Let's use the example of an iPhone 7 Plus.
If we wanted to know how many inches the display of an iPhone 7 Plus is, we'd have to know it's PPI.
In this case, according to Apple's specifications, it's 401 PPI.
Then, we'll need to know the pixel measurements in both width and height. The iPhone 7 Plus is 1080 pixels by 1920 pixels.
Therefore, our equation would be:
1080 (pixels) / 401 (PPI) = 2.7 inches (rounded up from 2.69)
1920 (pixels) / 401 (PPI) = 4.8 inches (rounded up from 4.78)
So, an Apple iPhone 7 Plus will be 4.8 inches in height by 2.7 inches in width.
In fact, if you own an iPhone 7 Plus, pull it out, grab some measuring tape, and see for yourself!
Inches to Pixel Example
Let's say you have a printed image (maybe a photograph) and want to know what the pixels would be when scanned.
Let's say the image is 8.5 inches by 11 inches, which is the standard size of a typical piece of printer paper.
Let's say that you also know the DPI of the printer used to print the photo is 600 DPI.
Our equation would be:
8.5 (inches) x 600 (DPI) = 5,100 pixels
11 (inches) x 600 (DPI) = 6,600 pixels
Therefore, your scanned image of the photograph would be 5,100 pixels by 6,600 pixels.