The basic formula: gross income minus deductions and credits

Federal income tax is calculated in steps, starting with your total income and working down through deductions and credits until you reach the amount you owe. Your employer does most of this math automatically through payroll withholding — they take an estimated amount from each paycheck based on the W-4 form you filled out when you were hired.

The actual calculation happens in this order: your gross income (all wages, tips, interest, and other money you earned) gets reduced by above-the-line deductions like student loan interest or contributions to a traditional IRA. That gives you your adjusted gross income, or AGI. From there, you subtract either the standard deduction or your itemized deductions (whichever is larger). The number left is your taxable income, and that is what the tax brackets apply to.

After you calculate tax owed on that taxable income using the tax brackets for your filing status, you then subtract any tax credits you are may have access to to. Credits directly reduce the tax you owe, dollar for dollar. The result is your final tax liability — the amount you actually owe to the federal government.

Key Takeaways

  • Your gross income is reduced first by above-the-line deductions, then by either the standard deduction or itemized deductions, to arrive at taxable income.
  • Tax brackets are progressive, meaning different portions of your income are taxed at different rates, not your entire income at one rate.
  • Your employer withholds an estimated amount each paycheck based on your W-4 form, and you settle up when you file your return.
  • Tax credits like the Earned Income Tax Credit or Child Tax Credit reduce your tax dollar-for-dollar, while deductions only reduce the income that gets taxed.
  • If too much was withheld, you receive a refund; if too little was withheld, you owe the difference when you file.

How tax brackets work — your rate is not one flat percentage

Many people believe that if you are in the 22% tax bracket, you pay 22% on all your income. That is not how it works. The United States uses a progressive tax system, meaning different portions of your income are taxed at different rates.

For 2024, if you are single, the brackets are roughly: 10% on income up to $11,600, then 12% on income from $11,601 to $47,150, then 22% on income from $47,151 to $100,525, and so on up to 37% on income over $578,100. Your actual tax is calculated by applying each rate only to the income that falls within that bracket. If your taxable income is $60,000, you pay 10% on the first $11,600, 12% on the next $35,550, and 22% on the remaining $12,850 — not 22% on all $60,000.

The brackets change every year based on inflation. The IRS publishes new brackets in October or November for the following tax year. Your filing status (single, married filing jointly, head of household, etc.) determines which bracket table you use, and married couples filing jointly have wider brackets than single filers at the same income level.

Deductions: standard versus itemized

After you calculate your AGI, you subtract one more layer before reaching taxable income: your deduction. You have two choices, and you pick whichever gives you the larger deduction.

The standard deduction is a flat amount set by the IRS each year. For 2024, it is $14,600 for single filers, $29,200 for married couples filing jointly, and $21,900 for heads of household. You do not have to list anything or prove anything — you simply subtract this amount from your AGI. Most people use the standard deduction because it is simpler and because the amount is often larger than what they could itemize.

Itemized deductions are specific expenses you can deduct if you list them out: mortgage interest, state and local taxes (capped at $10,000), charitable donations, and medical expenses above a certain threshold. You add up all your itemized deductions and use that total instead of the standard deduction — but only if the total is larger. If your itemized deductions add up to $18,000 and the standard deduction is $14,600, you use $18,000. If they add up to $12,000, you use the standard deduction of $14,600 instead.

Tax credits versus deductions — why credits are worth more

A tax credit and a deduction sound similar but work very differently. A deduction reduces the amount of income that gets taxed. A credit reduces the tax you owe directly.

If you have a $1,000 deduction and you are in the 22% tax bracket, that deduction saves you $220 in tax (22% of $1,000). If you have a $1,000 credit, it saves you $1,000 in tax, period. That is why credits are more valuable than deductions of the same dollar amount.

Common credits include the Earned Income Tax Credit (EITC), which can be worth up to $3,995 for single filers and up to $3,733 for married couples filing jointly in 2024, depending on income and whether you have children; the Child Tax Credit, worth up to $2,000 per child under 17; and the American Opportunity Tax Credit, worth up to $2,500 per student for education expenses. Some credits are refundable, meaning if the credit is larger than the tax you owe, the government sends you the difference as a refund. Others are non-refundable, meaning they can reduce your tax to zero but not below.

How withholding works and why your refund or bill happens

Your employer does not wait until April to collect federal income tax. Instead, they withhold an estimated amount from each paycheck based on the W-4 form you completed when you started the job. The W-4 asks about your filing status, number of dependents, other income, and whether you expect to itemize deductions. Your employer uses that information to calculate how much to withhold from each paycheck.

The withholding is an estimate. It is based on the assumption that your income and circumstances will stay the same all year. If your life changes — you get married, have a child, take a second job, or lose a job — your withholding may no longer be accurate. You can update your W-4 at any time during the year to adjust the withholding.

When you file your tax return in the spring, you calculate your actual tax liability for the year. If you had too much withheld, you receive a refund. If you had too little withheld, you owe the difference. The average refund in recent years has been around $2,500 to $3,000, though this varies widely by income and circumstances.

Self-employment tax and additional taxes on investment income

If you are self-employed or have significant income from investments, your tax calculation includes additional pieces. Self-employment tax covers Social Security and Medicare taxes for people who do not have an employer withholding these amounts. It is calculated on your net self-employment income (business income minus business expenses) and is roughly 15.3% — 12.4% for Social Security and 2.9% for Medicare. You can deduct half of your self-employment tax as an above-the-line deduction.

Investment income — interest, dividends, capital gains — is also taxed, but often at different rates than wages. Long-term capital gains (assets held more than one year) are taxed at 0%, 15%, or 20% depending on your income, which is usually lower than ordinary income tax rates. Short-term capital gains (assets held one year or less) are taxed as ordinary income at your regular bracket rate.

If your investment income exceeds certain thresholds, you may also owe the Net Investment Income Tax, an additional 3.8% tax on investment income. This applies to single filers with modified AGI over $200,000 and married couples filing jointly over $250,000.

State and local income tax — calculated separately from federal

Federal income tax is separate from state and local income tax. Not all states have an income tax — nine states (Alaska, Florida, Nevada, South Dakota, Tennessee, Texas, Washington, Wyoming, and New Hampshire) do not tax wage income at all. States that do tax income use their own brackets, deductions, and credits, and you calculate state tax independently from federal tax.

Your federal return does not automatically file your state return. You must file both separately, usually with the same documents (W-2s, 1099s, receipts). Some states allow you to deduct state income tax paid on your federal return (up to the $10,000 cap on state and local taxes), which slightly reduces your federal tax, but the state tax itself is still owed.

Frequently Asked Questions

Why do I owe money when I file if my employer was withholding taxes all year?

Withholding is an estimate based on the information you provided on your W-4. If your circumstances changed during the year — you got married, had a child, took a second job, or had significant investment income — your withholding may have been too low. You can update your W-4 mid-year to adjust future paychecks, but you will still owe the shortfall when you file.

Does the standard deduction apply to everyone?

Most people use the standard deduction, but some cannot. If you are claimed as a dependent on someone else's return, your standard deduction is limited. If you are self-employed, you must also account for self-employment tax. If you are over 65 or blind, you get an additional standard deduction amount. Check the IRS website or a tax professional for your specific situation.

Can I claim both a deduction and a credit for the same expense?

Generally, no. For example, if you use the American Opportunity Tax Credit for education expenses, you cannot also deduct those same expenses as a deduction. However, you can claim different credits and deductions for different expenses in the same year — for instance, the Child Tax Credit and the Earned Income Tax Credit together.

What happens if I do not file a tax return?

If you owe tax and do not file, the IRS can assess penalties and interest on the unpaid amount. If you are owed a refund and do not file, you simply do not receive it — there is no penalty, but you lose the money. The IRS generally has three years to collect unpaid tax, though that period can be longer in some cases.

How do I know if my withholding is correct?

The IRS provides a withholding calculator on its website that compares your expected tax to what you have had withheld so far. If the calculator shows you will owe or get a large refund, you can adjust your W-4 to change your withholding. Aim for withholding close to zero — a small refund or small amount owed is normal and means your estimate was accurate.