What aspect ratio is 3840×2160?

1.78:1
There are three main 4K resolution standards: UHD-1, or ultra-high-definition television (UHDTV), is the 4K standard for television and computer monitors. Resolution of 3840 ? 2160 (16:9, or approximately a 1.78:1 aspect ratio); UHD-1 is used in consumer television and other media, e.g. video games.