Their grandfather died and did not pay Zakah for years

My grandfather passed away and left behind an amount of money on which he apparently did not give Zakaah. We do not know the time since when he did not pay Zakaah. How many years should we pay Zakaah for? Moreover, should we calculate Zakaah for the first year and take it out from the money and then calculate Zakaah for the following year on the remaining amount, and so on?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad , is His Slave and Messenger.

If your grandfather has died and he did not pay Zakah for some years, his heirs have to pay this Zakah out of his wealth. Indeed, Zakah is a debt owed to Allaah The Exalted, and it is more worthy of repayment.

The years for which Zakah should be paid are those in which he did not give Zakah, whether few or many. In case there is doubt as to the number of years, caution should be adopted. For example, if there is doubt over whether the years are two or three, let them pay Zakah for three.

If you give Zakah for a year, the paid amount should be excluded from the calculation of Zakah for the year that follows, and so forth.

Allaah Knows best.