How is the Zakat calculated for a sum of money deposited in a bank for ten years, on which Zakat has not been paid, and is Zakat obligatory on the variable annual interest generated from this sum?

Funds deposited in banks fall into two categories: 1. Usurious banks (conventional banks): Zakat is obligatory only on the principal amount, and not on the usurious interest, because it is illicit wealth that must be disposed of. The money must be withdrawn immediately, and one must repent from usurious dealings. 2. Islamic banks: Zakat is obligatory on both the principal and the profit.

Zakat must be paid for previous years: The account should be reviewed at the end of each past year. Usurious interest should be separated if the bank is conventional, and included if it is Islamic. If the money reaches the nisab, 2.5% of it must be paid for each year.