No, you do not have to fill out Step 4(b) on the W-4. It is optional, and most employees leave it blank. You only need to complete it if your expected itemized deductions for the year will exceed the standard deduction for your filing status, or if you have above-the-line income adjustments like student loan interest or deductible IRA contributions. Leaving it blank tells your employer to withhold based on your filing status and the standard deduction, which is the right choice for roughly nine out of ten filers who do not itemize.{1Internal Revenue Service. Form W-4 2026 Employee’s Withholding Certificate}
What Step 4(b) Actually Does
The dollar amount you enter on line 4(b) reduces the annual taxable wages your employer uses to calculate withholding. Payroll spreads that reduction across every pay period, so each paycheck has slightly less federal income tax withheld and slightly more take-home pay.{1Internal Revenue Service. Form W-4 2026 Employee’s Withholding Certificate}
The default withholding calculation already assumes you will claim the standard deduction. Step 4(b) exists so people whose real deductions are larger can avoid overwithholding. Without it, they would give the government an interest-free loan all year and get it back as a refund.
When You Should Fill It Out
Two groups benefit from completing Step 4(b): people who itemize on Schedule A, and people who have above-the-line adjustments on Schedule 1 even if they take the standard deduction.
If You Itemize
If you plan to itemize and your total itemized deductions will exceed the standard deduction for your filing status, the difference goes on line 4(b).{2Internal Revenue Service. Deductions for Individuals: The Difference Between Standard and Itemized Deductions, and What They Mean} For 2026, the standard deduction amounts are:
- Single or married filing separately: $16,100
- Married filing jointly: $32,200
- Head of household: $24,150
These figures incorporate the 2026 inflation adjustments and amendments from the One, Big, Beautiful Bill.{3Internal Revenue Service. IRS Releases Tax Inflation Adjustments for Tax Year 2026, Including Amendments From the One, Big, Beautiful Bill} The taxpayers most likely to clear these thresholds are homeowners with sizable mortgage interest, people with significant charitable contributions, and residents of high-tax states with large state and local tax bills.
If You Have Above-the-Line Adjustments
The Deductions Worksheet also accounts for adjustments that reduce your taxable income whether or not you itemize. These include student loan interest, deductible traditional IRA contributions, educator expenses, and alimony under pre-2019 agreements.{4Internal Revenue Service. Instructions for Forms 1040 and 1040-SR} If you pay $2,500 a year in student loan interest and take the standard deduction, you can still end up with a positive number for line 4(b). This is easy to miss.
How to Come Up With the Number
The figure for line 4(b) comes from the Deductions Worksheet on page 4 of the W-4 instructions.{1Internal Revenue Service. Form W-4 2026 Employee’s Withholding Certificate} The worksheet stays with you; nothing about the underlying math goes to your employer or the IRS. In outline:
- Estimate your itemized deductions for the year: medical expenses above 7.5% of income, state and local taxes up to the cap, home mortgage interest, charitable gifts, and other Schedule A items.{}5Internal Revenue Service. Publication 502, Medical and Dental Expenses
- Subtract the standard deduction for your filing status. If the result is positive, that is your excess deduction amount.
- Add your above-the-line adjustments: student loan interest, deductible IRA contributions, educator expenses, and similar Schedule 1 items.
- Enter the combined total on line 4(b).
Even if you take the standard deduction, recent versions of the worksheet include a small default amount (for example, $750 for single filers) on the deductions line. Combined with above-the-line adjustments, that can still produce a meaningful figure. Running through the worksheet is worthwhile even if you are confident you will not itemize, particularly if you carry student loan debt or contribute to a traditional IRA.
Why This May Apply to You in 2026 Even If It Didn’t Before
Under the One, Big, Beautiful Bill, the state and local tax deduction cap rose from $10,000 to $40,000 starting in 2025, and increases by 1% each year through 2029. For 2026, the cap is approximately $40,400 for most filers and $20,200 for married filing separately.
The higher cap pushes more people over the itemizing threshold, especially homeowners in high-tax states who also have mortgage interest and charitable giving. If your itemizing status might have changed from last year, run the numbers before deciding whether Step 4(b) applies to you.
A phase-down applies to higher earners. The SALT cap starts shrinking when modified adjusted gross income exceeds roughly $505,000 for 2026 ($252,500 for married filing separately), and the deduction is reduced by 30 cents for every dollar above that threshold. If your income falls in that range, use the IRS Tax Withholding Estimator rather than the paper worksheet.{6Internal Revenue Service. Updated Tax Withholding Estimator Lets Millions of Taxpayers Take One, Big, Beautiful Bill Changes Into Account When Calculating Their Withholding}
The Risk of Overstating the Number
A larger number on line 4(b) means bigger paychecks now, but if your real deductions at tax time come in lower than what you claimed, you will have underwithheld all year. That produces a tax bill in April and can trigger an underpayment penalty.
You avoid the underpayment penalty if the balance you owe at filing is under $1,000, or if your withholding and estimated payments together covered at least 90% of your current year’s tax or 100% of last year’s tax, whichever is less.{7Internal Revenue Service. Underpayment of Estimated Tax by Individuals Penalty} If your prior-year adjusted gross income exceeded $150,000 ($75,000 if married filing separately), the 100% figure becomes 110%. The IRS charges interest on underpayments at 7% annually as of early 2026.{8Internal Revenue Service. Quarterly Interest Rates}
When in doubt, round your 4(b) figure down rather than up. A modest refund costs almost nothing in lost interest; an unexpected tax bill plus a penalty is a worse outcome.
When to Revisit It
The IRS recommends reviewing your W-4 each year and whenever your financial life changes.{9Internal Revenue Service. About Form W-4, Employee’s Withholding Certificate} Common changes that affect your 4(b) number:
- Buying or selling a home. Mortgage interest is one of the largest itemized deductions, so a new mortgage can push you into itemizing and paying one off can pull you back.
- Marriage or divorce. Your filing status and standard deduction both change, which moves the break-even point.
- Paying off student loans. If interest on those loans was producing your positive 4(b) number, that adjustment disappears with the balance.
- Starting or stopping deductible IRA contributions.
- Moving between states. State income and property tax differences can change whether the SALT deduction is large enough to make itemizing worthwhile.
You can submit a new W-4 at any time. Your employer must implement the change no later than the start of the first payroll period ending on or after the 30th day after receiving it.{10Internal Revenue Service. Topic No. 753, Form W-4, Employees Withholding Certificate} There is no limit on revisions, and your employer cannot refuse a properly completed official form.{11Internal Revenue Service. Withholding Compliance Questions and Answers}
What Your Employer Sees
Some people hesitate to complete Step 4(b) because they worry about revealing personal finances. Your employer sees only the final dollar figure on line 4(b), not the mortgage interest, medical bills, or charitable gifts behind it. The Deductions Worksheet is not submitted anywhere. Federal law treats W-4 information as confidential return information used for payroll withholding.{12Office of the Law Revision Counsel. 26 US Code 6103 – Confidentiality and Disclosure of Returns and Return Information} A single number on that line tells your payroll department nothing beyond the fact that you expect deductions above the standard amount.