Originally Posted by macropod

For Great Circle distances, the following give good approximations:
=ACOS(SIN(RADIANS(Lat1))*SIN(RADIANS(Lat2))+COS(RA DIANS(Lat1))*COS(RADIANS(Lat2))*COS(RADIANS(Long1-Long2)))*RadiusEarth
and more accurately for very small differences in Lat/Lon, particularly near the poles::
=2*ASIN(SQRT((SIN(RADIANS(Lat1-Lat2)/2))^2+COS(RADIANS(Lat1))*COS(RADIANS(Lat2))*(SIN(R ADIANS(Long1-Long2)/2))^2))*RadiusEarth
where the latitude and longitude coordinates are input as +/- decimal degrees and the earth's radius is given in miles or km, depending on which units you're using. When mixing hemispheres, enter Northern and Western coordinates as positive, and Southern and Eastern coordinates as negative.