Zakat is one of the few religious obligations anywhere that is calculated, and the calculation is the substance of the practice rather than an administrative detail attached to it.
The rate on savings, gold, silver and trade goods is one-fortieth, two and a half per cent, charged annually on wealth held for a full lunar year above a minimum threshold. The threshold is defined by a weight of gold or of silver, which means it has to be converted into current prices every year, and the two metals now give very different answers, a live disagreement, since the silver threshold is far lower and therefore brings many more people into liability. Different rates apply elsewhere: a tenth on crops watered by rain, a twentieth where irrigation required labour and expense, and separate schedules for livestock counted by head.
The critical feature is that it falls on accumulated wealth rather than on income. A person with a large salary and no savings owes nothing; a person with idle capital owes every year, so that over time the capital is either put to use or eroded. The traditional argument is that this is entirely deliberate and that hoarding is the target.
The recipients are fixed in the Quran as eight categories: the poor, the destitute, those employed to administer the fund, those whose hearts are to be reconciled, the freeing of captives, debtors, the path of God, and the stranded traveller. Most contemporary argument concerns how widely the seventh may be read, because a broad reading makes almost any communal project eligible and a narrow one restricts it to something close to armed defence.