How is zakat calculated on a sum of money, what is the zakat threshold in Saudi Arabia, and should one wait until the threshold decreases if the day is fixed and tomorrow it changes and becomes higher than the initial amount?

The method of calculating zakat on money is determined by knowing when it reaches the nisab (minimum threshold) according to the Hijri calendar. If a full lunar year passes without the amount falling below the nisab, then zakat becomes obligatory. The nisab for money is equivalent to 85 grams of pure gold or 595 grams of silver. The value of zakat is calculated based on the value of gold or silver at the time it becomes due, even if the the value changes afterwards. The amount that must be given is a quarter of a tenth (2.5%).