Image Resolution vs. DPI: How Many Pixels You Need for Web and Print

An image is really its pixel count; dpi is a print setting

A digital image is a grid of colored dots (pixels). A 3000 × 2000 image has 3000 dots across and 2000 down. That number is the image’s entire information content, and you can’t increase it after the fact.

DPI (dots per inch) specifies how many of those dots to print per inch (25.4 mm). The same 3000-pixel image prints 10 inches wide at 300 dpi and 20 inches wide at 150 dpi. In other words, changing the dpi doesn’t change the image data; it only changes how big the output is on paper.

Calculating the pixels you need from the print size

There’s just one formula: pixels needed = finished length (mm) ÷ 25.4 × dpi. One inch is 25.4 mm, so convert millimeters to inches and multiply by the dpi (if you already work in inches, simply multiply inches × dpi). Converting between inches and centimeters is covered in our inch to cm guide.

1. 1

   Check the finished size in mm

   A4 is 210 × 297 mm; a Japanese business card is 91 × 55 mm. If the print runs to the edge of the paper, it’s standard to add 3 mm of bleed on every side.

2. 2

   Decide the dpi you need

   Prints people hold in their hands need about 300–350 dpi; posters viewed from a distance need about 150–200 dpi. It depends on the use, so follow the print shop’s specification if there is one.

3. 3

   Plug the numbers into the formula

   To print the width of A4 at 350 dpi, you need 210 ÷ 25.4 × 350 ≈ 2894 pixels. The height is 297 ÷ 25.4 × 350 ≈ 4093 pixels. So you need an image of at least 2894 × 4093 px.

4. 4

   Compare with your image’s pixel count

   Check the width and height in pixels in the image’s properties or your editing software. If they exceed the calculated values, you’re fine. If they fall short, enlarging the image won’t make it sharp.

To find out how large you can print an image you already have, flip the formula: pixels ÷ dpi × 25.4 = printable length (mm). For example, a 5472 × 3648 px photo from a 20-megapixel camera prints 5472 ÷ 300 × 25.4 ≈ 463 mm wide at 300 dpi. That’s wider than A3 (420 mm), so you have room to spare up to A3.

DPI has nothing to do with web display

You’ll often read that “web images should be 72 dpi,” but that’s a relic of an earlier era. Browsers ignore the dpi value embedded in an image; all they use for display is the pixel count and the display size specified in CSS. At 72 dpi or 350 dpi, an image with the same pixel count looks exactly the same on screen.

What matters on the web instead is the device pixel ratio. On smartphones and high-resolution displays, one CSS pixel is actually drawn with two or three physical dots. If you place a 400 px image in a spot that CSS sets to 400 px wide, it looks blurry on a 2× screen.

• Prepare twice the display width: if CSS makes it 400 px wide, export the image 800 px wide. Going up to 3× is usually only worth it for crisp-edged assets such as logos.
• Don’t drop in oversized images as they are: putting a 4000 px-wide photo into a 400 px box looks the same but loads more slowly. Shrink it to the size you need before using it.
• Photos: resize and compress; diagrams and logos: SVG or PNG. Leaving photos as PNG makes files heavy. Just choosing the right format for each use visibly improves page speed.
• Shrink, but don’t enlarge: reducing the pixel count causes little visible loss, but increasing it adds no information, so the image goes soft.

Common mistakes and how to fix them

DPI problems get solved faster when you separate the causes. First determine whether you’re actually short of pixels or whether the specifications just don’t match.

The first row is by far the most common. No matter how high you set the dpi value in your editor, an image without enough pixels won’t become sharp. Software may enlarge it automatically, but that only fills in the missing dots by guessing from surrounding colors; no information that wasn’t there to begin with is created. The only shortcut is to calculate the pixels you need first and prepare material that meets that requirement. Once you know the pixel count, see the differences between PNG, JPEG, and WebP to choose a format.

FAQ

What’s the difference between dpi and resolution?

DPI is a print setting that says how many dots to print per inch. The image itself, on the other hand, is its width and height in pixels, which is also sometimes called resolution. Changing the dpi of the same image changes its size on paper, but not the information it contains. If someone asks you to “increase the resolution,” confirm the finished size in mm or inches to find out whether they need more pixels or just a different dpi value.

How many pixels do I need to print at A4 size?

About 2480 × 3508 px at 300 dpi, or about 2894 × 4093 px at 350 dpi. The formula is finished length (mm) ÷ 25.4 × dpi. A4 is 210 × 297 mm, so 210 ÷ 25.4 × 350 ≈ 2894. If the print runs to the edge of the paper, calculate with 3 mm of bleed added on every side.

Can I print a 72 dpi image if I change it to 350 dpi?

Yes, as long as the original has enough pixels. DPI is a print setting, so just rewriting the value changes the print size. If the pixel count falls short, though, raising the dpi value won’t make the image sharp, because software enlargement only fills in missing dots by guessing and can’t restore lost information. Start by calculating the pixels you need from the finished size and compare.

Should posters be made at 350 dpi too?

No. Posters A1 and larger are viewed from 1 m or more away, so 150–200 dpi is plenty. At 350 dpi the pixel count becomes huge and the file gets too heavy to handle. Banners and flags work fine even at 50–100 dpi. Remember: the farther away something is viewed, the lower the dpi it needs.