How to Calculate Zakah on Salary
Question

I am an employee with a monthly salary of 2000 Saudi Riyals. My family is all dependent on me and I pay all their expenses from my salary. I have a wife, daughter, father and brothers and sisters on whom I spend.
But my question is, how can I pay the zakah on my wealth when my only source of income is this salary, and all of my salary is spent on my family? 
So when should I pay my zakah? Some people say that a salary is like agricultural crops, and one should not wait until one full year has passed before paying it. So when does zakah become obligatory upon my salary?
Praise be to Allah.Is there zakah on salary?
If a person has a monthly salary and he spends all of it and has nothing left, so that at the end of the month he has spent all his money, then he does not have to pay zakah . For zakah to be obligatory, one full year must have passed (i.e., one full year from the date of his taking possession of the nisab , the minimum amount of wealth on which zakah is due). 
On this basis, you do not have to pay zakah, unless you save some of your wealth and that amount reaches the nisab (threshold or minimum amount), and one year has passed. 
Is zakah on a salary like zakah on agricultural crops
With regard to the person who told you that the zakah on a salary is like the zakah on agricultural crops and you do not have to wait until one full year has passed before paying it, what he said is not correct. 
As most people work for a salary, we think that it is a good idea to describe how zakah is to be paid on salaries.
How to calculate Zakah on salary 
Zakah on the salary of an employee:
The employee with a salary will find himself in one of the following two situations:

Either he spends all of it and does not save anything, in which case he is not obliged to pay zakah, as in the case of the person who asked this question;
Or he will save some of it, sometimes more and sometimes less. So how is zakah to be reckoned in this case?

 The answer is: if he insists on having all his rights and on not giving any charity to those who deserve it apart from what he is obliged to give, then he should make a schedule of his earnings and write down every amount and the date on which he took possession of it. Then he should pay zakah for each amount separately when one year has passed from the date on which he took possession of it.
But if he wants an easier method, and wants to be more generous and give precedence to the poor and others who are entitled to zakah over himself, then he can pay zakah on all the money he possesses when one year has passed from the date when his wealth first reached the nisab. This will bring a greater reward and raise him higher in status; it is easier for him and is more generous towards the poor and needy and others who are entitled to zakah. Whatever extra amount he may pay will be regarded as a “down payment” on the zakah for any wealth for which one year has not yet passed.  (Fatawa al-Lajnah al-Daimah, 9/280)
For example: a man takes his salary for Muharram, and saves one thousand riyals of it. Then he does likewise in Safar and the remaining months. When Muharram comes in the following year, he looks at all that he has and pays zakah based on that.
For more, please see these answers: 1119 , 231858 , 128167 and 296865 . 
And Allah knows best.