We just added the Aspect Ratio Calculator. Click your monitor’s name — FHD, QHD, UWQHD, 4K UHD, 8K UHD — and the ratio of the content you’re watching (a 21:9 video, an old 4:3 game, and so on), and it instantly shows how much letterboxing or pillarboxing you’ll get, plus exactly what percentage of the screen is actually used.
Open “More: resolution & PPI calculator” to find a ratio from a resolution like FHD, QHD, or 4K UHD, or solve for a missing width/height from a ratio.
Enter a monitor’s diagonal size (inches) and resolution to get its PPI (pixel density) and physical width/height in centimeters.