I'm assuming it's black bars and 1920x1080 in the middle?
If this is the case, you need to do a little trick if you want to convert to other games that use the whole monitor.
The calculator doesn't know that you have 1080p in the middle of a 32:9 monitor, so to make it work correctly switch FOV Type to Vdeg while you have 1920x1080 as the resolution (it will then show 70.5328), then change the resolution to 3840x1080. You will see an "Actual HFOV" in the calculations that are wider than your FOV is in reality, but this is so the calculator also includes the black bars in the calculation.