When is Zakat Due ?
1. Passage of One Lunar Year:
Zakat is obligatory after a time span of one lunar year passes with the money in the control of it’s owner. Then the owner needs to pay 2.5% (or 1/40) of the money as Zakat. (A lunar year is approximately 355 days).
2. Deduction of Debts:
The owner should deduct any amount of money he or she borrowed from others; then check if the rest reaches the necessary nisab, then pays Zakat for it.
If the owner had enough money to satisfy the nisab at the beginning of the year, then the money increased (in profits, salaries, inheritance, grants…etc.), the owner needs to add the increase to the nisab amount owned at the beginning of the year; then pay Zakat, 2.5%, of the total at the end of the lunar year. (there are small differences in the fiqh schools here)
Each Muslim calculates his or her own Zakat individually. For most purposes this involves the payment each year of two and a half percent of one’s capital. (Zakat Calculator)
A pious person may also give as much as he or she pleases as sadaqa, and does so preferably in secret. Although this word can be translated as ‘voluntary charity’ it has a wider meaning. The Prophet said ‘even meeting your brother with a cheerful face is charity.’
The Prophet said: ‘Charity is a necessity for every Muslim. ‘ He was asked: ‘What if a person has nothing?’ The Prophet replied: ‘He should work with his own hands for his benefit and then give something out of such earnings in charity.’ The Companions asked: ‘What if he is not able to work?’ The Prophet said: ‘He should help poor and needy persons.’ The Companions further asked ‘What if he cannot do even that?’ The Prophet said ‘He should urge others to do good.’ The Companions said ‘What if he lacks that also?’ The Prophet said ‘He should check himself from doing evil. That is also charity.’
Source : Islamicity