How can I calculate the zakat on money invested in the stock market?

Calculate the money that is actually in your possession, deduct the value of your debts and pay zakat on the remaining sum. Zakat is obligatory if the money meets the following conditions:
- It reaches nisab [minimum sum on which zakat is due]. Nisab equals the value of 85 grams of 21 carat gold according to the market price.
- It is surplus to your basic needs.
- It has been in your possession for one lunar year.
According to the opinion of Maliki scholars, it is obligatory to pay zakat on the debts owed to you for only one year after you recover them.

Stock shares
Zakat becomes obligatory on commercial shares according to their market value on the day they complete one lunar year after their purchase if they meet the above mentioned conditions. There is no zakat on service, production and industrial shares.
And God the Almighty knows best.