Why compress to 40KB?
Some forms set both a minimum and a maximum file size. A target near the middle of the range gives you a margin on both sides, so small differences in how a website measures kilobytes won't get your upload rejected.
Forty kilobytes is also enough for a good-looking color photo of a face at around 450 by 600 pixels, or a square profile photo around 500 pixels wide.
How to compress a photo to 40KB
Upload the image with the 40 KB target selected and JPG as the output format. The tool encodes the photo at the highest quality it can, checks the size, and searches downward until the file fits. The compare slider and the size readout update as soon as it finishes.
If the result is far below the limit, that's fine for a maximum. If the form also has a minimum, check the next section.
What if the file ends up below the minimum?
The target is a ceiling, not an exact size. A simple image, such as a small photo with a plain background, can come out well under 40 KB even at the default quality. If a form needs at least a certain size, switch to By quality and raise the quality until the file is large enough, or resize the photo to larger pixel dimensions first.
Avoid going to the other extreme. Doubling the dimensions of a small photo adds bytes, but not detail, so only enlarge as far as the form needs.
Common mistakes with 40KB uploads
Uploading a screenshot of a photo instead of the photo itself adds borders and lowers quality before you even start. Choosing PNG as the output means the target is ignored. And compressing an already compressed copy again and again makes it worse each time, so go back to the original file whenever you try a new setting.