KB to cm: how big a 20, 50 or 100 KB photo prints and scans

“KB to cm” is a search people make when a form gives a size in kilobytes and the photo studio talks in centimetres, or when a form gives both (“3.5 × 4.5 cm, under 50 KB”) and it is not clear how the two relate. They do not relate directly. Kilobytes measure the file; centimetres measure a print or a scan at a chosen resolution. The bridge between them is pixels, so the conversion is always two steps: KB to pixels (a range, for JPG photos) and pixels to centimetres (exact, once you pick a dots-per-inch value). This page walks through both steps with the numbers forms in Pakistan, India and Bangladesh actually use.

Step 1: KB to pixels

For a head-and-shoulders JPG at normal quality, a 20 KB file is about 300 by 375 to 400 by 500 pixels; 50 KB is about 450 by 560 to 600 by 750; 100 KB is about 600 by 750 to 800 by 1000; 200 KB is about 800 by 1000 to 1200 by 1500. The spread exists because a plain background compresses far better than a detailed one. The KB to pixels page gives the full table from 10 KB to 500 KB and explains why PNG does not follow it.

Step 2: pixels to cm

Centimetres = pixels ÷ dpi × 2.54. At 300 dpi, the usual photo-print setting, 100 pixels is 0.85 cm; at 200 dpi, common for document scans, 100 pixels is 1.27 cm; at 96 dpi, which screens assume, 100 pixels is 2.65 cm. The same 600 by 750 pixel file is therefore 5.1 by 6.4 cm at 300 dpi and 15.9 by 19.8 cm on a screen, which is why a “cm” figure without a dpi figure means nothing.

The passport photo case: 3.5 × 4.5 cm

Most forms that quote centimetres mean the standard passport photo, 35 by 45 mm. At 300 dpi that is 413 by 531 pixels (0.22 megapixels); at 600 dpi, which some photo labs use, it is 827 by 1063 pixels. Saved as JPG, the 300 dpi version is usually 25 to 60 KB and the 600 dpi version 80 to 200 KB. So a form that says “3.5 × 4.5 cm, under 50 KB” is satisfied by a 413 by 531 JPG at quality around 80, and a form that says “under 30 KB”, such as FPSC, needs the same pixels at a slightly lower quality or a very plain background. The passport photo maker on this site crops to 35 by 45 mm, sets the dpi, and keeps the file inside the KB limit you choose.

Other sizes forms quote in cm or inches

Scanning: set dpi, not cm

A scanner or scan app never asks for centimetres; it reads the paper at the dpi you set, and the pixel count follows from the paper size. A CNIC at 300 dpi is about 1011 by 638 pixels and, as a JPG, 150 to 400 KB because of the fine text and the hologram pattern. An A4 mark sheet at 200 dpi is 1654 by 2339 pixels and 300 to 600 KB. If the form’s KB limit is tighter than that, keep the dpi (so the text stays readable) and lower the JPG quality first; only if that is not enough, reduce the dpi to 150. Going below 150 dpi on a document makes the small print unreadable before the file is small enough to matter.