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PPI and DPI Explained

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If you’ve downloaded this tutorial, it’s probably because at some point you’ve become confused by the terms DPI and PPI, and the way they are often used interchangeably. We’ll explain what each term means, and describe why they are not interchangeable.

Introduction

The difference between PPI (pixels per inch) and DPI (dots per inch) continues to cause a great deal of confusion. Although PPI and DPI are often used interchangeably, they are not the same thing.

Simply put, PPI refers to the resolution of a digital image as you view it on a screen, and DPI refers to how an image is printed.

Unfortunately, many people, and even companies, continue to mistakenly use the terms interchangeably.

PPI (Pixels-Per-Inch)
PPI refers to screen resolution, and is easy to understand. A pixel is a tiny, square-shaped block of color. A digital image — whether it’s a photograph or another type of graphic — is composed of thousands or millions of individual pixels.
 

So, PPI refers to the number of pixels-per-inch in a digital image – on your computer monitor, or any other digital screen. Pixels are square and in contact with the adjacent pixels. The number of pixels-per-inch in your digital image will affect the print size of your photo and will affect the quality of the output. The more pixels-per-inch that a digital image contains, the higher its ‘resolution’ is said to be. More pixels-per-inch means more detail and crisper images.

How PPI Affects Print Size
Many people think if an image looks good on the screen, it will look when it’s good printed out. This is not always the case. If you plan to print your images, you want to keep them at the highest resolution (PPI) that you can.

When printing an image, it’s important to know how large you can print the image while still retaining an acceptable level of quality and detail.

The total amount of pixels that an image contains dictates the maximum size in inches it can be printed without a noticeable loss in quality. Print professionals recommend that, in order to achieve a high-quality print, an image should be saved at a minimum of 300 PPI.

So, for example, let’s say that you have a 3.2 megapixel image that you’ve saved at 300 PPI. If it measures 2,048 horizontal pixels x 1,536 vertical pixels (2,048 x 1,563 = 3.2 MP), an easy way to find out the maximum size image you can print (in inches) at 300 PPI is to simply divide each of the pixel dimensions by 300. 2,048 divided by 300 = 6.8, and 1,563 divided by 300 = 5.2. In this case, your 3.2-megapixel image can be printed at a maximum size of 6.8 inches horizontally x 5.2 inches vertically while still retaining photorealistic quality.

DPI (Dots-Per-Inch)
DPI refers to how a photo is printed on paper (and also to how a paper photo is scanned).

Traditional printing methods use patterns of dots to render photographic images on a printed page. Every pixel in a digital image is created on a print-out by a group of different colored dots. These dots are made by the different colored inks used in the printer, usually 4 colors - Cyan, Magenta, Yellow, and black (CMYK), although some photo printers use more. Printers combine these different colored dots together to make up all the colors of the image. Look at a photo in a magazine with a magnifying lens, and you’ll see the dots.

PPI to DPI
When a digital image is being printed, pixels are converted to dots. As we saw earlier, pixels are all next to each other with no spaces between them. Dot, however, have spaces between them. Thus DPI is always lower than PPI. A rough rule-of-thumb is to divide PPI by 2 to get DPI. So, 300 PPI becomes roughly 150 DPI. 150dpi is the accepted standard for printing photographic quality images.

In Summary

Remember, pixels-per-inch (ppi) refers to the number of square pixels within an inch of a digital image, while dots-per-inch (dpi) refers to printed dots within an inch of a printed image.