Pixels, DPI and Image Size: What the Numbers Mean

“Make it 300 DPI.” “Upload a 1080 pixel image.” “Under 2 megapixels.” Image sizes are described in several different ways, and they are easy to confuse. This guide explains what each number means and how they relate, with the arithmetic you need to get your own images right.

Pixels are the real size

A digital image is a grid of coloured dots called pixels. Its true size is the number of pixels across and down, such as 4,000 by 3,000. Everything else is derived from that. When someone says an image is “big” or “small” in a digital sense, pixel dimensions are what they should mean.

Megapixels are a shorthand for the total: width times height, divided by a million. A 4,000 by 3,000 photo has 12 million pixels, so it is a 12-megapixel image.

DPI is only a label

DPI means dots per inch, and PPI (pixels per inch) is the same idea. It is a number stored in the file that tells a printer how large to make each pixel on paper. It does not change the pixels themselves. A 3,000 by 2,000 pixel photo is exactly that size whether the file says 72 DPI or 300 DPI. Only the printed size changes, because the printer spreads the same pixels over a smaller or larger area.

This is why “make it 300 DPI” on its own is a vague request. What matters is whether you have enough pixels for the printed size.

The print arithmetic

For print, multiply the size in inches by the resolution you need.

• A 6 by 4 inch print at 300 DPI needs 1,800 by 1,200 pixels.
• A full A4 page (8.27 by 11.69 inches) at 300 DPI needs about 2,480 by 3,508 pixels.
• The same A4 page at 150 DPI needs about 1,240 by 1,754 pixels, which is acceptable for documents and fine for casual prints.

Photos for large posters can use lower resolutions, because they are viewed from further away. A 12-megapixel phone photo has plenty of pixels for a sharp A4 print.

The screen arithmetic

On screens, what counts is the pixel width relative to where the image is shown. A web page column that displays an image 600 pixels wide is best served an image at least 600 pixels wide. Many modern phones and laptops use high-density displays that put two or more physical pixels in each layout pixel, so for the sharpest result you provide about twice the width, here 1,200 pixels.

Sending far more than needed wastes bandwidth. A 4,000-pixel photo shown 600 pixels wide gives the visitor no benefit and makes the page slow. Reducing the dimensions is also the biggest single way to shrink a file, because halving both width and height leaves a quarter of the pixels.

Aspect ratio and cropping

The aspect ratio is the proportion of width to height. Common ones are 3:2 and 4:3 for cameras, 16:9 for video, 1:1 for squares and 4:5 for portrait posts on social networks. If the ratio you have is not the one you need, you have two honest choices:

• crop, which cuts away part of the picture to reach the right shape;
• resize, which scales the whole picture up or down.

Resizing to a different ratio without cropping stretches the picture and distorts faces and circles. Keep the aspect ratio locked when resizing, and crop first if you need a different shape.

Common targets

Platforms change their requirements, so check the current help page, but these have been stable for a long time:

• YouTube thumbnails: 1,280 by 720 pixels, a 16:9 image.
• Instagram feed posts: 1,080 pixels wide, with 1,080 by 1,080 for squares and 1,080 by 1,350 for 4:5 portraits.
• Passport and ID photos: specified by the issuing authority, usually in pixels for online forms or in centimetres for print.

Making images bigger

Enlarging an image cannot create detail that was not captured. Doubling the dimensions of a small photo creates new pixels by guessing from neighbours, which looks soft. If you need a bigger image, start from a bigger original. Shrinking, by contrast, is safe, and it is the normal direction for web and email use.

Putting it together

1. Decide where the image will be used and work out the pixel size needed.
2. Crop to the right proportions.
3. Resize to those pixel dimensions.
4. Save in a suitable format. Our guide to JPG, PNG, WebP and AVIF can help you choose.
5. If a file-size limit applies, compress last, as described in how to meet upload size limits.

The resize and crop tools on this site do all of this in your browser, so your pictures are not uploaded.