How do you calculate SP500 ?

1. I have 500 symbols (supposedly are constituents of the SP500 cash index).

2. I have %% weight of each in the SP500 (questionable quality data though but is close if you weigh them by Market Cap).

3. I calculate Sum(Last x Weight) and I get $120 instead of $180

what am I doing wrong?

 

wikipedia. there's a divisor needed

 

http://www.investopedia.com/ask/answers/05/sp500calculation.asp

Not a bad site for general financial info