It is eleven at night, the form closes at midnight, and the upload box says the photo must be between 20KB and 50KB. Yours is 180KB. You open an image editor, drag the quality slider down, save, and check. Now it is 12KB. You drag it back up. 94KB. You do this six more times.
This is not you being bad at computers. The relationship between JPEG quality and file size is genuinely unpredictable, and understanding why makes the whole thing less maddening.
## Quality 60 does not mean 60 percent of the size
The quality number in a JPEG encoder is not a percentage of anything. It selects a quantisation table, which decides how aggressively fine detail gets thrown away. How much that shrinks the file depends entirely on what is in the picture.
A passport photo against a plain white wall compresses enormously, because most of the image is one flat colour and the encoder can describe large areas very cheaply. The same photo taken against a bookshelf will not, because every book spine is detail the encoder has to spend bytes on.
So quality 60 might give you 22KB for one photo and 140KB for another. Two people following the same advice get different results and both assume they did something wrong.
## Dimensions matter more than quality
Most people reach for the quality slider first. Pixel dimensions usually have a bigger effect.
Halving the width and height of an image quarters the pixel count. A 2000 by 2000 photo has four million pixels. At 1000 by 1000 it has one million. That is a far larger reduction than anything the quality setting will do on its own, and it costs you nothing if the form only displays the photo at 200 pixels wide anyway.
Check the required dimensions before you touch quality. Many forms specify both, and people miss the pixel requirement entirely because the file size is the number that gets rejected loudly.
## The minimum is a real requirement
Here is the part that catches people out. A form that says 20KB to 50KB means both numbers.
A file under the minimum usually means the image has been compressed so hard that it is no longer legible when printed on an admit card. Portals reject these deliberately. If your signature scan comes out at 8KB and the form wants at least 10KB, compressing it further is the wrong direction. You need to go back and reduce the compression, or scan at a slightly higher resolution.
This is why "just make it smaller" advice fails. Roughly half the rejections we hear about are files that ended up too small.
## Renaming the extension does nothing
A PNG renamed to .jpg is still a PNG. Upload validators read the first few bytes of the file, called the magic number, not the name. A JPEG starts with the bytes FF D8 FF. A PNG starts with 89 50 4E 47.
If a portal says "invalid format" for a file that clearly ends in .jpg, this is almost always why. Open it in any image tool and use Save As or Export, choosing JPEG properly. That re-encodes the actual data instead of relabelling it.
## Doing it without the guessing
The reliable method is binary search: encode at quality 50, check the size, and move up or down by half the remaining range each time. Six or seven attempts will land within a kilobyte of almost any target. That is exactly what a person does by hand with the slider, only without the frustration of doing it by hand.
Our [image resizer](/resize-image) runs that search in the browser. It tries progressively, keeps the highest quality that still fits under your limit, and tells you the final size before you download. If the target is impossible without making the image unreadable, it says so rather than handing you a grey smudge that will be rejected anyway.
There is a specific page for the two sizes people ask for most: [10KB signatures](/resize-signature-to-10kb) and [20KB photos](/compress-image-to-20kb).
Nothing is uploaded. The resizing happens on your own device, which matters when the file in question is a photograph of your face attached to an identity document.
## A short checklist
Before you upload:
1. Read both numbers. Minimum and maximum. 2. Check whether pixel dimensions are specified separately. 3. Reduce dimensions before reaching for quality. 4. Confirm the format by re-saving, not by renaming. 5. If the result is under the minimum, compress less, not more.
Most rejections come from one of these five, and four of them take under a minute to check.