Answered by Ustadh Tabraze Azam
Question: My dad has not been working for several years. He didn’t pay zakat for several years when he was working. Do I need to pay his unpaid zakat for all those years? If so, how can i calculate it?
Another question is, can I give my zakat to my dad since he fell into one of the categories that eligible to receive zakat?
If i can give my zakat to my dad, should the money receive by him be used to pay his debt or to pay his unpaid zakat?
I pray that you are in the best of health and faith, insha’Allah.
 If he cannot remember the number of years of missed zakat payments he has, he should make a reasonable, yet safe estimate, and pay it off. You can pay it off for him with his permission. [see: http://seekersguidance.org/ans-blog/2012/06/06/how-do-i-determine-how-much-zakat-i-owe/]
 No, you cannot pay your zakat to your father. [see: http://seekersguidance.org/ans-blog/2010/03/11/zakat-how-to-calculate-who-to-give/]
 He should pay his debts before paying off his unpaid zakat. The general principle is that the rights of others are given precedence over the rights of Allah.
And Allah alone gives success.
Checked & Approved by Faraz Rabbani