Three ideas decide whether zakat is due, and how much: the threshold, called the nisab; the year, called the hawl; and the rate.

First, the threshold. Abu Sa'id al-Khudri reported that the Prophet, peace be upon him, said there is no zakat on less than five awaq of silver, in Sahih al-Bukhari, number fourteen hundred and forty-seven. Five awaq are two hundred dirhams. The letter of Abu Bakr about zakat, reported by Anas, says the same: if silver is less than two hundred dirhams, zakat is not required. Scholars converted the two hundred dirhams of silver, and the twenty dinars of gold, into modern weights: commonly about five hundred and ninety-five grams of silver, and about eighty-five grams of gold. Some classical sources give slightly different figures, such as those used in the Hanafi school.

Second, the rate. The letter of Abu Bakr states that for silver, the zakat is one-fortieth, which is two and a half percent. That is the rate on cash, gold, silver and trade goods.

Third, the year. Zakat is due once your wealth has been held above the threshold for one lunar year. A practical approach is to choose a fixed date each year, for example a date in Ramadan, and to review and calculate on that day.

Here is a simple example. If, on your zakat date, your qualifying wealth is ten thousand, in your own currency, and that is above the threshold, your zakat is two and a half percent of it: two hundred and fifty. To find the threshold in your currency, multiply the grams, five hundred and ninety-five for silver, or eighty-five for gold, by the current price per gram.

Scholars differ on some details. Because silver is much cheaper, the silver threshold is lower. Many contemporary scholars and zakat bodies advise using the silver measure for cash, since it benefits more of the poor and is the safer course, while others use the gold measure. On the year, the Hanafi school looks at whether the threshold is met at the start and the end of the year, while other schools require wealth to stay above it throughout. And the deduction of debts differs by school and by type of wealth.

Follow the guidance of your local scholars. The Worship page also has a simple zakat estimate, and if your situation is complex, confirm the result with a scholar.