KB to pixels: what size in pixels gives a 20, 50 or 100 KB photo

People search “KB to pixels” because a form says “photo must be under 50 KB” and the phone shows a picture that is 4000 by 3000 pixels and 3 MB. The honest answer is that kilobytes and pixels measure different things and there is no fixed conversion: the same 600 by 800 picture can be 40 KB or 400 KB depending on the file format, the JPG quality setting and how much detail is in the picture. What does exist is a reliable range for one common case, a head-and-shoulders photo saved as JPG at normal quality, and that range is what this page gives, together with the two things that break it.

Why there is no formula

Pixels describe how many dots the picture has; kilobytes describe how much space the file takes after compression. An uncompressed photo needs 3 bytes per pixel (one each for red, green and blue), so a 600 by 800 picture is 1,440,000 bytes, about 1.4 MB. JPG compression throws away detail the eye does not notice and typically keeps between 0.08 and 0.25 bytes per pixel for a photograph, which is why the same 600 by 800 file lands anywhere between 40 KB and 120 KB. A plain wall compresses to the low end; a patterned shirt, hair against a bright background or a grainy low-light picture pushes it to the high end. PNG does almost no lossy compression on photographs and stays near 1.5 bytes per pixel, so a PNG of the same picture is roughly 700 KB.

KB to pixels for a JPG photo

Read the table from the limit you were given. If a form says “under 30 KB”, a 400 by 500 pixel JPG is the safe target: it fits with room to spare and still prints acceptably at the passport size of 35 by 45 mm. If a form gives a lower limit too (JEE Main asks for 10 to 200 KB), aim for the middle of the range rather than the floor, because a file squeezed to the minimum is also the softest.

Documents are different from faces

A scanned CNIC, mark sheet or certificate is mostly small text on a light background, and JPG handles text badly: the sharp edges of letters cost many bytes. The same pixel count that gives a 100 KB face photo gives a 250 KB document scan. For documents the rule of thumb is to keep the width at about 1,200 pixels, which keeps the text readable, and then let the quality setting bring the size down; below 900 pixels wide a CNIC’s small print starts to blur before the KB target is reached.

How to hit an exact KB limit

1. Crop to the shape the form wants (passport 35:45, square, or the document’s edges) so that no pixels are wasted on background.
2. Resize to the pixel range in the table for your limit.
3. Save as JPG, not PNG. Start at quality 80. If the file is still over the limit, lower the quality in steps of 5; if it is far under, raise it.
4. Check the result at full size. Faces should be sharp at the eyes; documents should show every digit of the number.
5. The resizer on this site does steps 2 to 4 automatically: pick the form or type the KB limit, and it searches for the largest pixel size and quality that fit.

Related conversions

Pixels become centimetres only when you choose a print resolution, so “KB to cm” is two steps: KB to pixels (this page), then pixels to cm at 300 dpi for prints (413 by 531 pixels is 3.5 by 4.5 cm). The guide on KB, MB, pixels and DPI explains the units from the start, and the DPI to KB page has a calculator for scanned documents.