Compress an image to under 20KB

Why 20KB, and why it is usually a signature

20KB caps appear almost exclusively on signature upload fields rather than photograph fields. That distinction matters more than anything else on this page, because it changes what is achievable.

A signature is black line art on a white background. Large areas of flat, identical colour are what lossy compression handles best — there is very little information to preserve, so the encoder throws away almost nothing to reach a small file. At 20KB a scanned signature is not merely possible, it is usually comfortable.

A photograph at 20KB is a different matter entirely. Continuous tone, skin texture, hair, foliage and gradients are exactly what JPEG struggles with, and 20KB does not buy many of them. If a form is demanding 20KB for a photograph, expect to reduce dimensions substantially — and expect the result to be identifiable rather than flattering.

Measured on a real 4K phone photograph (3840 × 2160, 1.52 MB), 20KB came out at 540 × 304 pixels at quality 61. That is a small image, but a clean one. Forcing the same 20KB onto larger dimensions dropped the quality setting to 5, which is unusable for anything.

Getting a signature to 20KB properly

Most signature rejections are not size problems at all. They are scanning problems that only show up as size problems. Fix the capture and the compression stops being difficult:

1. Sign in black or dark blue ink on plain white paper. Light blue and gel pens scan into muddy greys that carry far more colour information than a signature needs, and compress badly as a result.
2. Photograph or scan it straight, in even light. No shadow falling across the paper, and no angle — a skewed signature often fails the dimension check even when the file size passes.
3. Crop tight to the signature. Border whitespace is wasted budget and wasted pixels. This is the single biggest win available.
4. Convert to greyscale. A signature has no use for colour channels, and dropping them frequently halves the file on its own.
5. Then compress to 20KB. By this point it is usually easy, and often you will land well under.

Do those in order and aggressive compression rarely enters into it.

When 20KB is not achievable, and what to do

If it is genuinely a photograph and the form will not budge:

• Crop to head and shoulders only — everything else is spending your budget on a wall
• Resize to roughly 200–300px on the long edge before compressing
• Convert to greyscale if the form permits it, which many ID forms do
• Accept visible softness. A document photo has to be identifiable, not beautiful

If the result is genuinely unrecognisable, the form's limit is wrong for what it is asking, and that does happen — particularly on portals where the photo field and the signature field were given the same cap by mistake. It is worth querying with whoever runs it rather than degrading the image further.

If 20KB is too aggressive

Check the requirement again before you accept the damage. Many forms that mention 20KB mean it only for the signature, and allow 50KB for the photograph — which is a far more comfortable target for a face. And if you are compressing for a web page rather than a form, 100KB is where photographs start looking genuinely good again.