W-4 Other Income: What to Enter in Step 4(a)

On Step 4(a) of Form W-4, enter the total dollar amount of taxable income you expect this calendar year that has no federal tax withheld at the source: interest, dividends, capital gains, rental profit, and similar income. Payroll reads that figure, adds it to your wages, and spreads the extra withholding across your remaining paychecks automatically. Getting the number close keeps you out of a surprise balance due and away from an underpayment penalty at filing time.

What Counts as Other Income on Line 4(a)

The W-4 itself describes Step 4(a) as the place for “other income you expect this year that won’t have withholding,” and lists interest, dividends, and retirement income as examples.¹ In practice, several other income types belong there too. Include any of the following that apply to you:

Bank interest reported on Form 1099-INT and dividends reported on Form 1099-DIV, including reinvested dividends from brokerage accounts.
Capital gains from selling stocks, mutual funds, real estate, or other assets during the year.
Net rental profit after deducting expenses like mortgage interest, repairs, and depreciation.
Traditional IRA or 401(k) withdrawals that don’t already have enough federal tax withheld at the source.
Alimony received under divorce or separation agreements finalized before 2019. Agreements executed after December 31, 2018, are no longer taxable to the recipient.²
Gambling and prize winnings. Casinos and lottery operators generally withhold federal tax only when winnings exceed $5,000, so smaller winnings arrive without any tax taken out.³
Cryptocurrency income from mining, staking, or receiving digital assets as payment.⁴
Taxable Social Security benefits. Depending on your combined income, up to 85 percent of your benefits may be taxable, and any shortfall between what’s withheld from your benefit check and what you’ll owe belongs in the Line 4(a) figure.

You put the annual amount on the line. Not a per-paycheck figure, not a tax amount. Just the income you expect to receive.

What Does Not Go on Line 4(a)

Two common income types look like they belong here but don’t.

Wages from a second or third W-2 job go in Step 2, not Step 4(a). The two steps solve different problems. When two employers each withhold independently, each one applies the lower brackets and standard deduction to your wages as if their job were your only income, which leaves you under-withheld overall. Step 2 corrects that. Step 4(a), by contrast, assumes zero tax has been collected on the amount you enter. If you put a second job’s wages on 4(a), the system over-withholds, because your second employer is already withholding on that money.¹

Self-employment income also fits poorly on Line 4(a). Freelance and contractor income carries a 15.3 percent self-employment tax on top of income tax, and the W-4 has no mechanism to collect that portion.⁵ Quarterly estimated payments through Form 1040-ES are the right tool for self-employment. You can use a hybrid approach if you want, covering income tax through paycheck withholding and self-employment tax through estimated payments, but Line 4(a) alone will leave you short.⁶

How the Number You Enter Becomes Withholding

Step 4(a) is mechanically simpler than it looks. You enter one number: the total dollar amount of other income you expect for the full calendar year. You don’t calculate a tax rate. You don’t figure a per-paycheck amount. The payroll system adds the figure to your wages and withholds against the combined total.¹

An example. You earn $70,000 at your job and expect $8,000 in dividend and interest income this year. Write $8,000 on Line 4(a). Payroll then withholds federal tax as if you earned $78,000, spreading the additional withholding evenly across your paychecks. The IRS instructions note that if you complete Step 4(a), you “likely won’t have to make estimated tax payments for that income,” which is exactly the point.¹

Your employer sees only the dollar figure. Payroll never learns what kind of income it is or where it came from.

Using the IRS Tax Withholding Estimator

For anyone with multiple income streams, itemized deductions, or a partway-through-the-year adjustment, the IRS Tax Withholding Estimator at irs.gov/W4App is worth the twenty minutes. It walks through each income source, applies the correct brackets, factors in credits and deductions, and produces a completed W-4 you can download and hand to your employer.⁷

Have a recent pay stub in front of you when you start. The Estimator needs to know how much federal tax has been withheld so far this year to calibrate the paychecks that remain. That matters most for mid-year changes.

Depending on your situation, the Estimator may put amounts on Step 4(a), Step 4(c), or both. Straightforward cases with a single job and modest investment income usually end up with just a figure on 4(a). More complex cases get recommendations across several lines.

When to Use Line 4(c) Instead or Alongside

Line 4(c), labeled “Extra withholding,” works differently from 4(a). You enter a specific dollar amount of additional tax to withhold from each paycheck, and payroll adds that flat amount on top of the standard calculation.¹ It gives you direct control over the dollar figure. Cases where 4(c) is the better choice:

Closing a known shortfall. If the Estimator tells you you’re on track to owe $1,200 with 20 paychecks left, $60 on Line 4(c) closes the gap exactly.
Handling income the standard formula treats imprecisely. Long-term capital gains taxed at preferential rates don’t fit the ordinary-income withholding formula cleanly, so putting the estimated tax on 4(c) can beat entering the income on 4(a).
Supplementing quarterly estimated payments. If you pay estimated tax on self-employment income but want your paycheck to carry the rest, a flat amount on 4(c) does that.

You can use 4(a) and 4(c) at the same time. The two amounts stack on top of the standard wage withholding.

Offsetting With Deductions on Line 4(b)

Line 4(b) is the mirror image of 4(a). Where 4(a) increases withholding to cover more income, 4(b) decreases it to reflect deductions above the standard amount. The W-4’s default assumes you’ll take the standard deduction and nothing more, so if your itemized or above-the-line deductions run higher, your paychecks are being taxed on income you won’t actually owe tax on.¹

The 2026 standard deduction is $16,100 for single filers, $32,200 for married couples filing jointly, and $24,150 for heads of household.⁸ The Deductions Worksheet on the W-4 walks you through the calculation. Above-the-line deductions like student loan interest, deductible IRA contributions, and educator expenses also belong there, since they reduce adjusted gross income whether or not you itemize.

The interaction with 4(a) matters. If you expect $8,000 in dividend income but also $5,000 in deductions above the standard amount, your net additional taxable income is roughly $3,000. Filling in both lines lets the withholding calculation account for both sides, rather than over-withholding on the income side while ignoring the deduction side.

Adjusting Mid-Year

Changing your W-4 in July is different from filing one in January. Fewer paychecks remain, so each one has to carry a larger share of the tax on your other income. The Estimator handles this automatically because it asks how much has already been withheld.⁷

The manual math is straightforward. Estimate the additional tax still owed for the year, then divide by pay periods remaining, and put that per-paycheck amount on Line 4(c). Suppose you started a side income in June, expect $2,400 in additional federal tax for the year, and $800 has already been covered by an earlier W-4 update. That leaves $1,600 across, say, 12 remaining biweekly paychecks, or roughly $133 on Line 4(c).

Once you submit a revised W-4, your employer must implement the new withholding no later than the start of the first payroll period ending on or after the 30th day after you turned it in.⁹ Many employers move faster, but plan for up to one pay cycle of lag. Don’t wait until December to fix a September problem. There’s no limit on how many times you can file a new W-4, so update it whenever a side income starts or stops, whenever you take a large gain or loss, or whenever retirement distributions begin.

Avoiding an Underpayment Penalty

The IRS charges an underpayment penalty when you haven’t paid enough tax throughout the year. Three safe harbors protect you regardless of how the shortfall happened:

Your total tax due after withholding and credits is under $1,000.
Your combined withholding and estimated payments cover at least 90 percent of this year’s final tax bill.
You paid at least 100 percent of last year’s total tax, or 110 percent if your prior-year adjusted gross income exceeded $150,000 ($75,000 if married filing separately).¹⁰

The prior-year safe harbor is the most useful when your other income is unpredictable. If last year’s total tax was $14,000 on an AGI under $150,000, withholding at least $14,000 this year guarantees no penalty even if your investment income doubles unexpectedly.

State Withholding Is Separate

The federal W-4 controls federal income tax withholding only. Most states with an income tax use their own withholding form, and even states that accept the federal W-4 may apply different rules to other income. If a federal adjustment on Line 4(a) is warranted, expect to make a parallel change on the state form. Your employer’s payroll department or your state tax agency can point you to the right one.

1
Internal Revenue Service. Form W-4 (2026) Employee’s Withholding Certificate
2
Internal Revenue Service. Topic No. 452, Alimony and Separate Maintenance
3
eCFR. 26 CFR 31.3402(q)-1 – Extension of Withholding to Certain Gambling Winnings
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Internal Revenue Service. Taxpayers Need to Report Crypto, Other Digital Asset Transactions on Their Tax Return
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Internal Revenue Service. Self-Employment Tax (Social Security and Medicare Taxes)
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Internal Revenue Service. Self-Employed Individuals Tax Center
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Internal Revenue Service. Tax Withholding Estimator
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Internal Revenue Service. IRS Releases Tax Inflation Adjustments for Tax Year 2026, Including Amendments From the One, Big, Beautiful Bill
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Office of the Law Revision Counsel. 26 USC 3402 – Income Tax Collected at Source
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Internal Revenue Service. Underpayment of Estimated Tax by Individuals Penalty