How is Zakat on money paid, what is the amount of money that necessitates its payment, and how is the Hawl (one-year period) calculated?

If the wealth (money and trade goods) reaches the value of 85 grams of gold or 595 grams of silver, then the Muslim has owned the Nisab of Zakat. Zakat becomes obligatory upon him if a lunar year ('hawl') passes over it, and his wealth has not decreased below the Nisab. The amount of Zakat that must be given out is a quarter of a tenth (2.5%) to the poor and the needy.