Taxable income appears on Line 15 of Form 1040. You get to that number by subtracting your total deductions on Line 14 from your adjusted gross income on Line 11b, and if the result is zero or less, you enter zero.1Internal Revenue Service. Form 1040, U.S. Individual Income Tax Return
Where Line 15 Sits on the Form
Line 15 is labeled “taxable income” on both Form 1040 and Form 1040-SR, the version used by taxpayers age 65 and older. It is the figure the IRS uses to determine how much federal income tax you owe. Once you fill it in, the return moves straight to Line 16, where you look up the actual tax.
Three earlier lines feed into Line 15: your adjusted gross income on Line 11, your standard or itemized deduction on Line 12e, and any additional deductions on Lines 13a and 13b. Those deduction lines are added together on Line 14, and Line 14 is what you subtract from AGI.
Adjusted Gross Income on Line 11
AGI is your total income from all sources — wages, self-employment earnings, rental income, interest, dividends, retirement distributions, and other reportable amounts — minus specific adjustments like student loan interest, educator expenses, and contributions to qualifying retirement accounts.2Internal Revenue Service. Adjusted Gross Income
On the form, Line 9 shows total income, Line 10 shows the adjustments pulled from Schedule 1, and Line 11a is where you subtract Line 10 from Line 9 to land on AGI. Line 11b carries that same AGI amount forward for use in the taxable income calculation.1Internal Revenue Service. Form 1040, U.S. Individual Income Tax Return
The Deductions That Reduce AGI
Standard or Itemized Deduction (Line 12e)
The single largest subtraction for most taxpayers goes on Line 12e. You choose either the standard deduction or itemized deductions on Schedule A, whichever gives you the larger benefit.3Office of the Law Revision Counsel. 26 USC 63 – Taxable Income Defined
For tax year 2026, the standard deduction amounts are:
- Single or married filing separately: $16,100
- Married filing jointly or qualifying surviving spouse: $32,200
- Head of household: $24,150
If you are 65 or older or legally blind, you qualify for an additional standard deduction on top of the base amount. For 2026 that additional amount is $1,650 per qualifying condition, or $2,050 if you are unmarried and not a surviving spouse.5Internal Revenue Service. Revenue Procedure 2025-32 You can claim the additional amount for both age and blindness if both apply, and married couples can each claim their own. Checkboxes on Lines 12a through 12d flag these situations.
Itemizing on Schedule A makes sense when your deductible expenses — state and local taxes (capped at $10,000), mortgage interest, charitable contributions, and medical expenses over 7.5% of AGI — add up to more than the standard deduction for your filing status.
Qualified Business Income Deduction (Line 13a)
If you earn income from a sole proprietorship, partnership, S corporation, or certain other pass-through business, you may qualify for a deduction of up to 20% of that qualified business income. It is calculated on Form 8995 or Form 8995-A and entered on Line 13a.6Office of the Law Revision Counsel. 26 USC 199A – Qualified Business Income Income thresholds and the type of business affect whether the full deduction is available.
Schedule 1-A Deductions (Line 13b)
Beginning with the 2025 tax year, the IRS introduced Schedule 1-A for new deductions created by recent legislation. The total from that schedule goes on Line 13b. It covers four categories: qualified tips, qualified overtime compensation, car loan interest, and an enhanced deduction for seniors. Each has its own eligibility rules and income phaseouts inside Schedule 1-A.7Internal Revenue Service. Schedule 1-A (Form 1040) Additional Deductions If none apply, leave Line 13b blank.
Doing the Math
Once each piece is filled in, the sequence is short:
- Find AGI on Line 11b.
- Add Line 12e, Line 13a, and Line 13b, and put the total on Line 14.
- Subtract Line 14 from Line 11b. That result is your taxable income on Line 15.
A quick example. Suppose your AGI is $75,000, you take the $16,100 standard deduction as a single filer, and you have nothing on Lines 13a or 13b. Line 14 is $16,100. Line 15 is $58,900.
When Deductions Exceed Your Income
If Line 14 is larger than Line 11b, the subtraction would produce a negative number. The form instructions say to enter zero on Line 15 instead.1Internal Revenue Service. Form 1040, U.S. Individual Income Tax Return Taxable income cannot go below zero. You may still qualify for refundable credits such as the Earned Income Tax Credit, which do not depend on a positive Line 15.
From Line 15 to the Tax You Owe
Line 15 feeds directly into Line 16. For taxable income under $100,000, you look up the tax in the IRS Tax Tables; for $100,000 and above, you use the Tax Computation Worksheet.8Internal Revenue Service. Instructions for Forms 1040 and 1040-SR – Line 16
Federal income tax is graduated. You pay a lower rate on the first portion of your taxable income and higher rates only on the income that falls within each higher bracket. Moving into a higher bracket does not push all of your income into that rate; only the amount above the bracket threshold is taxed at the higher rate.9Internal Revenue Service. Federal Income Tax Rates and Brackets
For the single filer with $58,900 in taxable income, the 2026 calculation runs in layers: 10% on the first $12,400, 12% on the next $38,000, and 22% on the remaining $8,500, for roughly $7,690 in tax before any credits.4Internal Revenue Service. IRS Releases Tax Inflation Adjustments for Tax Year 2026
Why the Number on Line 15 Has to Be Right
Errors on Line 15 that lead to underpaying your taxes can trigger IRS penalties, and the size depends on how the mistake happened. A substantial understatement — generally an understatement that exceeds the greater of 10% of the correct tax or $5,000 — carries an accuracy-related penalty of 20% of the underpaid amount, rising to 40% for gross valuation misstatements or undisclosed foreign financial asset understatements.10Office of the Law Revision Counsel. 26 USC 6662 – Imposition of Accuracy-Related Penalty on Underpayments
If the IRS determines the understatement was due to fraud, the penalty is 75% of the underpaid amount. Once the IRS establishes that any part of the underpayment involved fraud, the entire underpayment is presumed fraudulent unless you prove otherwise.11Office of the Law Revision Counsel. 26 USC 6663 – Imposition of Fraud Penalty
The best defense is to check that every number flowing into Line 15 — income, adjustments, and each deduction line — matches the supporting forms and schedules attached to your return.