Productivity

How to Compress an Image to 20KB (Signatures and Thumbnails)

NikolaLast updated on September 28, 2026 · 8 min read

Summary

20KB is a signature box, not a photograph. What actually fits, the dimensions that get you there, and why scanning at 600 DPI is working against you.

Three steps to a 20KB image: crop away the paper, resize to the pixels the field needs, then compress | ImagePDF.Tools | ImagePDF.Tools — Resize before you compress. In the other order you throw away quality, then throw away the pixels you paid for it with.

The upload box says maximum 20KB. Your scanned signature is 1.4MB. The form will not budge, and it will not tell you why your file keeps bouncing.

20KB is one of the tightest limits still in common use. It is almost always a signature field, a thumbnail, or a small ID photo on a government or university form, and the number is a legacy of a time when storage was priced very differently.

The good news is that 20KB is entirely achievable for the things that ask for it. Here is what actually fits, and how to get there without a grey smear where your signature used to be.

What 20KB Will and Will Not Hold

File size is driven by pixel count first and quality second. Before touching a quality slider, it is worth knowing what is physically plausible.

Content	Dimensions that fit	Comfortable?
A scanned signature	600 x 200 px	Yes, with room to spare
A passport style head and shoulders photo	About 300 x 400 px	Yes, at quality 70 to 80
A full page scanned document	Not realistically	No, the text goes to mush
A photograph with background detail	About 320 x 240 px	Tight, expect visible artefacts

Roughly what fits in 20KB as a JPEG at reasonable quality.

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If a form asks for a full page document under 20KB, read it again. It almost always means a signature or photo field, or it accepts a PDF for the document and a small image for the photo.

The Real Problem Is Usually the Scan, Not the Compression

Most people arrive at this problem holding a file that was never going to compress well, and no amount of quality reduction fixes the underlying choice.

A signature scanned at 600 DPI on A4 is roughly 5000 x 7000 pixels. That is 35 million pixels to describe a few pen strokes. Even squeezed hard, the file stays large, and the compression has to work so aggressively that the strokes break up.

The fix is to reduce the pixels before reducing the quality. A signature needs to be legible, not archival. 600 x 200 pixels is plenty, and at that size 20KB is easy rather than desperate.

Getting to 20KB Without Wrecking It

1.Crop first. Remove the white paper around the signature or the background around the face. Empty space still costs pixels.
2.Resize to what is needed. Use the resize tool to bring the longest edge down to 600px for a signature, or 400px for a photo.
3.Then compress. Open the 20KB compressor, which is already set to that target, and check the result.
4.Check it at full size. Open the result and look at it properly. A signature that is illegible is a rejected form, whatever its file size.

The Order Matters More Than the Settings

Compressing first and resizing afterwards wastes the compression: you throw away quality, then throw away the pixels you paid for it with. Resize, then compress, and you keep more of what is left.

For Signatures Specifically

A signature is not a photograph, and treating it like one is why it goes grey and fuzzy.

●Increase the contrast before compressing. Dark strokes on a white background compress far better than grey strokes on a cream background, because there is less detail to encode.
●Crop tight to the strokes. A 20% margin is generous. Paper is not information.
●Consider PNG if the form allows it. A high contrast signature with few colours can compress smaller as a PNG than a JPEG, and without the smearing.
●Do not scan at 600 DPI. 200 DPI on a cropped signature is more than enough, and starts you ten times closer to the target.

⚠️

Some portals check dimensions as well as file size, for example 140 x 60 pixels for a signature. Read the whole requirement before optimising for the byte count alone, or you will pass one check and fail the other.

When 20KB Genuinely Is Not Possible

If the form wants a readable full page document under 20KB, you cannot win it with compression. A page of text needs enough pixels to resolve the letterforms, and below that threshold the text stops being readable at any quality setting.

Two honest routes out. Convert the page to a PDF, where text stays as text and the file can be tiny, using image to PDF and then compressing it. Or split the requirement, because many forms have a separate, larger allowance for documents than for photos.

Why Do This in the Browser

The images that need to hit these limits are ID photos, signatures and scans of personal documents, submitted to government portals and universities. They are exactly the files worth not scattering across other people's servers.

A browser based compressor reads the file on your machine, re-encodes it there, and hands you the result. Nothing is uploaded, so there is no copy of your signature sitting in a queue somewhere.

Resize First, Then Squeeze

20KB is a small number but a reasonable one for what usually asks for it. Crop away the paper, bring the pixel count down to what the field actually needs, and only then reach for quality. Do it in that order and the result stays legible.

Check the dimension requirement as well as the size limit before you start. Open the compressor and drop your file in to see where it lands.

Frequently asked questions

How do I compress an image to 20KB?

Crop away empty space, resize the longest edge to around 600px for a signature or 400px for a photo, then compress. Resizing before compressing matters: it reduces the pixel count first, so the quality setting does not have to work as hard and the result stays legible.

Can a full page document fit in 20KB?

Not as a readable image. A page of text needs enough pixels to resolve the letterforms, and below that threshold no quality setting saves it. Convert the page to a PDF instead, where text stays as text, or check whether the form has a separate larger allowance for documents.

Why is my scanned signature so large?

Because it was probably scanned at 600 DPI on a full sheet, which is around 35 million pixels to describe a few pen strokes. Crop tight to the signature and resize to about 600px wide, and 20KB becomes easy instead of desperate.

Should a signature be a JPEG or a PNG for a 20KB limit?

Try PNG if the form allows it. A high contrast signature has few distinct colours, which is the case PNG handles efficiently, and it avoids the smearing that heavy JPEG compression puts around dark strokes on white.

Does 20KB mean the image will look bad?

Not for the things that usually ask for it. A cropped signature at 600 x 200 or a head and shoulders photo at 300 x 400 fits comfortably at a reasonable quality setting. It only looks bad when the pixel count was never reduced first.

The form rejects my file even though it is under 20KB. Why?

Many portals check pixel dimensions as well as byte size, and some check the format or the colour mode. Read the full requirement rather than the size limit alone, since passing one check and failing another produces the same unhelpful rejection.