DPI, cm, and KB: The Nightmare Triangle of Exam Photo Specs

⏱ 6 min read · ExamPhoto

Every exam application form asking for a photo contains three measurements: a physical size in centimeters (like 3.5cm × 4.5cm), a resolution in dots per inch (like 200 DPI), and a maximum file size in kilobytes (like 50KB). Most students treat these as three separate, independent requirements. They're not — they're interdependent, and misunderstanding how they relate is the source of most photo technical rejections.

Understanding DPI

DPI — dots per inch — is a measure of resolution: how many pixels are packed into one inch of a printed image. A photo at 200 DPI prints with 200 pixels per inch. This measurement matters for print quality. Higher DPI means more detail at the same physical size. For most exam applications and ID photos, 200 DPI is sufficient. Some require 300 DPI for higher-quality printing.

DPI only has meaning in the context of physical size. A digital file has no inherent DPI — the DPI is calculated at the point of printing or displaying at a specific physical size. An "image at 200 DPI" means the image has enough pixels to print well at the specified dimensions when those pixels are distributed at 200 per inch.

The Pixel Dimension Calculation

Here's the math most applicants need. The formula is: pixels = cm × (DPI ÷ 2.54). For a requirement of 3.5cm × 4.5cm at 200 DPI: width = 3.5 × (200 ÷ 2.54) = 3.5 × 78.7 ≈ 276 pixels; height = 4.5 × 78.7 ≈ 354 pixels. The correctly sized image is approximately 276 × 354 pixels. This is what the exam portal's validation expects.

File Size vs Quality

The maximum KB limit is separate from resolution. A 276 × 354 pixel JPEG can be compressed to many different file sizes depending on JPEG quality settings. At high quality, it might be 150KB — above many exam limits. At medium quality, the same image might be 40KB with no visible difference at the sizes it will ever be displayed.

JPEG compression is the solution. Most image editing tools let you specify a target file size or quality level. For exam photos, a quality setting of 60-80% typically produces excellent visual results well within the 50-100KB range most portals require.