What Is DPI? Pixels, cm and 300 DPI Explained

Pixels are the picture, DPI is a note about printing

A digital picture is a grid of coloured dots called pixels. A photo 600 pixels wide has 600 dots across, and that is all the detail it will ever have. DPI (dots per inch) is a separate number stored inside the file. It tells a printer how many of those pixels to place in one inch of paper. It does not change how the picture looks on a screen.

So the same picture prints at different sizes depending on its DPI. Here is one picture, 600 pixels wide, at three DPI values:

Higher DPI means a smaller, sharper print from the same pixels. Lower DPI means a bigger print where each pixel is larger and the picture looks softer.

Why changing DPI does not change the pixels

When you set a picture to 300 DPI with Change Image DPI, only that stored number is rewritten. In a JPG it is written in the JFIF header and in the EXIF data, because Windows and many photo apps read the EXIF one first. In a PNG it goes in a small block called pHYs. The picture data itself is copied as it is, so there is no quality loss and the file size changes by only a few bytes.

This also means a higher DPI cannot add detail. A small or blurry photo set to 300 DPI just prints smaller. It does not get sharper. If a form needs a certain number of pixels, you have to resize the picture, not change its DPI.

Many phone photos and screenshots store no DPI at all. The tool then says no DPI is set, and apps usually assume 72 or 96. WebP pictures have no place to store a DPI, so the tool saves them as a high-quality JPG first.

The cm to pixel formula

One inch is 2.54 cm. To turn a size in cm into pixels, change it to inches and multiply by the DPI, then round to a whole pixel:

pixels = cm ÷ 2.54 × DPI
cm     = pixels ÷ DPI × 2.54

3.5 cm at 300 DPI = 3.5 ÷ 2.54 × 300 = 413.4 → 413 px
4.5 cm at 300 DPI = 4.5 ÷ 2.54 × 300 = 531.5 → 531 px

For millimetres, divide by 25.4 instead of 2.54. For a size already in inches, just multiply by the DPI.

Common sizes in pixels at 200 and 300 DPI

For any other size, or for 72, 96, 150 or 600 DPI, use the cm to Pixel Converter. It works both ways, shows a table for the other DPI values, and can crop a photo to exactly that many pixels with the DPI written into the file.

What a form means by “200 DPI”

Forms are not always clear. Some really check the DPI number in the file. Many others only check the width and height in pixels and the size in KB, and mention DPI because the instructions were written for scanners. Read the form's own rules and do what they ask:

• If it gives a size in pixels, resize the picture to those pixels. The DPI then matters little.
• If it gives a size in cm and a DPI, work out the pixels with the formula above, make the picture that size, and set the DPI too.
• If it only asks for a DPI, set it with Change Image DPI. The picture stays exactly the same.
• If it also gives a KB limit, compress the picture last, after the size is right.

How to change the DPI of a picture

1. Open Change Image DPI and add one or more JPG, PNG or WebP pictures, up to 30 at a time and 30 MB each.
2. Each picture shows its size in pixels and the DPI it has now, or that none is set.
3. Choose 72, 96, 150, 200, 300 or 600, or type any whole number from 1 to 65535.
4. Check the print size shown for each picture in cm and inches.
5. Download each picture, or all of them in one ZIP. The new file name ends in -300dpi or the value you chose.

Everything runs in your browser, so your photos are not uploaded.

To check whether a photo is big enough to print well, divide its width in pixels by 300. The answer is the largest width in inches it will print sharply. A 1200 pixel wide photo is good for 4 inches (about 10 cm).