The basics
Zakat is due at 2.5% on qualifying wealth held for a full lunar year (hawl), once your total zakatable wealth exceeds the nisab (a minimum threshold, commonly pegged to the value of silver — the lower, more inclusive basis).
Active trader vs long-term holder
If you buy and sell shares to trade, they’re treated like inventory: zakat is due on the full market value of your holdings. If you hold shares long-term for their dividends and growth, many scholars assess zakat only on the zakatable assets within the company (cash, receivables, inventory) attributable to your shares — often approximated as a percentage of market value.
Keep it simple, then refine
A reasonable starting point for long-term holders is to apply zakat to roughly a quarter to a third of the market value (a common approximation of a company’s zakatable assets), or use a scholar-provided per-company figure where available. For traders, use the full value.
When in doubt, ask
Zakat calculations get nuanced fast. Treat any calculator — including ours — as an estimate, and confirm your situation with a knowledgeable scholar, especially for larger portfolios.