Wikipedia
In Sharia (Islamic Law) niṣāb (نِصاب) is the minimum amount for a Muslim net worth to be obligated to give zakat. Several hadith have formulas for calculating niṣāb, the most prominent of which declares that No Zakāt is due on wealth until one year passes. Zakat is determined based on the amount of wealth acquired; the greater one's assets, the greater the tax. Unlike income tax in secular states niṣāb is not subject to special exemptions.
In Islam niṣāb is 20 dinārs for gold and 200 dirḥams for silver . The dinār is a gold coin weighing one mithqal (4.25 grams) and the dirḥam is a silver coin weighing 0.7 mithqal (2.975 grams). The niṣāb is applicable to the cumulative stock of dinārs, dirḥam and any other zakatable valuables, such as merchandise that has been in store for at least one year. As long as the total value of the zakatable valuables exceeds the value of the niṣāb zakat must be paid.