How to calculate the value of an index on a particular day??

We often say things like -"The SENSEX opened at 15000 points",though not in today's bear market!The question is-"How to calculate the value of Sensex,Nifty or any other index on a particular day"??
For this you need to know about market capitalization.Market capitalization is a factor which determines the vastness of a company.It is the product of the total number of shares which are out and the current share price of that company.
For eg RIL,had a market-cap of about Rs 200000 crore,which is a big amount.Basically it is that amount,which one will have to spend in order to purchase all the shares of a company.
After the market-cap,we have something called asthe index factor.The index factor remains constant at a particular time for each index.Like Sensex will have a different index factor than Nifty.

After you get the market cap of all the companies listed in a particular index,simply add them up.Divide that value by the index factor.Now you have the value of that index right in front of you!