Summary
If you make 18,780,320 원 a year living in South Korea, you will be taxed 2,342,582 원. That means that your net pay will be 16,437,738 원 per year, or 1,369,812 원 per month. Your average tax rate is 12.5% and your marginal tax rate is 14.3%. 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 14.32 원, hence, your net pay will only increase by 85.68 원.
Bonus Example
A 1,000 원 bonus will generate an extra 857 원 of net incomes. A 5,000 원 bonus will generate an extra 4,284 원 of net incomes.