“Make the image bigger”, “it needs to be 300 DPI”, “upload a high-resolution photo”: image size instructions are confusing because three different things get called “size”. This guide separates them and tells you how many pixels you actually need.
Three kinds of image size
- Pixel dimensions: the width and height in pixels, such as 1920 × 1080. This is the real amount of detail in the image.
- File size: how much storage the file takes, in KB or MB. It depends on pixel dimensions, format and compression.
- Print size: how big the image comes out on paper, in centimetres or inches. It depends on pixel dimensions and the DPI you print at.
When someone says “resolution”, they usually mean pixel dimensions, sometimes DPI, occasionally file size. Ask which one if it isn’t clear.
Pixels are what matter on screens
A screen shows a fixed number of pixels. A 1920 × 1080 laptop display shows exactly that many, whatever DPI value is stored in the file. For anything shown on a screen, such as websites, social media, email and presentations, only pixel dimensions matter. The DPI number is ignored.
High-resolution displays, such as Apple Retina screens and most modern phones, pack two or three device pixels into every CSS pixel. An image shown 600 pixels wide on a web page looks sharpest at 1200 pixels wide on these screens. Going beyond twice the display size adds file size with no visible gain.
Common screen sizes
| Use | Pixel size |
|---|---|
| Full HD screen / video | 1920 × 1080 |
| 4K screen | 3840 × 2160 |
| Website hero image | 1920–2560 wide |
| Blog image | 1200–1600 wide |
| Square social post | 1080 × 1080 |
| Portrait social post | 1080 × 1350 |
| Story or reel | 1080 × 1920 |
| Profile picture | 400 × 400 or larger |
| Video thumbnail | 1280 × 720 |
Platforms change their recommendations from time to time, so check their help pages for critical work.
DPI matters only for print
DPI (dots per inch), or more accurately PPI (pixels per inch), tells a printer how densely to place the pixels on paper. The maths is simple:
Print size in inches = pixels ÷ DPI
So a 3000 × 2400 pixel photo prints at 10 × 8 inches at 300 DPI, or 20 × 16 inches at 150 DPI.
Quality guidelines for print:
- 300 DPI: photo prints and magazines viewed up close
- 200 DPI: still looks good for most home prints
- 150 DPI: posters viewed from a metre or more
- Under 100 DPI: visible softness up close
Pixels needed for common prints at 300 DPI
| Print size | Pixels needed |
|---|---|
| 6 × 4 in (15 × 10 cm) | 1800 × 1200 |
| 7 × 5 in (18 × 13 cm) | 2100 × 1500 |
| 10 × 8 in (25 × 20 cm) | 3000 × 2400 |
| A4 (8.3 × 11.7 in) | 2480 × 3508 |
| A3 (11.7 × 16.5 in) | 3508 × 4961 |
A modern 12-megapixel phone photo (4000 × 3000) prints sharply up to about A4 at 300 DPI, and larger at lower DPI.
Changing the DPI number doesn’t add detail
Many programs let you change an image from 72 to 300 DPI. If the pixel dimensions stay the same, nothing about the image changes except the printed size it suggests. If a print shop or form demands “300 DPI”, what they really need is enough pixels for the print size at 300 DPI.
Making images bigger
Enlarging an image can’t create detail the camera didn’t capture. Resampling fills in new pixels by blending neighbours, which looks soft at large enlargements. Modest enlargements, up to about 150%, usually look fine. For logos and graphics, the better fix is to export again from the original vector file. SVG files can be converted to PNG at any size with no loss.
Making images smaller
Shrinking is safe and almost always worth doing for screen use. Use a resizer with a high-quality resampling filter such as Lanczos, keep the proportions locked, and save in a suitable format. Fewer pixels also means a much smaller file. Halving width and height cuts file size roughly to a quarter.
Forms that ask for pixels and KB
Application and ID forms often specify both, for example “600 × 600 pixels, under 100 KB”. Do it in that order:
- Crop to the right shape, then resize to the exact pixel dimensions.
- Compress to the size limit.
Resizing first means the size limit is easy to meet at high quality, so the photo stays clear.