Compress an image to a specific size in KB

Why forms set size limits, and why guessing wastes time

Job portals, university applications, visa and government forms, exam registrations and many business systems cap uploads at a fixed size: 20 KB for a signature, 50 KB for a photo, 100 or 200 KB for a document. Ordinary compressors ask for a quality setting and leave you to guess, so you end up saving the same photo five times until it squeaks under the limit. This tool works the other way round. You give it the limit, and it works out the settings.

How it finds the right size

The photo is decoded in your browser and saved at the lowest quality first, to check the limit is reachable at the current dimensions. If it is, the tool searches for the highest quality that still fits, halving the gap each step, so within about eight attempts it lands just under your target at the best quality possible. If even the lowest quality is too large, the photo is scaled down in proportion to how far over it is, and the search runs again. The result is always the sharpest version that fits, never a blurry overshoot.

Each result shows its final size, the quality used and the dimensions, so you can see what was needed to hit the target.

KB, KiB and why the result is slightly under

Some systems count a kilobyte as 1000 bytes, others as 1024. The tool uses 1000, the stricter of the two, so a file made for a 50 KB limit is under 50,000 bytes and passes both kinds of check. The result will usually be one to five percent under the limit rather than exactly on it, because compressed image sizes move in small jumps as quality changes.

Getting the sharpest result at a small size

1. Crop first. Every pixel costs bytes. A passport photo that includes the whole room wastes most of its budget on the background. Crop to what the form needs with the crop tool, then compress.
2. Match the requested dimensions. If the form asks for 350 by 450 pixels, resize to that first. The tool then spends the whole budget on quality instead of shrinking the image itself.
3. Use a plain background. Flat colour compresses to almost nothing, busy patterns eat bytes.
4. Keep JPG for forms. WebP fits more quality in the same size, but most forms only accept JPG or PNG.

What fits in common limits

iPhone HEIC photos can be dropped in directly; they are converted and compressed in one step. Nothing is uploaded at any point, which matters when the photo is of your face, your ID or your signature.