Sheldon Brown's website has a universal calibration chart of sorts. I am assuming you need the circumference in millimeters. So if you go to that chart you see there is no 700X47. You need to split the difference between 700X44 (2224) and 700X50 (2293). That will get you (2259)

The answer ... 2259

http://sheldonbrown.com/cyclecomputers/index.html