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.