You should realize that zakat and tax are completely different things. Zakat is related to religion, while tax is related to finances. Zakat is something that is required to be paid for by Muslims. They need to pay this annually, but they will have different amounts to pay for depending on their income. Tax is something that citizens would need to pay for.
Some people pay for their taxes by getting the money from the salary that they get from work. There are also some people who pay for their taxes voluntarily, especially if they work from home. Zakat is not always considered to be required, but everyone who works is required to pay taxes.
Zakat and Tax are two different things entirely because they are not related in terms of what they stand for actually. Zakat is one of the five pillars of Islam, which mandates all Muslims to give a certain percentage of their income that will be used as a donation to the poor or the needy.
Tax, on the other hand, is the money imposed by a government on all its citizens, which will be used by the government for the development of its country. There is no law enforcing how and when zakat should be paid, which means it is not that compulsory, but Tax is something every citizen should be ready to pay because tax defaulters can be prosecuted, sanctioned by the government.
There is no general fixed rate as to how much people are expected to pay as Tax because there are regulations for this. On the other hand, 2.5 percent of your total income is expected to be paid as Zakat.