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How US Federal Income Tax Works

The federal income tax is the primary source of revenue for the United States government. It operates on a progressive system, meaning that higher levels of income are taxed at higher rates. Understanding how tax brackets, deductions, and credits interact is essential for accurate tax planning and for making informed financial decisions throughout the year.

Federal Income Tax Brackets and Marginal Rates

The US uses a marginal tax bracket system with seven rate tiers: 10%, 12%, 22%, 24%, 32%, 35%, and 37%. A common misconception is that moving into a higher bracket means all of your income is taxed at that higher rate. In reality, only the income that falls within each bracket is taxed at that bracket's rate. For example, if you are a single filer with $50,000 in taxable income, your first $11,600 is taxed at 10%, the income from $11,601 to $47,150 is taxed at 12%, and only the remaining amount above $47,150 is taxed at 22%. This layered approach ensures that everyone pays the same rate on the same portions of income, regardless of their total earnings.

Marginal vs. Effective Tax Rate

Your marginal tax rate is the rate applied to your last dollar of taxable income -- it corresponds to the highest bracket your income reaches. Your effective tax rate, on the other hand, is the average rate you actually pay across all of your income. It is calculated by dividing your total tax owed by your total gross income. The effective rate is always lower than the marginal rate because the first portions of income are taxed at lower brackets. For instance, a single filer earning $100,000 in gross income might have a marginal rate of 22% but an effective rate closer to 15%. Knowing both numbers is useful: the marginal rate helps you evaluate the tax impact of earning additional income, while the effective rate gives you a true picture of your overall tax burden.

The Standard Deduction and Itemizing

Before your income is subjected to the bracket rates, you are allowed to subtract a deduction. The standard deduction is a flat amount set by the IRS each year, and the majority of taxpayers -- roughly 90% -- choose it because it is simple and often provides more benefit than itemizing. Itemized deductions are worth pursuing when your qualifying expenses exceed the standard deduction amount. Common itemized deductions include state and local taxes (SALT, capped at $10,000), mortgage interest on the first $750,000 of loan principal, charitable contributions, and certain medical expenses that exceed 7.5% of your adjusted gross income. Choosing correctly between the standard deduction and itemizing can save you hundreds or even thousands of dollars.

Common Tax Credits and Deductions

Tax credits are especially valuable because they reduce your tax bill dollar-for-dollar, unlike deductions which only reduce your taxable income. The Earned Income Tax Credit (EITC) provides substantial refunds for lower- and moderate-income working individuals and families. The Child Tax Credit offers up to $2,000 per qualifying child under age 17. Education-related credits include the American Opportunity Tax Credit (up to $2,500 per student for the first four years of college) and the Lifetime Learning Credit. Additionally, contributions to tax-advantaged retirement accounts like a traditional 401(k) or IRA reduce your taxable income in the year of contribution, effectively deferring taxes until retirement when you may be in a lower bracket. Strategic use of these credits and deductions can significantly reduce your annual tax obligation.