From diagonal to real dimensions
Screens are sold by their diagonal measurement, but the diagonal alone does not tell you how wide or tall the display actually is. That depends on the aspect ratio. A 27-inch 16:9 monitor is about 23.5 inches wide and 13.2 inches tall, while a 27-inch 4:3 screen would be shorter and chunkier.
This calculator uses the aspect ratio to split the diagonal into true width and height in both inches and centimeters, and if you add the pixel resolution it also returns the PPI. That is handy for planning desk space, comparing two differently shaped screens, or checking whether a monitor will fit a shelf.
Check it at actual size
Once you know the physical width, you can confirm it against a real ruler. Open the online measuring tape or the CM ruler to sanity-check dimensions on your current screen.