Nisab is the legal term used to describe the minimum amount of constantly accessible surplus wealth that one must possess in order to be liable for Zakat.
In the Sunnah, it is designated as possession of either 20 gold Dinar or 200 gold dirhams. These amounts have been approximated in modern measurements as 84.9 grams of gold and 592.9 grams of silver. They are then multiplied by the current market value. This gives us the cash value of Nisab. You can easily do this by visiting this website.
Using this site, the nisab of gold for July 31st is equal to 4838.96 USD. For silver it is 539.34 USD. When calculating nisab for each metal separately, you use the respective nisab above. When calculating for cash and all marketable securities and surplus wealth, you use silver.
So if you own 539.34 or more in surplus wealth (including checking, savings, and investment accounts) then you will pay 2.5% of the sum total of that amount.