1080p is 2K. The value of the "K" coefficient is determined by the x axis, not the y axis. That's why 4K is 3840x2160.

16K = 15360x8640 8K = 7680x4320 4K = 3840x2160 2K = 1920x1080 1K = 960x540

(Every value is a doubling of the tier below it, or in the case of "1K" a halving.)