How to compress an image to an exact size in KB

Application forms, exam portals and government sites love a size limit. "Upload a photo under 100 KB." "Maximum 200 KB." The photo from your phone is five or ten times too big, and the site rejects it with a vague error.

Getting to an exact size is not guesswork. Two settings control it, and once you know how they trade off you can reach a target in a few tries.

The two settings that decide file size (link to section)

Quality is how much detail the encoder keeps. Lower quality throws away fine detail and gives a smaller file.

Dimensions are how many pixels the image has. Fewer pixels means less to store.

They work together. A large image at low quality and a small image at high quality can end up at the same size, but they will not look the same. The large one looks blocky and the small one looks soft.

What the numbers look like (link to section)

The table shows the highest JPG quality (MozJPEG) that stays under each limit, at four widths. The source is a 2400 by 1600 pixel test photo with texture and grain, so smooth real photos may need lower quality settings or land smaller.

Read across a row. For a 100 KB limit you can keep all 2400 pixels at quality 68, or drop to 1200 pixels and keep quality 92. The second is almost always the better photo, because most screens and form previews never show 2400 pixels anyway.

A method that works (link to section)

Why it rarely hits the number exactly (link to section)

Quality moves in whole steps, and each step changes the size by a few percent, so you land just under the limit rather than exactly on it. In the table the 100 KB rows came out at 99, 96 and 92 KB. That is fine. Forms check that a file is at or below the limit, not that it matches it.

If a form needs a minimum size as well, such as at least 50 KB, raise quality or dimensions until you are inside the range.

Tips for specific jobs (link to section)

• Passport and ID photos. Crop to the required shape first, then resize, then compress. Cropping after compressing wastes the work.
• Scanned documents. Convert to grayscale if the form allows it. It usually helps.
• Photos with plain backgrounds. They compress far better than busy scenes, so a studio style shot hits a small limit more easily.
• Screenshots. Use PNG or WebP instead of JPG, as described in the format guide.

Do it in the browser (link to section)

You can do all of this without uploading the photo anywhere. The Compress to KB tool is built for it: you set a target size, and it searches for the right quality.

If the form wants specific dimensions as well, such as a 35 by 45 mm photo, try the exam photo tool.