NISAAB AND RATE OF ZAKAAT
The amount of wealth which makes one liable for Zakaat is called Nisaab.
The payment of Zakaat is compulsory on the excess wealth or assets which is equal to/or exceeds the value of Nisaab, and which is possessed for a full Islamic year. If such wealth decreases during the course of the year, and it increases again to the value of Nisaab before the end of the year, the Zakaat then must be calculated on the full amount that is possessed at the end of the year.
The rate of Zakaat which was fixed by Rasulullah (Sallallaho Alaihe Wa Sallam) is 2.5% (1/40) i.e. 2.5 cents in a Dollar.