Nisab is a term that denotes the minimum amount of wealth Muslims must have before Zakat becomes obligatory upon them. They are required to donate 2.5% of their total wealth to those in need.
Nisab was set by Prophet Muhammad (صلی اللہ علیہ وسلم) at 20 Mithqaal of gold or 200 dirhams of silver. This is the equivalent of 87.48 grams (7.5 tola) of gold and 612.36 grams (52.5 tola) of silver, respectively.
Important Note: If someone possesses only gold, then the Nisab level for gold (87.48 grams) must be used. However, if a mixture of assets are possessed then the (lower) silver Nisab level must be used.