Hit an exact file size
For upload forms that demand a byte limit and will not say why.
About these tools
Upload forms impose byte limits for ordinary reasons: storage, bandwidth, a specification written years ago that nobody has revisited. Then they offer no help meeting them. The usual advice is to open an editor, guess a quality setting, save, check the size, repeat. That loop is slow mainly because each attempt has historically cost somebody a server round trip.
It costs nothing here. The encoder runs on your machine, so the tool can try: encode, measure, adjust, and narrow in on the highest quality that still fits under the number you gave. A handful of attempts takes about as long as one. You get the best-looking file that satisfies the limit rather than the first one that happens to.
Where quality alone cannot get there, the remaining lever is pixel dimensions, and the tool says so instead of returning something over the limit. Halving an image's width quarters its pixel count, usually a far bigger saving than any quality setting can find.