To calculate the marginal tax rate on the investment, you'll need to figure out the additional tax on the new income. In this example, $500 will be taxed at 15% and $500 at 25%.

How to Calculate the Corporate Marginal Tax Rate. The marginal income tax rate for a corporation refers to the rate at which the company's last dollar of taxable income is taxed.

How to Calculate Your Effective Tax Rate To calculate your effective tax rate, you need two numbers: the total amount you paid in taxes and your taxable income for that year.