Summary
If you make 3,900,000 บาท a year living in Thailand, you will be taxed 941,300 บาท. That means that your net pay will be 2,958,700 บาท per year, or 246,558 บาท per month. Your average tax rate is 24.1% and your marginal tax rate is 30.0%. This marginal tax rate means that your immediate additional income will be taxed at this rate. For instance, an increase of 100 บาท in your salary will be taxed 30 บาท, hence, your net pay will only increase by 70 บาท.
Bonus Example
A 1,000 บาท bonus will generate an extra 700 บาท of net incomes. A 5,000 บาท bonus will generate an extra 3,500 บาท of net incomes.