So, in video, you'll hear about 480i, 480p, 720p, 1080p, and 1080i. Those correspond, of course, to:
480i/480p: 720 x 480
1080p/i: 1920 x 1080
But, in the digital cinema notations of 2K/3K/4K, it's talking about the horizontal (x-axis) res. So:
2K = 2048 wide (2 x 1024, 1024 = 1K) so it's 2048 x 1152.
3K = 3072 wide (3 x 1024) for 3072 x 1728
4K = 4096 wide (4 x 1024) for 4096 x 2304.
Using these numbers, we can see that 2K is only slightly bigger than 1080P, even though "2K" (2048) sounds like a lot more than 1080, right? It's because the 2K refers to the horizontal, whereas the 1080P refers to the vertical. But once you do the math, you find it's 2048x1152, vs. 1920x1080. So 2K is about 14% larger than 1080P.