Who owes zakat, and how much?

Islam is built on five... and paying zakat.

Sahih al-Bukhari 8; Sahih Muslim 16.

Who: a Muslim who has held wealth equal to the nisab for a full year, surplus to their needs.

The nisab is stated by the weight of gold and silver — roughly 87.48 g of gold, or 612.36 g of silver (20 mithqal / 200 dirhams). Of this meaning in Sahih al-Bukhari 1447 and Sahih Muslim 979; the detailed weights were worked out by the jurists.

How much: a fortieth, 2.5% — on cash, gold, silver and trade goods. Crops, livestock and minerals have their own rates.

To whom: the Quran names eight categories itself — the poor, the needy, those employed to collect it, those whose hearts are to be reconciled, freeing captives, the indebted, the way of Allah, and the traveller (Quran 9:60). It is not given to one's own parents, grandparents or children, because their maintenance is already your duty.

And here is the part that gets left out

Today, is the nisab reckoned from gold or from silver? The two values differ greatly, and the effect is whether a person is a recipient or a payer. There is detail and disagreement here. The silver nisab is commonly advised as being to the poor's advantage, but that is not the answer in every case.

Also ask a mufti about: deducting debts, provident funds, shares, a rented house, and wealth that rose and fell during the year.

Zakat should not be worked out by guesswork. And on one who withholds it the Quran's words are severe — Quran 9:34-35.

This is educational, not a fatwa.