There's a calculator on the previous page for specific values, but here's a chart of Angular Field of View (Width, Height, Diagonal, in degrees) for many lens focal lengths ("Lens mm") on popular sensor sizes and common aspect ratios. One example of the chart use might be to compare the width of view on two cameras using same focal length lens.
The calculation is NOT for fisheye lenses (their view will be wider), and it is NOT for macro distances (that close is more narrow). Field accuracy will be better at a distance of at least a few feet, because the lens focal length number changes with focus at closer distance (reasonable accuracy if magnification is less than 0.1x). The stated value of focal length applies to Infinity focus, but is normally an insignificant difference up to the closest focus of a regular (non-macro) lens.
Cell phones and compact cameras use many sizes of 4:3 Aspect Ratio sensors. It is difficult to find their specifications. Probably best chance is that their image Exif data should show Focal Length, and also Equivalent Focal Length from which Crop Factor and then sensor size can be computed. A precise Crop Factor can be determined (see Determine Crop Factor, from either the sensor dimensions, or from Equivalent Focal Length ratio). Compact cameras that zoom probably instead show Equivalent Focal Length in the lens specifications in the manual or in advertising. Note that lenses that zoom report actual focal length used as the nearest of several coarse steps (far from precise), so we probably don't know precise focal length used for zooms except when actually at one of the end extremes.
Focal length: Not every computed sensor/lens focal length combination shown is realistically practical, for example 2 mm focal length on full frame, or 500 mm on a tiny phone camera. These are all the actual “real” focal lengths of the lens, NOT any Equivalent focal length. Equivalent is generally always on a different camera (convention is normally about a hypothetical 35 mm film camera of crop factor 1x), but it is NOT on the camera you are using.
The chart top row marked W×H shows the sensor width x height dimensions in mm, computed from the crop factor. Find your Sensor Size in the chart. See Crop Factor calculators which can compute sensor size.
FWIW, the size of our full Moon appears about 0.5 degrees (its size varies slightly in its elliptical orbit, 0.49 to 0.558 degrees).
For the same sensor, a 2x longer focal length shows a 2x larger subject view, cropped in a view dimension only half as wide. Or 4x is 4x size in a view 1/4 as wide, etc. But the numeric Angle of View number is not linear, meaning a 1/2x view is 2x field dimensions, but is NOT a 2x numeric angle (but is closer for small angles in long lenses).