KB vs Pixels vs DPI: Image Size Explained

Three different meanings of "size"

File size (bytes, KB, MB): how much storage the file uses and how long it takes to upload.
Pixel dimensions (for example 1080 × 1350): how many dots of colour the image contains across and down.
Print size (centimetres or inches) and DPI: how large the image appears when printed, and how densely the pixels are packed on paper.

They are related but not the same. You can change the file size without changing dimensions (by compressing), and you can change the DPI number without changing either.

Bytes, kilobytes and megabytes

One kilobyte is about a thousand bytes and one megabyte is about a thousand kilobytes. Some software uses 1024 instead of 1000, which is why the same file can show as 49 KB in one place and 50 KB in another. When a form gives a maximum, stay a little under it.

A rough sense of scale: a phone photo is 2–6 MB, a good web photo is 100–300 KB, a form photo is 20–50 KB, and a small signature is under 20 KB.

Pixels: what screens and websites care about

Screens show pixels, so websites and social networks care about pixel dimensions. Instagram, for example, displays posts at about 1080 pixels wide; uploading a 4000-pixel photo gains nothing because the app scales it down anyway. Doubling both width and height quadruples the number of pixels, which roughly quadruples the uncompressed data. That is why resizing is the most effective way to shrink a file. The Image Resizer changes dimensions, and Zoom Out Image fits a picture into a frame without cropping.

DPI: only matters when printing

DPI (dots per inch) is a note stored inside the file telling a printer how large to print it. A 1200 × 1800 pixel photo printed at 300 DPI comes out 4 × 6 inches; at 150 DPI it would be 8 × 12 inches, with visibly coarser detail. Changing the DPI field alone does not add or remove a single pixel, and it does not change the file size noticeably.

So when a form says "300 DPI", what usually matters is that the pixel dimensions are large enough to print at the stated physical size with 300 dots per inch.

Converting between centimetres and pixels

Pixels = (centimetres ÷ 2.54) × DPI. A few useful examples at 300 DPI:

Print size	Pixels at 300 DPI
3.5 × 4.5 cm (passport photo in many countries)	413 × 531
2 × 2 inches (US passport photo)	600 × 600
A4 page (21 × 29.7 cm)	2480 × 3508
6 × 4 inches (standard print)	1800 × 1200

Which number to change for each job

"Maximum 50 KB": change file size by compressing, and reduce dimensions if needed (compress to 50 KB).
"200 × 230 pixels": change dimensions with a resizer.
"3.5 × 4.5 cm at 300 DPI": resize to 413 × 531 pixels; the DPI note is secondary.
"Upload a high-resolution image": keep the original dimensions and avoid heavy compression.

Frequently asked questions

Does changing DPI reduce file size?

No. DPI is only a printing instruction stored in the file. To reduce file size, reduce the pixel dimensions or increase compression.

How many pixels is a 50 KB photo?

There is no fixed answer, because file size depends on content and compression too. A clear portrait of 400–700 pixels on the long side commonly fits in 50 KB as a JPEG.

More guides

Paul Digital guides are written from hands-on testing of the tools on this site and are reviewed when platforms or formats change. Requirements set by exam boards, governments and apps vary and change over time, so always check the official instructions for your case. Spotted an error? Email askpaulonline@gmail.com.