Photo Size ReducerI think the habit of chasing a limit down to the last byte causes more trouble than it solves. If your form says 40 KB, a file of 39.8 KB is not a win. It is a gamble on how the website measures. So when you compress image to 40KB, aim for something like 36 to 38 and enjoy the margin.
Any photo size reducer online will get you near the number, but only you can decide how much margin to keep under the image size limit. A 40KB image needs breathing room.
Why a margin helps
Websites do not all agree on what a kilobyte is. One counts 1024 bytes, another counts 1000. A file that reads 40.0 KB in one place can read 41 KB in another. On a strict portal, that difference is the gap between accepted and rejected, and you will not always get a helpful message.
There is a second reason. Some upload systems add a tiny amount of data of their own while processing. It is rarely more than a few hundred bytes, but with a hard limit even that can matter.
What a few kilobytes cost you
Almost nothing. The difference in look between 36 KB and 40 KB is very hard to notice, especially on a small face photo. The difference in risk is real. That is a good trade.
How to set it up here
Type 38 in the size box instead of 40, keep the unit at KB, and press Compress. The tool tries many JPG quality levels and keeps the best one that fits. If quality alone is not enough, it reduces the pixel dimensions and tries again. When it finishes, you see the final size beside the preview.
If you would rather see the effect yourself, run it twice, once with 40 and once with 36, and compare the previews side by side.
When I would not follow this advice
If the form has a minimum as well as a maximum, for example between 30 and 40 KB, then aim for the middle, around 35. Landing on the edge in either direction is the same gamble.
Also, if a portal shows you its own size checker as you upload, trust that over any rule of thumb. It knows how it counts.
Other sizes
If the number on your form is different, jump to 30 KB or 50 KB. And if you just want to reduce photo size to 40KB with no strict rule behind it, go with the target that looks best to you.
Nothing here is uploaded. The compression happens in your own browser tab and the picture is discarded when you close it.