When should I update my W-4?

Update your W-4 after any major life change: getting married or divorced, having a baby, buying a house, starting a second job, or if you received a large tax refund or owed a lot. You can submit a new W-4 to your employer at any time.

What happens if I don't fill out a W-4?

If you don't submit a W-4, your employer must withhold taxes as if you are single with no adjustments. This usually results in the maximum withholding, meaning less take-home pay but a larger refund at tax time.

What Is a W-4 Form?

Paycheck & Deductions3 min read·Updated for 2025

Quick Answer

The W-4 is a form you give your employer that tells them how much federal income tax to withhold from each paycheck. It asks for your filing status, whether you have multiple jobs, dependents, and any additional adjustments. Getting your W-4 right means your withholding closely matches your actual tax liability — so you avoid a surprise bill or an oversized refund.

The Five Steps of the W-4

The current W-4 (redesigned in 2020) has five steps:

Step 1: Personal Information (Required)

Enter your name, address, Social Security number, and filing status. Your filing status choice — single, married filing jointly, or head of household — sets the base withholding tables your employer uses.

Step 2: Multiple Jobs or Spouse Works (If Applicable)

If you work two jobs or your spouse also works, this step prevents under-withholding. You can use the IRS online estimator, the worksheet on the form, or simply check a box to withhold at the higher single rate.

Step 3: Dependents (If Applicable)

Claim $2,000 for each qualifying child under 17 and $500 for other dependents. This reduces the amount withheld from each paycheck.

Step 4: Other Adjustments (Optional)

4(a): Other income not from jobs (interest, dividends, side hustle income)
4(b): Deductions beyond the standard deduction (itemized deductions, 401(k), HSA)
4(c): Extra withholding per paycheck if you want more taken out

Step 5: Signature (Required)

Sign and date the form, then submit it to your employer.

Real Example With Actual Numbers

Chris is married, earns $95,000, and his spouse earns $55,000. They have one child. Without adjustments, Chris's employer might under-withhold because it does not know about his spouse's income.

Option A: Check the box in Step 2 Chris checks the "married, but withhold at higher single rate" box. This increases his withholding to account for the combined household income. His biweekly federal withholding goes from roughly $460 to $580.

Option B: Claim the child in Step 3 Chris enters $2,000 in Step 3 for his dependent child. This reduces his annual withholding by $2,000, spread across all paychecks (about $77 less per biweekly check).

Best approach: Chris does both — checks the box in Step 2 to prevent under-withholding, and claims the child credit in Step 3 for accuracy. Use the SalaryHog calculator with the married calculator to model different scenarios.

Common W-4 Mistakes

Not updating after marriage: Your withholding should change when you get married. The default "Married Filing Jointly" setting assumes your spouse has no income, which can lead to under-withholding.
Ignoring a second job: If you have a side hustle or second job, your primary employer does not know about it. You need to adjust Step 2 or request extra withholding in Step 4(c).
Claiming too many dependents: Each dependent reduces withholding. If your situation changes (child ages out, divorce), update your W-4.
Confusing W-4 with W-2: The W-4 is submitted to your employer at the start of employment. The W-2 is the year-end summary your employer sends you for tax filing.

How W-4 Affects Your Paycheck

The W-4 only affects federal income tax withholding. It does not change FICA (Social Security and Medicare), state tax withholding (that has its own form), or pre-tax deductions like 401(k) contributions.

See how different W-4 choices change your take-home pay by trying the SalaryHog calculator.