(1) The zakah is calculated as per the silver nisab which is 52.5 tola i.e. the price of 52.5 tola silver is the nisab of Zakah. If you own this amount of money for one year, you will have to pay its fortieth part (2.5%) as Zakah.
(2) Now, you should remember the month in which you paid the Zakah. The next year in the same month, if you have saved amount equal to nisab or more, then separate 2.5% of the total as Zakah i.e. the 2.5% which you took out will be considered Zakah.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband