Image Resolution, Pixels and Megapixels: What the Numbers Mean
A photo is a grid of tiny coloured squares called pixels. How many there are, and how big they are when shown or printed, decides how sharp the picture looks. This guide explains the numbers that describe an image, so you can pick the right size for a website, a social post or a printout.
Pixels and dimensions
An image's dimensions are its width and height in pixels, written as width × height. A picture of 4000 × 3000 pixels is 4000 pixels wide and 3000 pixels tall. The total number of pixels is the product: 4000 × 3000 = 12,000,000.
Megapixels
One megapixel is one million pixels. Our example image has 12,000,000 pixels, so it is 12 megapixels, which is what a modern phone camera commonly captures. More megapixels mean more detail and more room to crop, though sensor quality, the lens and lighting matter as much as the number.
Pixels per inch (PPI) and dots per inch (DPI)
These terms describe how tightly pixels or ink dots are packed. The picture file itself has no fixed physical size. Its size depends on the density you choose when printing or on the screen that displays it.
Print size = pixels ÷ PPI
For the 4000 × 3000 image printed at 300 PPI, a common standard for sharp prints:
- Width = 4000 ÷ 300 = 13.33 inches (33.9 cm)
- Height = 3000 ÷ 300 = 10.00 inches (25.4 cm)
At 150 PPI, which is acceptable for large posters viewed from a distance, the same file would print up to 26.7 × 20.0 inches. The pixels are spread wider, so the print looks less sharp up close.
Common sizes
| Use | Typical size | Megapixels |
|---|---|---|
| Full HD screen | 1920 × 1080 | 2.1 |
| 4K screen | 3840 × 2160 | 8.3 |
| Passport-size photo print (35 × 45 mm) at 300 PPI | about 413 × 531 | 0.22 |
| A4 print (8.27 × 11.69 in) at 300 PPI | about 2480 × 3508 | 8.7 |
Image size for the web
Screens do not need print resolution. A photo shown in a column 800 pixels wide does not need 4,000 pixels. Oversized images make pages slow, which hurts visitors on mobile data and search rankings alike. A good habit is to resize the image to about twice the width it is shown at (to look sharp on high-density screens) and no more.
File size and format
- JPEG compresses photographs well by discarding detail you cannot easily see. A quality setting of 70 to 85% usually looks fine at a much smaller size.
- PNG keeps every pixel exactly. It suits screenshots, logos and graphics with sharp edges or transparency, but photos become large.
- WebP and AVIF are newer formats that usually make smaller files than JPEG at the same quality. Most current browsers support them.
An uncompressed 12-megapixel image would take about 36 MB (three bytes for each pixel). A JPEG of the same photo is usually 2 to 6 MB, and a web-sized version can be under 200 KB.
Upscaling does not add detail
Making a small image larger adds pixels by guessing, not by recovering real detail, so it looks soft or blocky. If you need a bigger print, start with a larger original. Work in the opposite direction instead: shrink a big photo for the web, and keep the original file safe.
Check before you upload or print
Many forms ask for photos within a size and dimension limit, for example a signature of 140 × 60 pixels under 50 KB. Use our Image Pixel Checker to see the dimensions, megapixels, file size and the print size at different PPI values for any image. It reads the file in your browser and does not upload it. To pick exact colours from the same picture, try the Image Color Picker.