The crop factor is a much talked about, but ultimately minimal impact aspect of film making. There are several well written articles about this topic such that I do not feel the need to reinvent the discussion here.
Daniel Haggett provides an excellent overview of various crop factor and how they affect the apparent magnification of a given lens.
The following is the proper formula for calculating the crop factor against the 35mm sensor:
(sqrt((36^2)+(24^2))) / (sqrt((SW^2)+(SH^2)))
This is shorter version of the formula which when copy / pasted into Google’s search entry box with the WIDTH and HEIGHT replacing SW and SH respectively, provides the crop factor against the 35mm sensor:
sqrt((36*24) / (SW*SH))