Three numbers decide what an eyepiece will actually show in a given telescope, and all three come from arithmetic you can do on the back of an envelope: magnification, true field of view, and exit pupil. This page gives the formulas and what each result means in practice.
Magnification and true field of view
Magnification is the telescope's focal length divided by the eyepiece's. True field of view is the eyepiece's apparent field divided by that magnification. So a 1200mm telescope with a 25mm eyepiece of 52 degrees apparent field gives 48x and a true field of about 1.08 degrees, which is roughly two moon widths. That figure is what tells you whether a cluster will fit.
Exit pupil, the number nobody calculates and everybody should
Exit pupil is the aperture divided by the magnification, in millimetres, and it is the width of the cone of light leaving the eyepiece. A dark adapted adult eye opens to between five and seven millimetres depending on age. An exit pupil larger than your own pupil wastes light; one below about half a millimetre is where the view becomes dim and the floaters in your eye become visible.
Working an example through
Take the 76mm f/4 instrument in this index: 300mm focal length. With a 20mm eyepiece it gives 15x and an exit pupil of 5.1mm, which is close to ideal for a dark adapted eye and gives a wide, bright, low power view. With a 4mm eyepiece it gives 75x and an exit pupil of 1.0mm, which is still usable. A 2mm eyepiece would give 150x at 0.5mm, which is the practical floor for that aperture.
Questions people ask about telescope
How do I calculate true field of view?
Divide the eyepiece's apparent field of view, printed on its barrel, by the magnification it gives in your telescope. The result is in degrees; the full moon is about half a degree.
What is a good exit pupil?
Between about two and five millimetres for general observing. Above six most eyes cannot use all the light; below about half a millimetre the view gets dim and uncomfortable.
Why does my eyepiece show less than the calculator says?
Usually because the apparent field printed on the barrel is optimistic, or because the focuser or diagonal is narrowing the light path. Older 0.965 inch fittings vignette badly on wide eyepieces.