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Abhigyan Saikia
Abhigyan Saikia

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How to Compress an Image to an Exact KB Size in the Browser (Free, No Upload)

#javascript

Upload forms don't ask for "80% quality". They ask for "photo under 50KB". And every mainstream compressor hands you a quality slider and wishes you luck — compress, check size, adjust, repeat.

There are three ways to hit an exact KB target. Here's all of them.

Option 1: ImageMagick (one line)

If you're on a machine with ImageMagick installed:

magick input.jpg -define jpeg:extent=50KB output.jpg
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The jpeg:extent define binary-searches the quality for you. Change 50KB to whatever the form wants. Fast, scriptable, exact.

Option 2: Do it in the browser with Canvas

The same binary-search idea works client-side. Sketch of the approach:

async function compressToKB(file, targetKB) {
  const bitmap = await createImageBitmap(file);
  const canvas = document.createElement('canvas');
  canvas.width = bitmap.width;
  canvas.height = bitmap.height;
  canvas.getContext('2d').drawImage(bitmap, 0, 0);

  let low = 0.05, high = 0.95, best = null;
  for (let i = 0; i < 8; i++) {           // 8 rounds is plenty
    const q = (low + high) / 2;
    const blob = await new Promise(r => canvas.toBlob(r, 'image/jpeg', q));
    if (blob.size <= targetKB * 1024) { best = blob; low = q; }
    else { high = q; }
  }
  // If quality alone can't reach it, shrink dimensions and repeat.
  return best;
}
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Eight iterations converge fast. If the lowest quality still overshoots, halve the dimensions and run again — pixel count dominates file size.

Option 3: Use a tool that does this already (disclosure: I built it)

I got tired of doing the slider dance for exam forms, so I built ToolKB — you type the exact KB target (20KB, 50KB, 100KB, anything up to 10MB) and it runs the binary search above entirely in your browser.

Why in-browser matters: these are identity documents — passport photos, signatures. Nothing uploads anywhere; you can watch the Network tab and see zero outgoing transfers. No signup, no watermark, no daily limit.

Handy presets if you're filling Indian exam forms:

The rule of thumb

File size ≈ pixels × quality. If your 12MP phone photo won't squeeze into 50KB without turning to mush, don't just crush the quality — halve the dimensions first (that quarters the pixels), then compress. Sharper result, same file size.

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