If you make ₦18,420,000 a year living in Nigeria, you will be taxed ₦6,146,900. That means that your net pay will be ₦12,273,100 per year, or ₦1,022,758 per month. Your average tax rate is 33.4% and your marginal tax rate is 34.5%. This marginal tax rate means that your immediate additional income will be taxed at this rate. For instance, an increase of ₦100 in your salary will be taxed ₦34.50, hence, your net pay will only increase by ₦65.50.
A ₦1,000 bonus will generate an extra ₦655 of net incomes. A ₦5,000 bonus will generate an extra ₦3,275 of net incomes.