In the Name of Allah The All-Merciful, The Ever-Merciful.

 I am a trader of real properties and used cars. How do I calculate the Zakat and how do I determine its due time?

I may acquire a commodity today and sell it tomorrow, after a week, a month or may be after more than a year. So, should I calculate the due time of the Zakat of each commodity separately?

It is to be noted that sometimes I sell a property the night before the due time for paying Zakat or may be a week or so before? Should I pay Zakat for all what I own or only that on which a lunar year has passed?

Praise be to Allah; Prayers and Peace of Allah be upon the Messenger, his household and his companions.

At first, I ask Allah to bless your trade and render it a way for you to paradise. A trader, my dear brother, starts to calculate the lunar year from the date he owns the basic minimum (nisab). He should evaluate the merchandise he has at the start of the year at the current price and add to it whatever sum he has of liquid money and any debts expected to be liquidated (a debt is expected to be liquidated if the debtor is solvent and is willing to pay the debt). He should subtract from this total any debts he has to pay, then calculate the Zakat on the rest of the money as quarter-tenth (2.5%). It is not necessary that the a year should pass on each and every part of the wealth or every deal of the trade. The due time of Zakat on the money gained during the year is that of the origin, when the origin reaches nisab. I ask Allah to grant us success and rightness. And Allah knows best.