Zakat is a mandatory duty on all able Muslims who posses wealth at or above a certain threshold. This threshold is called the Nisab. You only pay Zakat if your net assets equal or exceed the Nisab threshold. Zakat should be calculated on 2.5% of all net savings one possesses that is above the nisab value.
Net savings is the total maintained wealth for one lunar year before Zakat is due. If net yearly savings falls below this amount, he or she is exempt from paying Zakat. Only those people whose net capital exceeds the nisab values are obliged to give zakat.