Ratio, pitch, size and PowerPoint slides.
An aspect ratio, pixel pitch and display geometry calculator that runs entirely in the browser. Resolution × pixel pitch = physical size, so any two give you the third. It names ratios the way the trade does, so 3440×1440 is 21:9 rather than 43:18, and it'll tell you 1366×768 is 0.049% off 16:9 instead of pretending it's spot on. It sizes PowerPoint slides on the same arithmetic, works round the 56-inch slide limit, and tells you when the better answer is to leave the deck alone and raise the export DPI.
Aspect Calc runs in the browser and needs JavaScript.