Zakaah that has not been paid for several years
Question

                For several years I did not pay zakaah on my wealth. It is nearly five years. What should I do? How should I pay it this year? What is the nisaab (minimum amount of wealth) on which zakaah should be paid? May Allaah reward you with good and may He help you to continue serving this religion and its followers.
Praise be to Allah.To find out the nisaab on 
    which zakaah must be paid, please see question no. 
    2794. 
With regard to paying 
    zakaah for past years, whoever is obliged to pay zakaah and delays it with 
    no excuse has committed a sin, because the evidence of the Qur’aan and 
    Sunnah indicates that zakaah must be paid immediately at the time when it 
    becomes due. So he has to repent sincerely to Allaah. 
Secondly, whoever is 
    obliged to pay zakaah and does not do so at the specified time has to pay it 
    later on, even if he has delayed it for a number of years. 
“If he is certain about 
    the amount then he should pay it, and if he is not certain about the amount 
    then he should pay an amount with the intention of paying zakaah, until he 
    thinks that what he has given is most likely to cover the amount of zakaah 
    that he owes. Basing actions on what is most likely to be correct is one of 
    the basic principles of sharee’ah.” 
(The Standing Committee. 
    See Fataawa Islamiyyah, vol. 2, p. 95) 
He should make an 
    estimate if he does not know the exact amount, and pay that. He also has to 
    repent. And Allaah is the Source of strength.