Using approximation in calculating Zakah
8-2-2011 | IslamWeb
Question:
Do we really need to calculate Zakat?What we do we give money almost every month for Zakat and then charity our Niat is that we calculate approximately for Gold and some money we have for whole year.Do you think approximation is acceptable or we need to calculate exactly.
Answer:
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.
A person who pays Zakah is obligated to calculate the Zakah due on his money in a way that he is sure that he pays the obligatory amount and not less than it. The Sharee’ah determined a fixed amount for Zakah and did not open the door to assumption and guessing regarding its estimation.
Shaah Waliyyullaah Ad-Dahlawi, in his book “Hujjatu Allaah Al-Baalighah” said: “There was a need for determining the amount of Zakah, because was the amount not determined, some people would be negligent regarding it, and others would exaggerate in paying it.”
Therefore, paying the Zakah by just assumption and guessing may lead to paying less than the obligatory amount, so this is not permissible unless one is sure that he pays the obligatory amount and not less than it.
Indeed, Shaykh Abdur-Razzaaq ‘Afeefi 
was asked: “Is it permissible to pay Zakah without calculating it?”, and he replied: “It is permissible to pay an amount by which one is sure that he fulfills the obligation or more than it.”
However, paying the Zakah by estimation is only permitted by the scholars in regard to fruits, and they call this method Khars (i.e. estimation), though they hold different opinions regarding the kinds of fruits on which one may pay the Zakah by Khars, but they did not differ regarding the two currencies: gold and silver and other items that serve instead of it, like paper currencies.
Ibn Qudaamah said: “Chapter: The Zakah on dates and grapes is calculated by Khars for the Hadeeth that we reported about these two items, and we have not heard of Khars in (calculating the Zakah of) anything other than them…” Therefore, there is no estimation in paying Zakah on money unless you are sure that you pay the amount that you are obligated to pay and not less than it.
As regards what you mentioned about paying (Zakah) each month, then if you mean that you do so for the Zakah that has become due on the money you have at hand, then this is wrong because you have to pay the entire Zakah immediately and not in installments, and if you mean that you pay Zakah which will be due in the future and has not yet become due, then this is paying Zakah in advance before its due time and this is permissible according to many scholars.
With regard to your statement “Zakah and then charity”, then if you mean that you intend the amount that exceeds the Zakah to be optional charity, then this is permissible and you will get a reward for it, Allaah willing, but as we mentioned, you must pay an amount which you think it is not less than the obligatory amount.
Finally, you should look to the Zakah that you previously paid, if it equals the obligatory Zakah or more, then you are acquitted from sin, but if you are not sure that it equals the obligatory amount or more than it, then it is an obligation on you to pay the remaining amount of Zakah. For more benefit, please refer to Fataawa 82016 and 81977.
Allaah Knows best.