How can a person liable for zakat (muzakki) make up for past zakat on wealth that they did not accurately calculate, given that they do not remember the amounts saved or how to calculate the zakat due on them?

One must repent to Allah for any negligence in calculating and paying zakat. Accuracy in calculation is essential to ensure the discharge of one's responsibility, by identifying the time when the wealth reached the nisab, then accumulating savings from salary and deducting expenses. Zakat (2.5%) is obligatory on the remaining amount if the wealth has not fallen below the nisab in previous years. Any amounts previously paid with the intention of zakat should be accounted for; if the total is sufficient, the obligation is discharged. If it is less, the difference must be paid. If a full year has passed over the wealth, zakat is due for each year.

Sheikh Al-Uthaymeen mentioned that repentance absolves one of Allah's right, but the right of the zakat beneficiaries remains, and thus the zakat must be handed over to them. In estimating zakat, the muzakki (one who pays zakat) should exert their best effort. For example: the zakat on ten thousand is two hundred and fifty, so this amount is paid for each year, while accounting for any increases or decreases.