Should he pay zakaah on the expenses that his father gives to him?
Question

                I am still studying, and my father is spending on me. I do not spend a lot, so, Alhamdulillah, I do not think that my wallet was empty of money for some time. I think more than a year passed, but I do not remember exactly how long. This money I have increases and decreases during the year. My father provides me with money every so often. Sometimes I have about 1000 Saudi Riyal, other times I have 5000 or more. Do I have to pay Zakat on this money, knowing that my father does not give in specific times or specific amounts?  

My question is:  

In case I should pay Zakat, how shall I calculate it? Should I give it out to the Zakat treasury? 

Another question:  

What is the nisaab of money (the amount that determines whether Zakat is obligatory if a full year turns on it; anyone who has more than this is obliged to pay Zakat)?.
Praise be to Allah.
    Firstly: 

    Zakaah is due on cash if you have full possession of the 
    nisaab (minimum threshold) and one full hijri year has passed. 

    The expenses that a father gives to his son is something that 
    the son takes full possession of and disposes of as he wishes, so that is 
    subject to zakaah. 

    Secondly: 

    It is well known that cash in the form of bank notes was not 
    known at the time of the Prophet  (peace and blessings of Allaah be upon 
    him), but the scholars have ruled that it is subject to zakaah by analogy 
    with gold and silver. 

    The nisaab for gold is 85 grams, and the nisaab for silver is 
    595 grams. 

    If the cash reaches the value of the nisaab for gold or the 
    value of the nisaab for silver, then it has reached the minimum threshold. 

    As silver is of a lower price nowadays, the nisaab for cash 
    may be worked out on the basis of the nisaab for silver, because that is 
    more on the safe side and is more beneficial to the poor. 

    Based on the price of silver today –  12 Rabee’ al-Aakhir 
    1428 AH/29 April 2007 CE – the nisaab for cash is approximately 1093 Saudi 
    Riyals. 

    If you own this amount of money and one full hijri year has 
    passed since you acquired this amount and it has not dipped below the nisaab 
    during this year, then zakaah must be paid on it, at a rate of 2.5 %. 

    But if the cash that you possessed dipped below the nisaab 
    during the year, then zakaah does not have to be paid on it, until it 
    reaches the nisaab again and you start to reckon a new year from the time it 
    reached the nisaab. 

    If the amount by which it dipped below the nisaab was small, 
    then in order to be on the safe side you may pay zakaah, and carry on 
    counting the year. That is because the price of silver fluctuates and does 
    not remain constant throughout the year.  

    We should not forget to commend your keenness and concern 
    with regard to zakaah, even though you are talking about pocket money that 
    you take from your father, but you are paying attention to Allaah’s right to 
    it and have asked about the Islamic ruling on it at a time when many rich 
    people neglect this pillar of Islam and do not acknowledge Allaah’s rights 
    over their wealth or spend a lot or a little of it, and they spend their 
    days accumulating and hoarding and seeking to acquire more, but at the 
    reckoning on the Day of Resurrection their wealth will be a cause of loss 
    and regret. 

    Allaah says (interpretation of the meaning): 

    “And those who hoard up gold and silver (Al‑Kanz: the 
    money, the Zakaah of which has not been paid) and spend them not in the way 
    of Allaah, announce unto them a painful torment.

    35. On the Day when that (Al‑Kanz: money, gold and silver, 
    the Zakaah of which has not been paid) will be heated in the fire of Hell 
    and with it will be branded their foreheads, their flanks, and their backs, 
    (and it will be said unto them:) ‘This is the treasure which you hoarded for 
    yourselves. Now taste of what you used to hoard’”

    [al-Tawbah 9:34-35]

    We ask Allaah to bless your wealth for you and to grant you 
    abundant halaal provision. 

    And Allaah knows best.