Sensex Calculation Methodology
Sensex is calculated using the "Free-float Market Capitalization" methodology.
As per this methodology, the level of index at any point of time reflects the Free-float market value of 30 component stocks relative to a base period.
The market capitalization of a company = Price of its stock * Number of shares.
The free-float market Capitalization. =This market capitalization * the free-Float factor to determine
The base period of Sensex is 1978-79 and the base value is 100 index points.
This is often indicated by the notation 1978-79=100.
The calculation of Sensex involves dividing the Free-float market capitalization of 30 companies in the Index by a number called the Index Divisor.
The Divisor is the only link to the original base period value of the Sensex. It keeps the Index comparable over time and is the adjustment point for all Index adjustments arising out of corporate actions, replacement of scrips etc.
During market hours, prices of the index scrips, at which latest trades are executed, are used by the trading system to calculate Sensex every 15 seconds and disseminated in real time.
Example:-
Suppose the Index consists of only 2 stocks: Stock A and Stock B.
Suppose company A has 1,000 shares in total, of which 200 are held by the promoters, so that only 800 shares are available for trading to the general public. These 800 shares are the so-called 'free-floating' shares.
Similarly, company B has 2,000 shares in total, of which 1,000 are held by the promoters and the rest 1,000 are free-floating.
Now suppose the current market price of stock A is Rs 120. Thus, the 'total' market capitalisation of company A is Rs 120,000 (1,000 x 120), but its free-float market capitalisation is Rs 96,000 (800 x 120).
Similarly, suppose the current market price of stock B is Rs 200. The total market capitalisation of company B will thus be Rs 400,000 (2,000 x 200), but its free-float market cap is only Rs 200,000 (1,000 x 200).
So as of today the market capitalisation of the index (i.e. stocks A and B) is Rs 520,000 (Rs 120,000 + Rs 400,000); while the free-float market capitalisation of the index is Rs 296,000. (Rs 96,000 + Rs 200,000).
The year 1978-79 is considered the base year of the index with a value set to 100. What this means is that suppose at that time the market capitalisation of the stocks that comprised the index then was, say, 60,000 (remember at that time there may have been some other stocks in the index, not A and B, but that does not matter), then we assume that an index market cap of 60,000 is equal to an index-value of 100.
Thus the value of the index today is = 296,000 x 100/60,000 = 493.33
This is how the Sensex is calculated.
The factor 100/60000 is called index divisor.
