The IRS starts with your total income, subtracts deductions, and applies tax rates to what remains

Federal income tax is not calculated all at once. The IRS works through your income in layers: first, it counts all the money you earned. Then it lets you subtract certain expenses and personal deductions. What is left is called taxable income. The IRS then applies tax rates to that taxable income — and the rates increase as your income goes up. Finally, it subtracts any tax credits you are may have access to to. The result is the tax you owe.

The process follows the same path whether you file on paper or use tax software. Understanding each layer helps you see where your money goes and why two people earning the same salary might owe different amounts.

Key Takeaways

  • The IRS calculates tax by starting with all income you received, then subtracting deductions to find your taxable income.
  • Tax rates are progressive, meaning higher portions of your income are taxed at higher percentages — you do not pay one flat rate on everything.
  • Tax credits reduce your tax bill dollar-for-dollar, while deductions only reduce the income that gets taxed.
  • Your filing status (single, married, head of household) determines your tax brackets and the standard deduction amount you can claim.
  • The IRS compares the tax you owe to the tax already withheld from your paychecks or paid through estimated payments to determine your refund or balance due.

Step 1: Add up all your income sources

The IRS defines income broadly. It includes wages from your job, interest from a savings account, dividends from investments, rental income, self-employment income, and many other sources. If you received a W-2 from an employer, that is income. If you received a 1099-INT from a bank, that is income. If you sold something for a profit, that is income.

You report this income on your tax return using forms that match the source. W-2 income goes on the main return. Interest and dividends go on Schedule B. Self-employment income goes on Schedule C. The total of all these sources is your gross income.

Some income is excluded entirely — for example, gifts and inheritances do not count as income for federal tax purposes, and neither do most state and local tax refunds. But the default rule is that money you received counts unless a specific law says it does not.

Step 2: Subtract adjustments to income

Before the IRS applies deductions, it lets you subtract certain expenses directly from your gross income. These are called adjustments to income or "above-the-line" deductions. They reduce your income before anything else happens.

Common adjustments include contributions to a traditional IRA (up to the annual limit), student loan interest (up to $2,500 per year), and self-employment tax (half of what you owe as a self-employed person). If you are self-employed, you also subtract the business expenses you incurred to earn that income. A freelancer subtracts office supplies, software subscriptions, and mileage. A rental property owner subtracts mortgage interest, repairs, and property taxes.

After you subtract all adjustments, you arrive at your adjusted gross income, or AGI. This number matters because many other tax rules depend on it — for example, whether you can claim certain credits or deductions.

Step 3: Choose between the standard deduction and itemized deductions

Next, the IRS lets you subtract a large deduction from your AGI. You have two choices: take the standard deduction or itemize deductions. You pick whichever is larger.

The standard deduction is a flat amount that depends on your filing status and age. For 2024, a single person under 65 can deduct $14,600. A married couple filing jointly can deduct $29,200. These amounts change each year. The standard deduction is straightforward — you just claim it on your return with no paperwork.

Itemized deductions are specific expenses you paid during the year: mortgage interest, property taxes, charitable donations, and medical expenses above a certain threshold. You add them up and deduct the total instead of the standard deduction. Itemizing requires you to keep receipts and file Schedule A with your return. Most people take the standard deduction because it is larger than their itemized total, but homeowners with high mortgage interest or people who made large charitable gifts often itemize.

After you subtract either the standard or itemized deduction from your AGI, you have your taxable income. This is the number the IRS uses to calculate how much tax you owe.

Step 4: explore tax rates to your taxable income

The federal tax system uses progressive tax brackets. This means different portions of your income are taxed at different rates. You do not pay one flat percentage on all your income.

For 2024, the brackets for a single filer are roughly: 10% on the first $11,600, 12% on income from $11,601 to $47,150, 22% on income from $47,151 to $100,525, and so on, up to 37% on income over $578,100. Married couples filing jointly have higher bracket thresholds. Head of household filers have different thresholds again.

Here is how it works in practice: if you are single with $50,000 in taxable income, you pay 10% on the first $11,600 ($1,160), then 12% on the next $35,550 ($4,266), then 22% on the remaining $2,850 ($627). Your total tax is $6,053. You do not pay 22% on all $50,000.

The IRS publishes tax tables and worksheets that do this calculation for you. Tax software does it automatically. The result is your tax before credits.

Step 5: Subtract tax credits

Tax credits are different from deductions. A deduction reduces the income that gets taxed. A credit reduces the tax itself, dollar-for-dollar. A $1,000 credit saves you $1,000 in tax. A $1,000 deduction saves you tax only at your bracket rate — if you are in the 22% bracket, it saves you $220.

Common credits include the Earned Income Tax Credit (EITC) for lower-income workers, the Child Tax Credit for parents, the American Opportunity Credit for students, and the Saver's Credit for retirement contributions. Some credits are refundable, meaning if the credit is larger than your tax, the IRS sends you the difference. Others are non-refundable, meaning they can only reduce your tax to zero.

You subtract all credits you are may have access to to from your tax before credits. The result is your total tax liability — the amount you owe.

Step 6: Compare to tax already paid

Throughout the year, your employer withheld federal income tax from your paychecks. If you are self-employed, you made quarterly estimated tax payments. The IRS compares the total tax you owe to the total tax already paid.

If you paid more than you owe, you get a refund. If you paid less, you owe the difference. If you paid exactly the right amount, you break even. This is why people with the same income can owe different amounts — it depends on how much was withheld or paid during the year.

Why your filing status and personal situation matter

Two people earning $60,000 can owe different amounts of tax because the tax brackets, standard deduction, and available credits all depend on filing status and circumstances. A single person and a married person filing jointly have different bracket thresholds. A parent with two children can claim the Child Tax Credit; a single person with no dependents cannot. Someone over 65 gets a higher standard deduction than someone under 65.

This is why the IRS asks for your filing status, age, number of dependents, and other details on your return. Each piece of information changes the calculation. The tax system is designed to account for these differences, but it means you have to provide accurate information for the calculation to be correct.

Frequently Asked Questions

Why do I pay more in taxes if I earn more money?

You pay more in total tax, but not at a higher rate on all your income. The progressive bracket system means each additional dollar you earn is taxed at a higher rate than the previous dollar. If you earn $10,000 more, you do not pay 22% on all your income — you pay 22% only on that extra $10,000. Your effective tax rate (total tax divided by total income) goes up, but it is always lower than your marginal rate (the rate on your last dollar).

Does the standard deduction reduce my tax or just my taxable income?

The standard deduction reduces your taxable income, which then reduces your tax. If you are in the 22% bracket and claim a $14,600 standard deduction, your tax goes down by $3,212 (22% of $14,600). The deduction itself is not a tax reduction — it is a reduction in the income that gets taxed.

What is the difference between a tax credit and a tax deduction?

A deduction reduces the income that gets taxed. A credit reduces the tax itself. A $1,000 deduction in the 22% bracket saves you $220 in tax. A $1,000 credit saves you $1,000 in tax. Credits are more valuable, which is why they are usually harder to claim.

Can I owe federal income tax if my employer withheld money from my paycheck?

Yes. Withholding is an estimate based on the W-4 form you filled out. If you did not withhold enough — because you have a second job, a spouse who also works, or investment income — you can still owe tax even though money came out of your paycheck. The IRS compares what you actually owe to what was actually withheld, not to what should have been withheld.

Do I have to file a return if I did not earn much income?

It depends on your income and filing status. For 2024, a single person under 65 generally does not have to file unless their income exceeded $14,600 (the standard deduction). But you may want to file anyway if you had tax withheld, because you could get a refund. The IRS website has a filing requirement tool that tells you whether you must file based on your situation.