Pixels, DPI and print size, explained

Two numbers describe a digital photo, and people mix them up constantly. One is how many pixels it has. The other is a “DPI” figure that shows up in print dialogs and photo editors. They answer different questions, and only one of them matters for most of what you do.

Pixels are the photo

A pixel is one dot of color. A photo that is 4,000 × 3,000 pixels has 12 million of them. That count is the photo’s real resolution: it sets how much detail exists and how big the file will be.

On a phone, laptop or web page, a photo is shown by mapping its pixels to the pixels of the screen. Inches never come into it. A 1,200-pixel-wide photo is the same 1,200 pixels whatever number is written in its settings.

DPI only matters on paper

DPI means dots per inch. For print, it says how many of the photo’s pixels are squeezed into each inch of paper. Some people say PPI (pixels per inch), which is the more exact term for an image. In practice the two get used interchangeably.

The relationship is simple:

print size in inches = pixels ÷ DPI

Take that 4,000 × 3,000 photo. At 300 DPI it prints at about 13.3 × 10 inches. At 150 DPI, the same pixels spread across 26.7 × 20 inches. Fewer pixels per inch means a larger print, and a softer one.

Why changing the DPI number does nothing on screen

Image files can carry a resolution label, and a print dialog can read it. Changing that label rewrites a number in the file and leaves every pixel as it was. So:

• On screen, the photo looks identical.
• The file is the same size.
• A print will come out at a different size, because the printer now spreads the same pixels over more or less paper.

That is why “convert to 300 DPI” usually accomplishes nothing for a web upload. If a site demands a pixel size, set pixels. If a print shop asks for 300 DPI, check the pixel size they need for the print dimensions.

How many DPI do you need?

It depends on how close people will stand.

• About 300 for photos held in the hand or viewed at arm’s length: prints, cards, albums.
• About 150 for larger prints on a wall, seen from a bit further away.
• 100 or less can be fine for big posters and banners, which are viewed from a distance.

More is not better past a point. Beyond about 300 DPI, most people cannot see the difference on a normal print.

A worked example

Pixels needed at 300 DPI for common print sizes:

Print size	Pixels needed
4 × 6 inches	1,200 × 1,800
5 × 7 inches	1,500 × 2,100
8 × 10 inches	2,400 × 3,000
11 × 14 inches	3,300 × 4,200

Now check your photo the other way. A phone photo of 4,000 × 3,000 pixels can fill an 8 × 10 at 300 DPI and have room to spare, though the shape will not match exactly. A photo is 4:3, an 8 × 10 print is 5:4, so some of the edges have to be trimmed. For help with shape, see how to resize a photo without stretching it.

A photo of only 800 × 600 pixels would print at just 2.7 × 2 inches at 300 DPI. Stretched to 8 × 10 it would drop to about 80 DPI, and look soft.

Using the resizer for this

The image resizer works in pixels, because that is what is real. Pick a size by longest side, width, height, percentage or an exact width and height, and it gives you a copy at exactly that many pixels. To prepare a photo for a print size, work out the pixels from the table above or the formula, and enter them as the exact size. It can make photos up to 8,000 pixels on a side, and checks the result opens at the size you chose.

If the file is then too heavy for a form or an email, use the compressor.