If you make ₦18,000,000 a year living in Nigeria, you will be taxed ₦5,548,400. That means that your net pay will be ₦12,451,600 per year, or ₦1,037,633 per month. Your average tax rate is 30.8% and your marginal tax rate is 32.0%. 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 ₦31.98, hence, your net pay will only increase by ₦68.02.
A ₦1,000 bonus will generate an extra ₦680 of net incomes. A ₦5,000 bonus will generate an extra ₦3,401 of net incomes.