If you make S$18,120,000 a year living in Singapore, you will be taxed S$3,974,950. That means that your net pay will be S$14,145,050 per year, or S$1,178,754 per month. Your average tax rate is 21.9% and your marginal tax rate is 22.0%. This marginal tax rate means that your immediate additional income will be taxed at this rate. For instance, an increase of S$100 in your salary will be taxed S$22, hence, your net pay will only increase by S$78.
A S$1,000 bonus will generate an extra S$780 of net incomes. A S$5,000 bonus will generate an extra S$3,900 of net incomes.