If a person has shares of a factory, he has to give zakah if he has the amount of nisab. It is calculated as follows:

First, the total value of all of the shares are calculated. Then, the value of the factory, the office building, machinery, tools and the furniture are subtracted from the total. The total of the remaining tradable goods, raw materials and the money are divided by the amount of the shares. The number (product) is divided by forty and the amount for zakah is obtained. 

In other words, everything except the factory building, machinery and furniture must be calculated and divided by the amount of the shares. For instance, if the amount for each share is TL one million, the zakah for each share is TL twenty-five thousand.

However, if the intention of the person who holds the shares is buying and selling shares, then he should calculate and give one-fortieth of the values of the shares as zakah.

The shares of the companies are calculated based on their current value and their zakah is given based on it. (Fiqhul-Islam wa adillatuhu, v. 2, p. 774).