Summary
If you make 7,800,000 円 a year living in Japan, you will be taxed 2,045,634 円. That means that your net pay will be 5,754,366 円 per year, or 479,530 円 per month. Your average tax rate is 26.2% and your marginal tax rate is 31.1%. This marginal tax rate means that your immediate additional income will be taxed at this rate. For instance, an increase of 100 円 in your salary will be taxed 31.14 円, hence, your net pay will only increase by 68.86 円.
Bonus Example
A 1,000 円 bonus will generate an extra 689 円 of net incomes. A 5,000 円 bonus will generate an extra 3,443 円 of net incomes.