Compress an image to 20 KB
The editor above is set to a 20 KB limit. Open your image and it searches for the highest quality whose file still fits. At this limit a photo usually has to lose pixels as well as quality, so when it cannot fit, or fits only by looking awful, the editor works out a smaller size for you and puts it on a button.
Twenty kilobytes is about a hundredth of what a phone photo weighs. It is still enough for a legible signature or a recognisable face — just not at the size the camera took it.
What fits in 20 KB, measured
We ran the editor’s own resize and size search on our sample photograph, a detailed lakeside landscape, at six widths and at its full size:
| Pixels | Megapixels | WebP quality | JPEG quality | Result |
|---|---|---|---|---|
| 200 × 133 | 0.03 | 99 | 98 | No visible loss |
| 300 × 200 | 0.06 | 92 | 91 | No visible loss |
| 400 × 267 | 0.11 | 82 | 77 | Clean |
| 500 × 333 | 0.17 | 67 | 57 | Clean, slightly soft |
| 600 × 400 | 0.24 | 42 | 37 | Blocky in fine detail |
| 800 × 533 | 0.43 | 18 | 18 | Smeared; not usable |
| 1920 × 1280 | 2.46 | Never under 45 KB | Never under 21.5 KB | Does not fit at all |
The last row is the one most people meet first. At its full 1920 pixels the photo cannot be squeezed under 20 KB at any quality setting: WebP bottoms out at 45 KB, JPEG at 21.5 KB. Lowering the slider further does nothing, because there is nothing further to lower. Only fewer pixels will do it, and about 400 px wide is where a detailed photo still looks clean.
When the editor offers a smaller size
If the limit cannot be met, or is met only below quality 30, a button appears under the size field: Resize to 480 × 320 px, for example. Nothing changes until you press it.
The suggestion is not a guess. During the search the editor has already encoded your image at quality 50 and knows how large that file was. File size grows roughly in step with the number of pixels, so it scales the pixel count to fit 80% of your limit at that quality, keeps your aspect ratio, and rounds the width down to a multiple of ten. On our sample at full size, that offer was 480 × 320 for WebP, and the search then landed on quality 73. For JPEG it offered 460 × 307 and landed on quality 66.
If a form gives exact pixel dimensions, ignore the offer and type those into the Resize panel instead; the search runs again either way.
Signatures and initials
Most 20 KB limits are for a scanned or photographed signature. Ink on white paper is almost all flat background, which costs very few bytes, so a signature fits far more easily than the landscape above. What eats the budget is everything around it:
- Crop tight. A phone photo of a signature is mostly desk, shadow and paper grain. Cropping to a thin strip around the ink removes all of that before compression begins.
- Use daylight, not the flash. An even background compresses to almost nothing; a gradient from a lamp or a flash hotspot does not.
- Keep it wide enough to read. 300 to 400 px across is plenty for a signature box. Much narrower, and thin pen strokes can thin out to grey; check the result at 100% zoom.
- Choose JPEG unless the form says otherwise. Almost every portal accepts it.
When a form also sets a minimum
Some forms ask for a file between, say, 10 KB and 20 KB. The search always takes the highest quality that fits, so it already uses as much of the allowance as it can. If the result is still below the minimum, which a small, clean signature can be even at quality 100, there are no more bytes to be had from quality. The only way to make the file larger is to give it more pixels: increase the width in the Resize panel until the readout passes the minimum.
Why 19 is the safe number
CropShrink counts a kilobyte as 1,024 bytes, so a 20 KB target allows up to 20,480 bytes. A form that counts in thousands rejects anything over 20,000. The panel prints the exact byte count of every result, so you can check; or set the target to 19, which is 19,456 bytes and passes under either rule.
Questions about 20 KB limits
Can any photo be compressed to 20 KB?
Not at full size. Our 1920-pixel sample cannot go below 45 KB as WebP or 21.5 KB as JPEG at any quality. Resized to about 400 px wide it fits at quality 82 as WebP and looks clean.
Why does the editor offer to resize my image?
Because the limit was missed, or met only at a quality below 30. It sizes the offer from the file it encoded at quality 50 during the search, so the next search lands close to that quality. It only resizes when you press the button.
What size should a signature be for a 20 KB limit?
Crop tight around the ink and keep it 300 to 400 px wide. Unless the form gives exact dimensions, that is readable and fits 20 KB comfortably as a JPEG.
Is my signature uploaded anywhere?
No. The crop, the resize and the size search all run in your browser, and the file is saved straight from it. The privacy policy shows how to confirm that nothing is sent.
Other limits
For a photograph rather than a signature, 50 KB is the more common ceiling: the 50 KB page measures the same photo with more room. Larger limits have pages of their own at 100 KB and 200 KB.
More image tools
Every one of these is the same editor, opened for a different job — and all of them run on your device.
- Compress to 50 KBGet a photo under a tight upload limit.
- Compress to 100 KBHit a hard upload limit without guessing at the quality slider.
- Crop an imageTrim to what matters, freeform or to a fixed ratio.
- Resize an imageChange the pixel dimensions with Lanczos3, Catmull-Rom or Mitchell.
- Remove EXIF dataStrip the GPS location, camera and serial number from a photo.
Or start from the full editor, which does all of it at once.