing. It also supports a full alpha channel and can go lossless, neither of which JPEG can do at all.

One detail this tool checks that many do not: after encoding, the returned data is verified to actually be WebP. A browser that cannot encode WebP does not throw an error when asked to — it silently returns a PNG instead. The result would be a file named .webp containing PNG data, which opens fine locally and is rejected by whatever you were uploading it to. If that substitution happens, you get an explicit message instead of a broken file.

When you'd use this

Website performance is the main reason, and the numbers are worth stating plainly. Images are typically 60 to 70 percent of a page's total weight. Cutting them by a third cuts the whole page by around 20 percent, which moves largest-contentful-paint measurably on mobile connections. Since page speed feeds into search rankings, this is one of the few optimizations that is both cheap and directly measurable.

Storage and bandwidth is the second case. A photo library, an image-heavy documentation site, or a media server all benefit proportionally, and the conversion is a one-off cost.

Email newsletters are a third, though check your platform first — a few still strip or refuse WebP attachments.

When not to bother

If the images are going to someone else as files rather than being displayed on a web page, stay with JPG. The compatibility gap is real and a recipient who cannot open your attachment will not care that it was 30 percent smaller.

If the images are already small — icons under a few kilobytes, thumbnails — the absolute saving is negligible and not worth managing two formats for.

And if the JPGs are oversized for where they are displayed, resize them first. That will save more than the format change, and doing both together as this tool allows is better than doing either alone.