How much zakat do I owe?

What zakat comes to on what you hold after debts, and whether what is left reaches the threshold the rule sets at all. It covers money, gold and what is owed to you, not business stock, livestock or crops, and it uses the gold nisab, and an authority that sets the threshold in silver puts it far lower.

Zakat is 2,500 on 100,000, one fortieth of it, against a nisab here of 25,500, on wealth held a full lunar year.

Two rules, and nothing inferred past them. Egypt's Dar al-Ifta states both in one line: "The legal minimum amount is the equivalent in value to 85 grams of 21 carat gold", and "The value of zakat equals one quarter of a tenth, which is 2.5%."

So the calculation is a threshold and then a multiplication. What is due now comes off what you hold; if what remains reaches the value of 85 grams of gold, zakat is one fortieth of it. If it does not, nothing is due — and that is the part worth dragging the slider for. There is no partial band. The rate is not levied on the amount above the line; it is levied on the whole, or on nothing.

The nisab is a quantity of gold rather than a quantity of money, which is why this tool asks for a gold price instead of holding a number. A threshold pinned to a currency would be wrong in that currency within a year and wrong everywhere else immediately.

2.5% of (what you hold − what is due now), where that reaches the nisab

The nisab is the value of 85 grams of gold at the price you enter. Below it nothing is due, and above it there is no partial band — the rate applies to the whole.

What it assumes

• What you enter has been held for a full lunar year. Wealth that arrived last month is measured on its own anniversary rather than on this one, and the year counted is the lunar one, which is eleven days shorter than the calendar year.
• Debts are the ones due now. A long instalment plan is not deducted in full — what comes off is what has fallen due, and a figure entered as the whole outstanding balance will take you below the threshold when nothing has.
• The carat the nisab is priced at is not settled the same way everywhere. Dar al-Ifta sets it at 85 grams of 21 carat gold, on what people commonly trade; IslamWeb, publishing for the Gulf, sets it at 85 grams of pure gold. Enter the price at the carat your own authority names, and the threshold moves with it.
• Recognised authorities differ on what counts as zakatable and on whether the threshold is set by gold or by silver. The silver nisab is materially lower and makes more wealth liable. This uses the gold nisab throughout. A specific case — a business, a pension, property held to let — belongs with a specialist rather than with a calculator.