Photo size terms explained: DPI, pixels, cm
Exam form photo instructions throw around DPI, pixels, cm, mm and KB as if everyone already knows what they mean — and the confusion these terms cause is one of the most common reasons a photo gets sent back for not meeting spec. Here's what each one actually is and how they connect.
Pixels — the actual building blocks
A digital photo is a grid of tiny coloured squares called pixels. "600×600 pixels" means a grid 600 squares wide and 600 squares tall. Pixels are the one measurement a computer actually stores and checks — everything else (cm, mm, DPI) is a way of describing physical size, which only matters once you're printing or thinking in real-world units.
DPI — dots per inch, the bridge between pixels and physical size
DPI tells you how many pixels are packed into one inch when the image is printed. The formula is simple: pixels = inches × DPI. So a 2-inch-wide photo at 300 DPI is 600 pixels wide (2 × 300 = 600). The same 2-inch photo at 72 DPI would only be 144 pixels wide — far too low-resolution to look sharp when printed, even though the "inch" size is identical. This is exactly why two photos described as "2×2 inch" can have wildly different pixel counts.
CM and mm — the same physical measurement, different units
| cm | mm | inch |
|---|---|---|
| 1 cm | 10 mm | 0.39 inch |
| 2.5 cm | 25 mm | 1 inch |
| 3.5 cm | 35 mm | 1.38 inch |
| 5.08 cm | 50.8 mm | 2 inch |
These are simply the same physical length in different units — an Indian form might state cm, an international one might use inches, but they describe the same real-world size.
KB — how much data the file takes up, separate from all of the above
KB (kilobytes) measures file size — how much storage the image takes up, not how big it looks. Two photos with identical pixel dimensions can have very different KB sizes depending on how much they've been compressed: a lightly-compressed version keeps more detail and is larger; a heavily-compressed version is smaller but shows more visible quality loss. This is the number most exam portals actually enforce a strict limit on, separate from the pixel/inch/cm specification.
Why pixels and DPI are often confused with each other
Pixel dimensions describe how many tiny squares of color make up the image file itself, while DPI (dots per inch) describes how densely those pixels get printed onto physical paper — the two are related only once you decide on a print size, and a file with no plan to be printed doesn't really need a DPI value at all, which is why most exam and ID portals specify pixel dimensions and KB size, not DPI. A 600×600px photo has exactly the same pixel data whether its metadata says 72 DPI or 300 DPI; what changes is only how large or small it would appear if printed at that setting, not the sharpness of the underlying digital file.
This confusion usually comes up when someone tries to satisfy a 300 DPI requirement without also being told a target print size, since DPI alone, without dimensions, doesn't actually describe a usable image size.
Frequently asked questions
If a form asks for pixels, do I need to worry about DPI separately?
No — once you know the exact pixel dimensions a form wants, that's the number that actually gets checked; DPI is only relevant for converting between physical size and pixels.
Does a higher DPI photo automatically look better on screen?
Not on screen — DPI only matters for print. On a screen, what matters is the pixel count relative to the display, not DPI.
Why does changing DPI in my photo editor sometimes not change the file size?
Because DPI is just metadata describing intended print size — it doesn't add or remove actual pixel data unless the editor also resamples (changes) the pixel dimensions.
Does changing a photo's DPI make it sharper?
No — DPI only affects how large the image appears if printed; the underlying pixel data and sharpness stay exactly the same regardless of the DPI value in the file's metadata.