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In Sharia (Islamic Law) niṣāb (نِصاب) is the amount one's net worth must exceed for the Muslim owner to be obligated to give zakat. Several hadith have formulas for calculating niṣāb, the most prominent of which declaring that No Zakāt is due on wealth until one year passes. Zakat is then determined based on the amount of wealth acquired; the greater the assets, the greater the tax. It is not subject to special exemptions. (Unlike Income Tax in secular states.)
In Islam (not in Quran) Nisab is 20 for gold (dinārs) and 200 for silver (dirḥams)[dubious ]. Dinar is a gold coin with the weight of the (mithqal) which is 4.25 gr. and the dirham is a silver coin with the weight of 7/10 of the mithqal which is 2.975 gr. The nisab is applicable to the cumulative stock of dinars, dirhams and any other zakatable valuables, such as merchandise in store over a year. As long as the total value of the zakatable valuables exceeds the value of the nisab, zakat must be paid.
The Value of the Nisab 
The value of the nisab is calculated in dinar coins and dirham coins. The relation between 20 dinars and 200 dirhams which is part of the definition of nisab reflects that the exchange value between the dinar and the dirham in the early days of Islam was 1/10. In later times the exchange between the two Sharia coins changed according to market values. Exchange values of 1/11 and 1/12 are seen in early fiqh literature. This exchange rate has changed even more in the last few centuries. The change in exchange rate meant that in later times 20 dinars became more expensive in market value than 200 dirhams. Yet the definition of nisab has not been ever altered throughout Islamic history and therefore a choice is created between the value of 20 dinar or the value of 200 dirhams. Both values are accepted in Shariah, therefore everyone could accept anyone of them.
The Value of the Coins and the Value of the Metal 
The value of one dinar is not identical to the value of the gold content of one dinar (the same with silver dirham). A gold dinar is more valuable that its gold content. Naturally a coin is a manufactured product therefore it has an added value in the market than a similar amount of gold material. There is also difference in value between dinars depending on their quality and their acceptability. Although all gold coins and gold products must be exchanged by weight and not by value (also with silver products)in order to prevent riba al-fadl, different coins may have different market value as it is reflected in the fiqh literature when it is said that some coins are not popular (makruhi)[See Imam Malik's al-Muwatta]. Therefore the value of the dinar and the value of the dirham are entirely different than their gold and silver content. This represents a problem to modern Muslims because dinar and dirham coins are difficult to obtain.
A Practical Approach to the Lack of Dinars and Dirhams 
Some modern scholars have accepted that since we do not have dinars and dirhams anymore the best solution is to calculate the nisab using the mithqal (4.25 gr) in pure gold as reference (instead of the dinar. And the same with the dirhams. Although this is not an exact calculation, they considered it a valid approximation given our circumstances. In some countries Muslim communities have minted their own dinars and dirhams following the original standards and they are being used as means of payment and means to pay zakat. World Islamic Mint is an world organisation of Muslim communities which has been minting dinars and dirhams since 1992.
One Year Rule 
In order to be liable for zakat, the Muslim must possess wealth in excess of the nisab level for one lunar year (354 days).]</ref> This year begins on the date the wealth is obtained; so long as the assets are in its owner's possession at the beginning and end of the lunar year, the zakat tax is applicable. In many [modern] societies, niṣāb is considered equivalent to a governmentally determined poverty threshold.