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%.

For long-term capital gains, you fall into the 15% tax bracket, so you calculate your long-term capital gains tax separately as 15% of $10,000: $1,500. For 2024, your tax bill is roughly $12,053.

To calculate your effective tax rate, you need two numbers: the total amount you paid in taxes and your taxable income for that year. You can find both numbers on your tax return.