Resize Image to KB

Because it works backwards. There is no formula from quality to file size — it depends entirely on what is in the picture — so the tool encodes the image repeatedly at different quality settings and keeps the best-looking one that still fits under your limit. Most compressors ask for a quality percentage and tell you the size afterwards, which is the same job in the direction nobody needs.

Because 20 KB is very little data for a large photograph, and all of it is being spent describing millions of pixels crudely. Switch on the dimension cap: at 600 or 800 pixels on the longest side there are far fewer pixels to describe, so each one gets more data and the result looks dramatically better at the same file size. Forms that ask for 20 KB are not expecting a full-resolution photo.

That means even the lowest quality setting overshoots, which happens with very large or very detailed images. Cap the dimensions, and prefer JPG over PNG — PNG is lossless and has no quality dial to turn, so the only way it gets smaller is fewer pixels.

JPG for photographs and anything a form asks for, because it compresses smoothly to a target. PNG only if you need a transparent background or perfectly sharp text and lines. WEBP compresses better than both but some older upload forms still reject it, so check what the form accepts before choosing it.

No. It is read with the File API and re-encoded on a canvas in your own browser. There is no request carrying the image, which is why this works offline once the page has loaded.