I ran across this wind chill web site that shows the formulae for both the old method and the new method.

http://www.onlineconversion.com/windchill.htm

The formulas used to calculate the wind chill index are:

Old Wind Chill Index (Pre-November 2001) =
91.4 - (0.474677 - 0.020425 * V + 0.303107 * sqrt(V)) * (91.4 - T)

New Wind Chill Index =
35.74 + 0.6215T - 35.75(V0.16) + 0.4275T(V0.16)

where:
V = wind speed (mph)
T = temperature (F)