Summary
If you make 20,618,000 원 a year living in South Korea, you will be taxed 2,629,254 원. That means that your net pay will be 17,988,746 원 per year, or 1,499,062 원 per month. Your average tax rate is 12.8% and your marginal tax rate is 15.8%. This marginal tax rate means that your immediate additional income will be taxed at this rate. For instance, an increase of 100 원 in your salary will be taxed 15.76 원, hence, your net pay will only increase by 84.24 원.
Bonus Example
A 1,000 원 bonus will generate an extra 842 원 of net incomes. A 5,000 원 bonus will generate an extra 4,212 원 of net incomes.