When it comes to image size and quality, the acronyms DPI and PPI are used often. The problem is, there’s a lot of confusion about them. The terms are used interchangeably when they aren’t the same thing. What is DPI? How are DPI and PPI different? And how do they apply to digital and print photography?
This guide will answer these questions. We’ll explain the difference between DPI and PPI and clear up the confusion between the two. Understanding what DPI and PPI mean for your work will help you produce quality photo prints; optimize your images for the internet; and in the long run, save yourself a lot of time.
What is PPI?
The acronym PPI stands for pixels per inch. It describes the resolution of a digital image. A pixel is a tiny colored square that is the smallest building block for a photo. When combined with other pixels, a photograph is formed. Zoom in to any image on your computer and you’ll eventually see the colored squares that make up the photo — these are the pixels.
So, when we’re talking about PPI, we’re referring to the density of pixels within a given image.
When to use PPI
Use PPI whenever you are working with a digital image, not a print. It’s most useful when you’re preparing a digital file for printing. Part of the reason for the confusion between PPI and DPI is that printers use DPI (we’ll explain this below) and image editing software has a DPI setting. This setting is just to make the conversion to print easier. However, it had the consequence of making users erroneously believe that PPI and DPI were the same thing and could be used interchangeably.
An image with a higher PPI tends to be of higher quality because it has more pixels per inch. But a general rule of thumb is to export your digital photos at 300 PPI for high-quality prints.
Higher PPI images have larger file sizes, so if you’re using your image for the web you’ll want to save your image with a low PPI. PPI doesn’t matter when it comes to images that will be displayed on a screen. That’s because monitors have fixed pixel density. In other words, an image with 1 PPI and an image with 5,000 PPI will display the same size as long as their pixel count is the same.
How to change PPI
If you don’t have enough pixels in your image to make a large print, you can resample it. Resampling is the process of adding pixels to an image. In Photoshop, navigate to Image > Image Size. From the popup, you’ll have the option to change the width, height, and PPI of your photo. If you click the check box that says “Resample,” the software will add extra pixels by spreading the existing pixels apart and then guessing what the new pixels should look like based on the neighboring pixels.
Resampling to increase a file’s PPI is not recommended. It results in loss of image quality and loss of detail.
You can also use Image Size to decrease the PPI of an image and make it suitable for web or email. While downsizing can also degrade an image since you’re removing pixels, it is a matter of compromise between faster loading speeds and image quality.
What is DPI?
The letters DPI stand for dots per inch. It has nothing to do with your digital image but refers to the resolution of a physical printer. The “dots” are tiny dots of ink on paper that combine to create your print. In fact, it takes many dots to create one pixel and that’s why PPI and DPI shouldn’t be used interchangeably.
When to use DPI
If you’re going to print your photo, the printer will use the term DPI. Every printer has its own DPI — home inkjets are usually 300 to 720 DPI while some photo printers can go as high as 2,400 DPI or more.
Note: When someone tells you they want a “300 DPI image” what they really mean is they want a 300 PPI image. They’re using the term interchangeably (and as you’ve learned, incorrectly). Additionally, the phrase 300 PPI (or DPI) is useless without including the image size, too. A more useful request would be “I need an 8”x10” image at 300 PPI.”
Understanding Digital Image Size
When discussing DPI and PPI, it’s essential to understand digital image size or pixel count, too. This is the size of the image created by your camera. For example, my Canon 6D Mark II captures RAW images that are 6420 pixels x 4160 pixels.
What is 300 DPI in pixels per inch?
We’ve mentioned a few times that DPI is often used interchangeably with PPI. If you’re asking what 300 DPI is in pixels per inch, you’re likely thinking about PPI and not DPI. So, the answer is 300.
Finding the number of pixels you need to be able to export a 300 PPI image for print can be done with an online pixel calculator or by using a simple equation.
To figure out the pixel size for an 8×10” image, you simply do 8×300=2400. Then 10×300=3000. So, your pixel size needs to be 2400×3000 pixels.
Here’s another example. How many pixels would you need to print a 4×6 photo at 300 DPI?
4×300=1200px and 6×300=1800px, so you would need an image that was 1200×1800 pixels. Easy, right?
Inch-to-pixel conversion chart
Here are some common image print sizes and their corresponding pixel sizes:
|Image Size||Pixel Size @ 300 PPI|
To recap, PPI describes the resolution of a digital photo and determines its print size. DPI defines how a printer works and has nothing to do with a digital file. However, the term DPI is misused so often when the person using the phrase actually means PPI that the two have become interchangeable. Having an understanding of these two terms — what they mean and how they differ — will go a long way in making you better able to communicate your needs with a printer; save image files efficiently for print or web; and, most importantly, save you time and headaches!