Khums is a divinely-ordained financial obligation. It is the fifth of the yearly income that remains unspent. So, you should determine a specific day to calculate your profits in coordination with the Marja’ or his appointed agent. Every year on this specific day, you calculate the fifth of all the net income that you possess plus the fifth of the worth of items that were not used during the year.
The khums amount is divided into two equal amounts: one is to be given to needy people amongst the offspring of Hashim – the great-grandfather of the Prophet (peace be upon him) – and the second is to be spent with the coordination of the Marja’ by referring to him or his appointed agent to spend it in the ways of serving the religion and fulfilling the necessities of the believers.

The government collects tax for their own purposes and objectives, whereas the khums is a tax legislated in the Sharia taken on the yearly profits that remains after deducting the yearly expenses. The khums is legislated for specific goals which the Holy Legislator intended. Those goals are totally different from the uses of the government imposed taxes. It is natural then for the duty-bound individual to be obligated by the Sharia to pay khums regardless of whether he is paying other taxes or not.