TL;DR
- ✅ 4a — non-job income (freelance, dividends, rentals): increases withholding
- ✅ 4b — deductions above the standard deduction: decreases withholding
- ✅ 4c — flat extra amount per paycheck: the most flexible tool on the form
- ✅ All three are optional — leave blank if your situation is straightforward
Step 4a — Other income (annual)
Enter the annual total of income you receive that isn't subject to withholding — income where no employer is automatically taking out taxes on your behalf.
Entering income in 4a doesn't pay the tax directly — it tells your employer to withhold at a higher rate to cover the income tax on that amount. Note: 4a covers federal income tax only, not self-employment tax. See the guide on W-4 for freelance income for how to handle SE tax through Step 4c.
Step 4b — Extra deductions (annual)
Most people take the standard deduction — $15,000 for single filers and $30,000 for married filing jointly in 2026. If you don't, and you itemize on Schedule A, Step 4b lets you reduce your withholding to account for those deductions.
Important: enter only the amount by which your itemized deductions exceed the standard deduction — not your total itemized deductions.
📊 Step 4b example — homeowner who itemizes
Mortgage interest: $18,000
Property taxes + state income tax (SALT): $14,000
Charitable donations: $3,000
Total itemized: $35,000
Single standard deduction: $15,000
Step 4b entry: $35,000 − $15,000 = $20,000
With the 2026 SALT cap raised to $40,000 under OBBBA, more homeowners in high-tax states can now benefit from itemizing. See buying a home and your W-4 for details.
Step 4c — Extra withholding per paycheck
Step 4c is the most flexible line on the W-4. Enter a flat dollar amount and your employer adds it to every paycheck's withholding — no questions asked, no explanation required.
It's the right tool when you know you need more withholding but want to control the amount precisely — rather than letting the IRS tables determine it.
Cover a gap from the IRS estimator
If the estimator says you'll owe $780 and you have 13 paychecks left, enter $60 in Step 4c.
Cover self-employment tax on side income
Step 4a covers income tax; Step 4c covers the 15.3% SE tax. Calculate ~15.3% of net side income ÷ pay periods.
Safety buffer
Even $25–$50/paycheck over a full year can prevent a surprise April bill.
Fine-tune after using 4a or 4b
If 4a and 4b don't quite get you where you want, 4c is the precision adjustment.
Calculate your Step 4 amounts — free
Enter your other income and deductions to get the exact amounts for Steps 4a, 4b, and 4c.
Open the W-4 Calculator →When to leave all of Step 4 blank
If you have a single W-2 job, take the standard deduction, and have no non-wage income, you can leave Steps 4a, 4b, and 4c entirely blank. The IRS withholding tables will produce a reasonable result for your filing status, and you'll likely get a small refund or owe a small amount — both within normal range.
Key terms in this article
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