Assuming Lat1, Lon1 is the first point and Lat2, Lon2 is the 2nd point.
Remember that Westerly latitudes and Southerly Longitudes are NEGATIVE.

X = (Sin(Val(Lat1) / 57.2958) * Sin(Val(Lat2) / 57.2958)) + (Cos(Val(Lat1) / 57.2958) * Cos(Val(Lat2) / 57.2958) * Cos(Val(Lon2) / 57.2958 - Val(Lon1) / 57.2958))

MILES = 3963 * (Atn(-X / Sqr(-X * X + 1)) + 2 * Atn(1))
NAUTICALMILES = 3438 / 3963 * MILES
KILOMETRES = 6378 / 3963 * MILES