What it does
Drop one SVG or a whole folder's worth, or paste the code. Each file is run through SVGO, the optimiser most build tools use, in a background worker in this page. It takes out what a browser does not need to draw the picture: comments, <metadata>, the extra data Inkscape, Illustrator and Sketch leave behind, default values, empty groups and hidden shapes. Then it rewrites what is left more briefly: numbers are rounded to the Decimal places you choose, path commands are shortened, colours like #ff0000 become red and IDs are shortened. Every change is applied to every file at once, and the sizes update as you go.
Checking the result
Pick a file in the list to see it before and after, side by side, with both sizes and how much was saved. Both previews are drawn as images, so a file's scripts never run here. If the optimised one looks different, the usual cause is too few decimal places on a detailed drawing: raise it a step or two. Turn off Merge paths or Shorten IDs if a script or stylesheet elsewhere relies on particular shapes or IDs. Copy the markup to paste into HTML, download one file, or download them all in a ZIP with their original names.
What stays
The viewBox is always kept, so the picture still scales. The title is kept unless you remove it, because screen readers use it as the picture's name. Drop width and height leaves only the viewBox, so the SVG fills whatever box your CSS gives it. Scripts are left alone unless you switch on Remove scripts.
Questions
Are my files uploaded?
No. SVGO runs in your browser, in a worker on your device. The files are never sent anywhere, and nothing is kept once you close the page.
How much smaller will it get?
It depends on where the file came from. A drawing saved from Inkscape or Illustrator often loses half or more, mostly metadata and long decimals. An icon that was already hand-tidied may shrink by only a few percent. Served with gzip or Brotli, as most websites do, the saving on the wire is smaller again.
Why does my SVG look slightly different?
Rounding moves points by a fraction of a unit. On a 24 by 24 icon that cannot be seen, but on a large detailed drawing with a small viewBox it can. Raise Decimal places until the two previews match.
Can it make a PNG or trace a picture?
No, it only rewrites SVG code. To turn a PNG or JPG into an SVG, use Image to SVG.