FOV Calculator
Horizontal to vertical FOV and back, for any aspect ratio.
The conversion
vFOV = 2 atan(tan(hFOV / 2) / aspect)
FOV converts through tangents, not proportions, because the camera projects onto a flat plane. 90° horizontal at 16:9 is 58.7° vertical, not the 50.6° a linear scale would suggest.
Why games disagree about 'FOV 90'
Some engines mean horizontal FOV, some vertical (Unity's inspector, most Unreal templates), and some horizontal-at-4:3 that scales with the monitor (the Half-Life/Source lineage). Vertical-locked FOV gives ultrawide players extra side vision; horizontal-locked crops it. This is why competitive players convert their sensitivity and FOV together when switching games.