Pixels, KB, cm and DPI Explained: Photo Size for Online Forms

Photo instructions in online forms can look like a puzzle: “3.5 cm x 4.5 cm”, “200 x 230 pixels”, “maximum 50 KB”, “200 DPI”. They describe different things, and once you understand each one, choosing the right settings becomes easy.

Pixels: the size of the picture on a screen

A digital picture is made of tiny dots called pixels. The size “200 x 230 pixels” means the picture is 200 dots wide and 230 dots tall. More pixels mean more detail and a bigger file. A phone camera makes photos with thousands of pixels in each direction, while a form photo needs only a few hundred.

KB and MB: the weight of the file

KB (kilobyte) and MB (megabyte) tell you how much storage the file takes. 1 MB equals 1024 KB. A photo may have the correct pixel size and still be heavy, because the quality setting also affects the KB. That is why forms ask for both pixels and a KB limit.

cm, mm and inches: the size on paper

Centimetres (cm), millimetres (mm) and inches are physical sizes, used for printed photos. Here are the conversions worth remembering:

• 1 inch = 2.54 cm = 25.4 mm
• 10 mm = 1 cm

A photo of “3.5 cm x 4.5 cm” is the same as “35 mm x 45 mm”, a common passport-style size.

DPI: the link between paper size and pixels

DPI means dots per inch. It says how many pixels fit in one inch of the printed picture, and it connects the paper size to the pixel size:

pixels = size in inches x DPI

Since 1 inch is 25.4 mm, you can also write it as:

pixels = size in mm / 25.4 x DPI

Worked example: 35 x 45 mm at different DPI

DPI	Width in pixels	Height in pixels
100	about 138	about 177
200	about 276	about 354
300	about 413	about 531

The same paper size gives a different pixel size for each DPI. So if a form gives a size in cm or mm and also mentions the DPI, use that DPI. If it does not mention any, 300 DPI is the usual default for good print quality. You can enter the size, the unit and the DPI in our Photo Resizer, and it does this calculation for you.

Does DPI change my photo on the screen?

On a computer or phone screen, only the pixel size matters. Two photos with the same pixels but different DPI labels look the same on the screen. DPI matters when the picture is printed, and when a form uses it to work out the pixel size from a cm or mm size.

Aspect ratio: the shape of the photo

Aspect ratio is the width divided by the height. A photo of 413 x 531 pixels has the same shape as 35 x 45 mm. A photo of 200 x 230 pixels is a little wider, so the two sizes are not the same shape. Always follow the exact numbers in your form instead of guessing from a similar size.

1. Crop it. This cuts the extra part and keeps the face natural.
2. Stretch it. This forces the photo into the size, but it makes faces look wide or thin, and can lead to rejection.

Cropping is nearly always the better choice.

How the four things work together

Term	What it measures	Example in a form
Pixels	Picture size on screen	200 x 230 px
KB / MB	File weight	Maximum 50 KB
cm / mm	Physical size on paper	3.5 x 4.5 cm
DPI	Pixels per inch, linking paper size to pixels	300 DPI

A simple way to get it right

1. Read your form’s instructions and note the size, the unit, the KB limit and the format.
2. Open the Photo Resizer and enter the same size and unit. Add the DPI only if the unit is cm, mm or inches.
3. Enter the KB limit and press Resize.
4. Check the pixel size and KB shown under the preview, then download the file.
5. If the upload still fails, see our guide on photo and signature upload problems.

Frequently asked questions

Which is more important, pixels or KB?

Both. A form can reject a photo for either one, so check both.

If I increase the DPI, will my photo get bigger?

Not on the screen. It only changes the printed size, or the pixel size worked out from a cm or mm size.

Why is my 50 KB photo blurry when I zoom in?

A small photo has few pixels. It looks fine at its normal size and blurry when enlarged.

Can I trust the sizes given on other websites?

Sizes differ between forms and years. Always follow the official instructions of the form you are filling.

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Sarkari Photo Tools is an independent website and is not connected to any government department. Please always check the official instructions of your form.