Thanks everyone for replies. Most of the mentioned software (e.g. newastro) look at how an image is compared with the size of the CCD camera sensor.
This is not what I am looking for. In other words, the said software draws a rectangular on the night sky image, instead of a circle (which would be the case when using an eyepiece).
What's more, the said software does not allow to overlay the circles representing eyepieces of different focal lengths and of different AFOVs. Thus, I cannot compare different eyepieces.
I did more search, and found that the right keywords are "telescope FOV simulator". That's "simulator", not "calculator".
Following this search, I discovered the website which is closest to what I want, i.e. it allows to key in the focal length and aperture of the telescope and of EP, and voi la ! -- the appropriately sized map of the sky appears. But it is still does not do what I need -- it does not allow to compare several eyepieces.
This is the website,
http://www.12dstring.me.uk/fovcalc.php
EDIT. Aha, I finally found it !!!!
http://astronomy.tools/calculators/field_of_view/