The actual field of view of an eyepiece roughly equals its apparent field (the angular diameter of the circle of light you see in the eyepiece when you look through it by itself) divided by the magnification provided by the telescope and eyepiece combination. For example, if a 50° apparent field eyepiece gives you 100x on a particular telescope, it will have a 1/2° actual field of view (50/100) - about the diameter of the full Moon.
Keeping just two simple formulas in mind you can easily figure the true field of view for any given eyepiece and telescope combination. Wide views are preferred for observing larger objects many requiring a very wide field of view to be seen completely, and they can provide a sense of being in a galactic fishbowl with views as far as your eye can see.
Eyepiece manufacturers will commonly provide three call outs for eyepiece specifications: the focal length (provided in millimeters), the apparent field of view (provided in degrees) and the barrel size of the eyepiece (typically provided in inches).
The formula for calculating magnification: Telescope focal length/ eyepiece focal length.
The formula for finding true field of view: Eyepiece apparent field of view/ magnification.