How to Compress an Image to 20KB
20KB works only for small pictures. If your image is a phone-sized photo, you'll need to cut the pixels, not just the quality, and PNG will usually be too big.
What fits in 20KB
20 KB is 160,000 bits. Here is how much that leaves per pixel:
| Image size | Pixels | Bits per pixel at 20 KB |
|---|---|---|
| 140 × 60 (a signature) | 8,400 | 19 |
| 200 × 230 | 46,000 | 3.5 |
| 600 × 600 | 360,000 | 0.44 |
| 1080 × 1350 | 1,458,000 | 0.11 |
Small images have plenty of room. Anything near phone size does not. A 600 × 600 photo at 20KB is already past what a JPEG can do without visible blocks, and a full photo is out of the question. If a form really requires 20KB, it is almost always asking for a small image, such as a signature or a stamp-sized photo, and it normally states the dimensions too.
Steps
- If the form gives dimensions, use KB + Dimensions. It crops or fits to the exact size first, then finds the best quality that is safely under your limit.
- If it doesn’t, scale the picture to something small with Resize Image, then use Compress Image with a 20 KB maximum.
- For a signature, run it through the Signature Cleaner first so the paper and shadows aren’t eating the budget. A plain white background with dark ink compresses extremely well.
Why PNG struggles
PNG is lossless: it stores every pixel exactly, so the main ways to make it much smaller are fewer pixels or fewer colours. A photo as PNG is almost never near 20 KB unless it’s tiny. A flat graphic with few colours can be. If the form allows JPG, convert and compress as JPG. Otherwise keep the PNG small in dimensions.
The default floors can stop you
Compress Image stops before it ruins a picture. Its default floors are JPEG quality 50, at least 55% of the original size on each side, and never below a 400 px long edge. If a limit is out of reach above those floors, it tries harder compression once and keeps the result only if it still measures at least 90% similar to the original. Otherwise you’ll see “Target too aggressive” and the best result at the floor.
On a generated 3000 × 4000 test image, a 20 KB maximum came out at 17.9 KB in Chromium, after cutting the picture to 1132 × 1509 pixels. Safari’s engine could not get there and stopped at 203 KB. Neither is a usable result for a full photo, which is the point: at 20 KB, resize to a small fixed size first. Your other options are to raise the limit, or to turn on “Allow lower quality” under Advanced and check the result carefully. Don’t submit something you can’t read.
The opposite problem: a minimum size
Forms that say “10–20 KB” also have a minimum. A small, plain image can come out below 10 KB. That isn’t a compression failure; the file is simply too light. KB + Dimensions can raise a too-small file (it tells you which method it used) and still re-checks every rule. See how to reduce a signature file size in KB for the details.