Pixels, DPI and KB — what photo specs actually mean
Form instructions mix three different measurements: physical size (cm or inches), pixels and file size (KB). Knowing how they relate makes most upload errors easy to fix.
The three numbers
| Measure | What it describes | Example |
|---|---|---|
| Pixels | How many dots make up the image | 413 × 531 px |
| DPI | How many of those dots fit in an inch when printed | 300 DPI |
| Physical size | Pixels ÷ DPI — the printed size | 3.5 × 4.5 cm |
| File size (KB) | How much storage the file takes, mostly decided by JPEG compression | 10–200 KB |
Converting cm to pixels
Pixels = centimetres ÷ 2.54 × DPI. So a 3.5 × 4.5 cm photo is:
- at 300 DPI: 413 × 531 pixels
- at 200 DPI: 276 × 354 pixels
If a form gives cm and DPI, you need both to get the right pixel size. If it gives only pixels, DPI usually doesn't matter for an upload.
Why file size is the usual problem
Portals limit KB to keep their databases small. JPEG compression trades quality for size: the converter searches for the highest quality that still fits under the maximum, so you get the sharpest file allowed. Some forms (NEET, UPSC, IBPS, Sarathi) also set a minimum, to stop people uploading tiny, blurry images. If the image is too simple to reach that minimum, the converter adds harmless padding data to the file.
Common mistakes
- Stretching the photo to fit. Changing the shape (aspect ratio) squashes your face. Crop to the right shape first; the converter always crops instead of stretching.
- Using a screenshot. Screenshots of photos lose quality and often have the wrong size. Use the original photo.
- Photo of a printed photo. It looks dull and blurry, and live-capture exams treat it as a violation.
- Wrong format. iPhone photos are often HEIC; many portals need .jpg. The converter always outputs JPEG.
- Swapping photo and signature uploads. Check each box before submitting.
Frequently asked questions
Does DPI change the file size?
No. DPI is just a number stored in the file that says how big to print it. File size depends on pixel dimensions and compression.
Why is my file too small even though the photo looks fine?
Small images (like 200 × 230 px) compress very well. If a form has a minimum KB, the converter raises quality and adds padding data to reach it.
Is PNG better than JPEG for forms?
For photos, no — most portals only accept JPEG, and PNG files are much larger.