Field of view (FOV) is one of the most misunderstood settings in FPS games. A lot of players crank it up for "more visibility" without realizing how it actually distorts the image or how it interacts with aspect ratio. I built a calculator to handle the conversions and wanted to share the reasoning behind it.
Most games report FOV as either horizontal or vertical, and this trips people up when comparing settings between titles. Source engine games (CS, TF2) traditionally use horizontal FOV, while Unreal Engine titles often default to vertical FOV. The conversion isn't 1:1 — it depends on aspect ratio:
Horizontal FOV = 2 × arctan(tan(Vertical FOV / 2) × aspect ratio) So a 90° vertical FOV on a 16:9 screen is a very different horizontal FOV than the same 90° on an ultrawide.
A wider FOV shows more of the map but shrinks enemy models on screen, which affects flick accuracy and target acquisition. There's a real tradeoff between situational awareness and precision, and it's personal — but it helps to know what number you're actually changing when a game's slider just says "FOV."
If you play on an ultrawide or switch between monitors, the "same" FOV number can look and feel completely different. This is the part most in-game settings menus don't explain at all. I put together a free calculator that converts between horizontal/vertical FOV and adjusts for aspect ratio, so you can match settings across games or monitors without doing the trig by hand: FOV Calculator Pro
Happy to get into specifics on any engine's FOV implementation in the comments.