We would say 800 x 600 = 4:3 (Aspect ratio).

600 (vertical resolution) x 4 = 2400

2400 /3 = 800 (horizontal resolution) or simply 800 x 600.

Same as 1024 x 768 = 4:3 (Aspect ratio).

768 (vertical resolution) x 4 = 3072.

3072/3 = 1024 (horizontal resolution) or simply 1024 x 768.

So for 16:9 Aspect ratio if your desired vertical resolution is to be 1800;

1800 x 16 = 28800

28800/9 = 3200 should be your horizontal resolution or simply 3200 x 1800.

If on the other hand you want your horizontal resolution to be 2880:

2880 x 9 = 25920

25920/16 = 1620 (vertical resolution) or simply 2880 x 1620.

At 1:1.00 render resolution.