About the FOV Calculator

Most games store a single FOV number but interpret it against a fixed reference aspect ratio. The Source engine (CS, TF2, Portal) assumes 4:3, while Unreal-based games assume 16:9. If you set 90° in a Source game on a 16:9 monitor, you are actually seeing about 106° of horizontal view — the conversion is done by projecting through the vertical FOV.

The math behind the conversion

First the base horizontal FOV is converted to its vertical equivalent: vFOV = 2·atan(tan(hFOV/2) · h/w). The vertical FOV is invariant, so the true horizontal FOV for any other ratio is hFOV' = 2·atan(tan(vFOV/2) · w'/h'). Ultrawide users gain real peripheral vision (a 21:9 screen at the same setting shows roughly 13° more), while portrait-oriented setups lose it — which is why some games cap FOV on ultrawide to prevent an aiming advantage.