How is the percentage of weights in SENSEX calculated ?

     The method used for calculating Sensex is the market capitalisation weighted method in which weights are assigned according to the size of the company. Larger the size, higher the weightage . The base year of Sensex is 1978-79 and the base index value is set to 100 for that period.

        The construction technique of index is easy to understand if we assume that there is only one stock in the market. In that case, the base value is set to 100 and let’s assume that the stock is currently trading at 200. Tomorrow the price hits 260 (30% increase in price) so, the index will move from 100 to 130 to indicate that 30% growth. Now let’s assume that on day 3, the stock finishes at 208. That’s a 20% fall from 260. So, to indicate that fall, the Sensex will be corrected from 130 to 104(20%fall).

       As our second step to understand the index calculation, let us try to extend the same logic to two stocks – A and B. A is trading at 200 and let’s assume that the second stock ‘B’ is trading at 150. Since the Sensex follows the market capitalization weighted method, we have to find the market capitalization (or size of the company- in terms of price) of the two companies and proportionate weightage will have to be given in the calculation.

Regards .