How is Zakat to be paid on wealth for which Zakat has not been paid for five years, and what is the Nisab of Zakat?

Whoever is obligated to pay Zakat (definition) and does not pay it at its appointed time has sinned and must repent. He must also pay it later, even if it is years overdue. If he does not know the exact amount, he should take an amount from his wealth with the intention of it being Zakat, until he is reasonably certain that what he has paid is sufficient to cover the Zakat obligation upon him.