How to use this screen size calculator
Pick the tab that matches your question. The first one takes an advertised diagonal, say 55 inches, and an aspect ratio, and returns the real width and height in both inches and centimetres, along with the area of the picture. If you are checking whether a set will fit an alcove or a media unit, add a bezel allowance per side and the tool adds it to the outer dimensions. The comparison field answers the question everyone asks in the shop: a 65-inch screen has about 40% more area than a 55-inch one, not 18% more, because both width and height grow.
The second tab works in reverse. Measure the visible picture with a tape, type the width and height, and you get the diagonal and the detected aspect ratio, which is handy for second-hand monitors with no label. The PPI tab combines a resolution with a diagonal to give pixel density, dot pitch and the distance at which the pixels stop being visible. The last tab turns a diagonal into recommended seating distances using the THX and SMPTE viewing-angle guidelines, and tells you how close you need to be for 4K to look sharper than 1080p.
Why the numbers matter
Diagonal sizes are a marketing convention that hide the shape of the screen. A 34-inch ultrawide monitor is wider than a 32-inch 16:9 television but noticeably shorter, and a 12.9-inch 4:3 tablet has more screen area than a 13.3-inch 16:9 laptop. Working from the actual width, height and area lets you compare devices honestly and avoid buying a stand that is two inches too narrow.
Pixel density and viewing distance are linked. A 27-inch 1440p monitor at 109 PPI looks crisp at arm's length but shows visible pixels if you lean in, while a 65-inch 4K TV at 68 PPI is only sharper than a 1080p set if you sit within about 8.5 feet. Knowing those thresholds before you spend money on a higher resolution, or on a larger panel, means the upgrade is one you can actually see from your sofa or desk.