Do I have to pay Zakat on my stocks that are listed on the stock exchange and if yes, how would zakat be calculated especially if the market value of the stocks falls below the purchase price?

Zakat is due on the commercial shares with the percentage of 2.5% according to the current market price if it reaches nisab [minimum amount upon which zakat is due]. Nisab equals 85 grams of 21 carat gold according to the current market price.

However, there is no zakat on industrial, production or service shares.