You can specify the size of an image by choosing the number of pixels that will make up that image: n pixels wide by n pixels high. Easy! But there’s lots more to know about how to create a high resolution image so that it prints or displays properly on a web page. Let’s start off with saving images for print.
Sizing images for printing
Unless you’re printing something huge like a poster, trade show banner, or billboard, you’ll almost certainly be printing on a 300 DPI (dots per inch) printer, simply because this is the resolution of most printers. That means we have to make sure the resolution of your image matches the resolution of your printer.
300 DPI means the printer is going to spit out 300 little dots of ink for every inch of your photograph. So your image file should contain the same number of pixels per inch that your printer is going to be printing per inch.
When somebody asks you for a “300 DPI file,” what they want is a file with enough pixels to print that image on a 300 DPI printer.
Getting a 300 DPI file
Since we know the printer is going to print at 300 dots per inch, all we need to do is figure out how many pixels to give the printer. To do that, just multiply 300 by the number of inches the print is going to be. That means in order to create a 4 x 6 print on a 300 DPI printer, you want 1200 x 1800 pixels.
If you don’t feel like doing the math, here’s a handy chart:
Size vs. Shape
Your image needs to be the same aspect ratio (shape) as the paper you are printing on, otherwise things get either distorted or cut off. If your image is already the same shape as your paper, you can make this adjustment with the Resize tool.More often, your image is a different shape than the print you plan on making. In that case, you’ll be looking for the Crop tool rather than Resize.
Displaying images on a screen
Monitors and screens use pixels much more sparingly than printers. While a printer gobbles up 300 of your hard-earned pixels per inch, your miserly monitor might only use 72. That means a 2400 x 3000 pixel image used to create an 8 x 10 print would display at about 2 ¾ feet (yes, we’re talking feet here, folks, not inches) on a 72 PPI monitor.
Sizing for the web would be pretty easy if all screens were 72 PPI. Trouble is, they aren’t—not by a long shot. They used to be, and 72 PPI is still used as a reference for designers, but most modern PC monitors are 96 PPI. Retina displays can be around 220 PPI. This complicates selecting an image size for the web.
You may have noticed that when we were talking about the resolution of printers, we used the term DPI, and now when we’re talking about monitors, we’ve switched to PPI (pixels per inch). Good eye! Monitors don’t deal in dots of ink, they have pixels that light up to create your image.
Sizing for sharing on the web
When saving images for a website or blog, the site layout that will often dictate how many pixels high and wide you need to size your image. On our blog, the images within the posts are around 600 pixels wide. Header images that span the entire page might be larger—in the neighborhood of 1200 pixels—which leaves it up to the browser to stretch it if the browser window is larger.
If you over-accommodate for large displays, you’ll end up serving images that slow your site down. It’s a balancing act. You want the smallest file that works for your purpose. When you’re not matching an image to a particular layout, you can be fairly loose with the size. Most people consider a “high resolution” share on the internet to be in the neighborhood of 1200 – 1600 pixels wide.
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