How to determine the amount of zakah on trade goods.

The Shaykh : And how is the amount of its zakah determined? We tell him to estimate it in terms of money when the lunar year has completed, and look at what its value is, then divide the value by forty; whatever results is the zakah, because trade goods are subject to a quarter of a tenth. The reason for this is that the purpose of trade goods is the same as the purpose of trade goods: to increase wealth in terms of value. For this reason, you will find that the owner of trade goods does not intend the item itself. He may buy the item in the morning, and if he profits from it by the end of the day, he sells it, or he does not, he sells it. This is unlike the person who has an item for keeping, for he will keep this item and not sell it, because he has a purpose in the item itself. As for the owner of trade goods, his purpose is to increase wealth in terms of value. And thus, we can infer the obligation of zakah on trade goods from the saying of the Prophet (peace and blessings be upon him): For the owner of trade goods does not want anything but the value in reality. Now, that is good.