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Your tax bracket – a.k.a. your marginal tax rate – is the highest percentage of your income that Uncle Sam will expect you to cough up in federal taxes.

The tax brackets range from 10% to 39.6%—and the more you earn the higher your bracket. You can find your 2015 bracket here. But be aware that the rate is not applied flat across all your income. Portions of your income fall into different brackets, which means your actual tax rate is likely to be much lower than your bracket.

Let’s say you're single and your household income is $89,000 in 2014. That puts you in the 25% tax bracket, but the first $9,075 of that is taxed at 10%, and everything between that and $36,900 is taxed at 15%. So only the last $43,025 of your income is taxed at 25%, which is why the tax bracket is sometimes defined as the rate you’ll pay on the last dollar you earn.

The actual percentage of your total income you’ll pay in taxes—in IRS-speak, your effective tax rate—will likely be much lower than your tax bracket, in part because the tax is calculated in the proportional way described above. Also, any deductions and credits that you are eligible to take will reduce your effective tax rate even more.

Read next: What is FICA?