For various reasons gadget specifications and even reviews only give the diagonal measurement of screens, and not the total screen area. (They typically provide resolution and PPI as well.) This makes sense only when you’re comparing devices with the same aspect ratio. However, phones (and TVs) are typically 16:9, tablets and e-readers are typically 4:3. This means a strict comparison of diagonal measurements is not very useful. Even with devices that share the same aspect ratio, the fact that screen area increases geometrically while the diagonal measurement increases linearly, means that breaking out the screen area can be useful for seeing the real-world differences between different screen sizes (for example, how much text can fit on a screen).
iPhone 6S Plus, 12.96 square inches
Kindle Paperwhite, 17.28 square inches
Kindle Fire 7", 22.02 square inches
iPad Mini 4, 29.6 square inches
iPad, 45.16 square inches
iPad Pro 12.9", 80.3 square inches