Three different measurements, one small image
Pixels are the actual grid of dots that make up the image — width × height, like 300 × 400. This is what determines how much detail the image can hold.
Physical size (centimetres or inches) is what the image represents in the real world — a 3.5 × 4.5 cm passport photo, a signature strip 4 × 2 cm. On its own, physical size says nothing about pixels.
DPI (dots per inch, sometimes written PPI) is the bridge between the two: it says how many pixels represent one inch of the physical size. The formula, using centimetres directly:
pixels = (centimetres ÷ 2.54) × dpiSo a 3.5 × 4.5 cm photo at 200 dpi is (3.5 ÷ 2.54 × 200) ≈ 276 px wide and (4.5 ÷ 2.54 × 200) ≈ 354 px tall.
Common sizes at different resolutions
| Physical size | at 100 dpi | at 200 dpi | at 300 dpi |
|---|---|---|---|
| 3.5 × 4.5 cm (passport-style photo) | 138 × 177 px | 276 × 354 px | 413 × 532 px |
| 4.0 × 2.0 cm (signature strip) | 157 × 79 px | 315 × 157 px | 472 × 236 px |
A form that gives pixel dimensions and a physical size together is implicitly telling you the DPI it expects — use the formula above to check whether the two actually agree before you crop.
Why hitting the pixel size doesn't guarantee the KB target
This is the part that trips people up: file size (KB) is not determined by pixel dimensions alone. A 276 × 354 px JPEG can be 8 KB or 45 KB depending on how much JPEG compression is applied and how much fine detail the photo has (a plain studio headshot compresses more than a textured one). So a form that asks for "276 × 354 px, 20–50 KB" is really asking for two separate, somewhat independent things to both be true at once: crop and resize to the pixel dimensions first, then adjust the JPEG quality (most phone and photo-editing apps expose this as a quality or compression slider) until the file lands inside the stated KB range.
Ranges commonly seen on forms
Based on what's published across various application and exam portals, typical ranges look roughly like this — these vary by issuer, so treat them as a starting point, not a rule:
- Signature strips: often 10–20 KB
- Passport-style application photos: often 20–50 KB
- Larger photo uploads: sometimes up to 100–200 KB
Exam application photo and signature sizes (SSC, UPSC, NEET)
A few Indian exam boards publish specific figures for photo and signature uploads. The numbers below are compiled from coaching and exam-prep websites, not from the exam boards' own official notifications, and exam requirements change from year to year:
| Exam | Photo | Signature |
|---|---|---|
| SSC (OTR profile) | JPEG, 20–50 KB, 3.5 × 4.5 cm | JPEG, 10–20 KB, 4.0 × 2.0 cm |
| UPSC | JPG, 20–200 KB | JPG, 20–100 KB |
| NEET UG | 10–200 KB (passport and postcard sizes) | 4–30 KB |
Treat this table as a starting point only. Before you submit anything, re-check the current, official notification published by the exam body itself (ssc.gov.in, upsc.gov.in, nta.ac.in, or the relevant year's own notice) — requirements and formats are revised often enough that a secondary source can be out of date.
Getting the crop and signature right
For a signature, sign in dark ink on plain white paper, photograph or scan it flat under even light with no shadow across the strokes, then crop tightly to the signature itself with nothing else in frame. For a photo, follow whatever background and framing the form specifies (usually plain white or light-coloured) before worrying about KB — a reviewer rejecting a photo for the wrong background won't care that the file size was correct.
What pdf300 can and can't do here yet
pdf300 doesn't currently have a dedicated "resize this photo to an exact KB" slider — that's a different kind of tool from the PDF utilities on the site today. What is here: Crop can isolate a signature or photo region out of a scanned page, and PDF to Image can then export that region as a JPG or PNG at a pixel size you choose. Getting the final file into a specific KB range still takes a bit of manual adjustment of quality in whatever tool you crop with — this guide tells you the target to aim for, even where the last step is manual.
If you later need that photo or signature placed back onto a PDF — for example, signing a form you received as a PDF — Sign PDF places a PNG signature directly onto the page in the browser.
Quick checklist
- Work out pixel dimensions from physical size and DPI using the formula above.
- Crop to the correct physical proportions first, pixel size second, KB size third.
- Adjust JPEG quality, not just dimensions, to hit a KB range.
- For exam forms, verify the figures against this year's official notification before submitting.
- Plain background, even light, tight crop — for both photos and signatures.