How Income Tax Is Calculated: AGI, Brackets, Credits

Federal income tax is calculated in a set sequence: start with everything you earned, subtract adjustments to get your Adjusted Gross Income, subtract either the standard deduction or itemized deductions to get taxable income, apply the graduated tax brackets to that figure, subtract any credits, and finally compare the result to what you already paid in through withholding or estimated payments. Whatever is left is either your balance due or your refund. Understanding how income tax is calculated is really about following that chain, because each step narrows the number the next step works with.

Step 1: Start With Gross Income

Gross income is the widest bucket. It includes wages reported on your W-2, freelance or gig earnings, interest, dividends, rental income, retirement distributions, unemployment benefits, and business profits.¹ The general rule is simple: if money came in and no specific law excludes it, it counts.

A handful of receipts are excluded by statute. Gifts and inheritances don’t count. Interest on most municipal bonds doesn’t. Life insurance proceeds paid to a beneficiary generally don’t, and neither do qualifying scholarships used for tuition and fees.¹ Everything else that qualifies gets totaled up. This total sets the ceiling for the rest of the calculation.

Step 2: Subtract Adjustments to Get Your AGI

Before you get to any deduction, certain items come off the top. These are called above-the-line adjustments because they appear above the AGI line on Form 1040, and you get them whether or not you itemize later.²

The most common ones:

Traditional IRA contributions, up to $7,500 for 2026 or $8,600 if you’re 50 or older, with a phaseout at higher incomes if you or your spouse are covered by a workplace retirement plan.³
Student loan interest, up to $2,500, subject to income phaseouts.⁴
Health Savings Account contributions, up to $4,400 for self-only coverage or $8,750 for family coverage in 2026, if you have a qualifying high-deductible health plan.⁵
Half of self-employment tax, if you work for yourself.

Subtract these from gross income and you have your Adjusted Gross Income. AGI matters beyond this one calculation because it drives eligibility for a lot of credits and deduction phaseouts that come later. A smaller AGI can unlock benefits that a higher one closes off.

Step 3: Choose a Deduction to Reach Taxable Income

AGI still isn’t the number that gets taxed. From it, you subtract either the standard deduction or your itemized deductions, whichever produces the bigger reduction.⁶ The result is taxable income.

The Standard Deduction

Most people take the standard deduction because it’s simple and often larger than what they could itemize. For 2026:⁷

Single or married filing separately: $16,100
Married filing jointly or qualifying surviving spouse: $32,200
Head of household: $24,150

Taxpayers 65 or older get an additional $2,050 (single) or $1,650 per qualifying spouse (joint). A separate $4,000 senior deduction created by the One Big Beautiful Bill Act is available regardless of whether you itemize, phasing out above $75,000 of income for single filers or $150,000 for joint filers.⁸

Itemized Deductions

Itemizing means listing specific expenses on Schedule A.⁹ It only pays off if your total exceeds your standard deduction. The main categories:

State and local taxes (SALT): state income or sales taxes plus property taxes, capped at $40,000 for most filers starting in 2025 ($20,000 if married filing separately). The cap phases down for modified AGI above roughly $500,000 but never drops below $10,000.¹⁰
Mortgage interest on up to $750,000 of acquisition debt for a primary or second home ($375,000 if married filing separately).¹⁰
Charitable contributions to qualified organizations, within AGI-based percentage limits.
Medical expenses, but only the portion above 7.5% of AGI.¹⁰

The medical floor trips people up. On an $80,000 AGI, the first $6,000 in medical bills yields no deduction; only what’s above that counts. Itemizing for medical expenses alone rarely pencils out unless something major happened during the year.

Step 4: Apply the Brackets

Once you have taxable income, the federal rates come in. The U.S. uses a progressive structure with seven brackets. Reaching a higher bracket does not mean every dollar you earned gets taxed at that rate. Only the dollars falling inside that bracket do.

For 2026, a single filer’s brackets are:¹¹

10% on taxable income up to $12,400
12% from $12,401 to $50,400
22% from $50,401 to $105,700
24% from $105,701 to $201,775
32% from $201,776 to $256,225
35% from $256,226 to $640,600
37% on income above $640,600

For married filing jointly, the lower brackets are roughly double:¹¹

10% up to $24,800
12% from $24,801 to $100,800
22% from $100,801 to $211,400
24% from $211,401 to $403,550
32% from $403,551 to $512,450
35% from $512,451 to $768,700
37% above $768,700

A Worked Example

Take a single filer with $100,000 in taxable income for 2026. The tax is not 22% of $100,000. Each slice gets its own rate:

10% on the first $12,400 = $1,240
12% on the next $38,000 (from $12,401 to $50,400) = $4,560
22% on the remaining $49,600 (from $50,401 to $100,000) = $10,912

Total: $16,712. The marginal rate is 22%, because that’s the rate on the last dollar earned. The effective rate is about 16.7%. If you’re weighing whether to take on extra work, the marginal rate is what tells you how much of each additional dollar goes to the government.

Where Different Rates Apply

Two boundaries are worth flagging. Profits from investments held longer than a year are taxed at preferential long-term capital gains rates of 0%, 15%, or 20%, based on separate thresholds, not the ordinary brackets above. High earners can owe an additional 3.8% net investment income tax on top.

A parallel system called the Alternative Minimum Tax also exists. It disallows certain deductions and applies its own rates; you owe whichever is higher, regular tax or AMT. For 2026, the AMT exemption is $90,100 (single) or $140,200 (joint), phasing out at $500,000 and $1,000,000.⁷ It catches far fewer people than it once did, but exercising incentive stock options or claiming large SALT deductions can trigger it.

Step 5: Subtract Credits

Credits come off the tax figure produced in Step 4, dollar for dollar. That makes them stronger than deductions of the same size. A $1,000 deduction saves you $220 at the 22% bracket. A $1,000 credit saves you $1,000.

Nonrefundable credits can zero out your tax but nothing more. Refundable credits can drive your tax below zero, meaning the IRS pays you the difference.¹² The three most commonly claimed:

Child Tax Credit. Up to $2,500 per qualifying child under 17 for 2026, following the One Big Beautiful Bill Act increase.⁸ The main credit is nonrefundable, but a portion known as the Additional Child Tax Credit is refundable for lower-income families. You need at least $2,500 in earned income to start qualifying for the refundable piece.¹³

Earned Income Tax Credit. Fully refundable and aimed at low-to-moderate-income workers. The amount depends on income and number of qualifying children, ranging from a few hundred dollars for workers with no children to over $8,000 for families with three or more.¹⁴ If your income is modest and you skip filing because you assume you owe nothing, you may be walking away from a real refund.

American Opportunity Tax Credit. Covers qualified higher education expenses for a student’s first four years of college. The maximum is $2,500 per eligible student: 100% of the first $2,000 in expenses plus 25% of the next $2,000. Up to $1,000 is refundable.¹⁵

Subtracting all applicable credits from the Step 4 figure gives you your final tax liability for the year.

Step 6: Compare What You Owe to What You Already Paid

Your final liability covers the whole year, but most people have been paying toward it as they go. Employees have federal income tax withheld from each paycheck based on Form W-4.¹⁶ Self-employed people and those with substantial non-wage income make quarterly estimated payments on Form 1040-ES.

At filing time, add withholding and estimated payments together and compare to the final liability. Paid more than you owe? The excess comes back as a refund. Paid less? You owe the difference by the filing deadline, which for 2025 returns is April 15, 2026.¹⁷ Payment can go through IRS Direct Pay from a bank account, EFTPS, or debit or credit card through approved processors.¹⁸

If You File Late or Underpay

Missing the deadline or coming up short brings penalties and interest.

The failure-to-file penalty is 5% of unpaid tax per month, capped at 25%.¹⁹ The failure-to-pay penalty is 0.5% per month, also capped at 25%, and runs separately. When both apply in the same month, the filing penalty is reduced by the payment penalty so you aren’t hit twice. So even if you can’t cover the full balance, filing on time avoids the steeper of the two penalties.

Interest accrues on top of penalties. For the first quarter of 2026, the individual underpayment rate is 7% per year, compounded daily.²⁰

If you pay quarterly estimates, you can avoid the underpayment penalty by hitting one of two safe harbors: pay at least 90% of the current year’s tax, or 100% of last year’s tax (110% if your prior-year AGI was above $150,000, or $75,000 if married filing separately). You also avoid the penalty if you owe under $1,000 at filing.²¹

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