W-4 Line 4c: When and How to Add Extra Withholding

Line 4c on Form W-4 is where you tell your employer to withhold a specific extra dollar amount from every paycheck on top of the normal calculation. You use W-4 Line 4c for extra withholding when your tax picture is more complex than one job with no outside income and the built-in adjustments in Steps 2 through 4(b) don’t fully close the gap between what’s withheld and what you’ll owe. The usual triggers are a second job, a working spouse, meaningful investment or freelance income, and large bonuses withheld at a flat rate below your real bracket.¹

How Line 4c Works

The W-4 labels Line 4c “Extra withholding” and asks for any additional tax you want withheld each pay period.¹ Payroll runs its normal withholding calculation based on your wages and your other W-4 entries, then adds the flat amount from Line 4c on top. That combined figure is what comes out of each check.

The number stays constant regardless of hours or overtime. Enter $50 and get paid biweekly, and you’ve added $1,300 of withholding across 26 pay periods. The same $50 on a semimonthly schedule produces $1,200 across 24. Your pay schedule matters when you back into the right figure.

Line 4c Versus Line 4a

Step 4 of the W-4 gives you two tools that are easy to confuse. Line 4a asks for the annual total of other income you expect that won’t have its own withholding, such as interest, dividends, or retirement distributions.¹ When you enter a number on 4a, payroll adds that amount to your wages before running the withholding tables, so the extra withholding scales proportionally.

Line 4c doesn’t change the tax calculation. It bolts a flat dollar amount onto whatever the system already computed. Use 4a when you can estimate non-job income as a round annual figure and want payroll to do the math. Use 4c when you need precise control, when the shortfall comes from something 4a doesn’t cover (like a bracket mismatch from a second job), or when you’ve already run your own numbers and want to plug in the exact amount.

When You Need Extra Withholding

Multiple Jobs or a Working Spouse

Each employer withholds as if its wages are your only income, applying the lower brackets and full standard deduction separately. On the combined total, that almost guarantees under-withholding. Step 2 of the W-4 and its Multiple Jobs Worksheet try to fix this and can generate a Line 4c figure, but those shortcuts use rough estimates. If your household income is uneven across jobs or runs well past the 22% bracket ($105,700 for single filers, $211,400 for married filing jointly in 2026), the standard corrections tend to fall short.²

Line 4c closes the remaining gap. You only need to add it on one W-4. Most people put it on the higher-paying job so the per-check impact is proportionally smaller.

Investment, Rental, and Other Non-Wage Income

Interest, dividends, capital gains, and rental income don’t carry payroll withholding. When those amounts are modest and predictable, Line 4a handles them cleanly. When they’re large or hard to pin down, Line 4c gives you more flexibility: pick a dollar amount that covers your best estimate and adjust mid-year if the actual income runs higher or lower.

Self-Employment and Gig Income

Freelance and gig income creates a double problem. You owe income tax and self-employment tax of 15.3% (12.4% Social Security on net earnings up to $184,500 in 2026, plus 2.9% Medicare on all net earnings).³⁴ Line 4a only addresses income tax. Line 4c lets you cover both the income tax and the self-employment tax in one adjustment. You can deduct half of the self-employment tax when calculating adjusted gross income, which slightly trims the income tax side.⁵

If you have a W-2 job alongside freelance work, running everything through Line 4c is often simpler than juggling quarterly estimated payments.

Bonuses Withheld at the Flat Rate

Employers typically withhold federal tax on bonuses and commissions at a flat 22% (37% on the portion above $1 million in a calendar year).⁶ If your real marginal rate is 24%, 32%, or higher, that 22% leaves a gap. A $10,000 bonus withheld at 22% shortchanges you by $200 at a real rate of 24%, and by $1,000 at 32%. Line 4c lets you increase withholding on regular checks before or after the bonus to make up the difference.

The Underpayment Penalty You’re Trying to Avoid

The IRS charges an underpayment penalty when you haven’t paid enough tax throughout the year. You avoid it by hitting any one of three safe harbors:⁷

Less than $1,000 owed after subtracting withholding and credits.
Withholding and credits covering at least 90% of this year’s tax.
Withholding equal to 100% of last year’s total tax, or 110% if your prior-year AGI exceeded $150,000 ($75,000 if married filing separately).⁸

The prior-year safe harbor is the easiest to aim at because you already know the number. Pull line 24 from last year’s Form 1040 and make sure your current-year withholding will at least match it (or exceed it by 10% if your AGI was above $150,000). If there’s a shortfall, Line 4c closes it.

Why Extra Withholding Often Beats Estimated Payments

When you owe tax on non-wage income, you can either make quarterly estimated payments on Form 1040-ES or increase paycheck withholding through Line 4c. Both satisfy the IRS. Withholding has a built-in timing advantage.

Federal income tax withheld from your pay is treated as paid in equal amounts across the four quarterly due dates, even if the actual withholding happened unevenly or late in the year.⁹ Estimated payments must land on their specific due dates of April 15, June 15, September 15, and January 15.¹⁰ Miss one and you rack up a penalty for that quarter even if you catch up later.

That rule makes Line 4c especially valuable if you realize mid-year that you’re behind. Bumping withholding in September or October spreads the credit back across the whole year for penalty purposes. Catching up on estimated payments that late doesn’t give you the same retroactive benefit. Estimated payments still make sense if you don’t have a regular paycheck, if your non-wage income is very large relative to your salary, or if you want tighter control over cash flow timing. For most people with a W-2 job and side income, Line 4c is the simpler path.¹¹

How to Calculate the Right Amount

Using the IRS Tax Withholding Estimator

The IRS offers a free Tax Withholding Estimator that walks through your income, deductions, and credits, then recommends a specific Line 4c amount. It factors in year-to-date withholding, so the recommendation adjusts based on how far into the year you are.¹²

Before you start, gather your most recent pay stubs (showing year-to-date withholding), last year’s tax return, and details on any non-wage income you expect.¹³ If you’re married filing jointly, pull the same for your spouse. The tool doesn’t ask for your name, Social Security number, or bank information.

One limit worth knowing: the IRS says people with alternative minimum tax exposure, long-term capital gains, or qualified dividends should use the Publication 505 worksheets instead of the estimator.¹³

Doing the Math Yourself

The manual version is straightforward:

Estimate total income for the year: wages from all jobs, expected bonuses, freelance income, investment income, and anything else taxable.
Subtract the standard deduction. For 2026 that’s $16,100 for single filers, $32,200 for married filing jointly, and $24,150 for head of household. Use itemized deductions if they’re higher.²
Apply the federal brackets to your taxable income.
Add self-employment tax if applicable: 15.3% of net self-employment earnings after the 92.35% adjustment. Half of this comes back as an AGI deduction.⁵
Subtract credits such as the Child Tax Credit or Credit for Other Dependents.¹⁴
Compare to current withholding. Look at year-to-date on a recent pay stub and project forward to year-end. The gap between projected tax and projected withholding is your annual shortfall.
Divide by the number of paychecks remaining in the year. That’s your Line 4c amount.

Mid-Year Catch-Up

Finding a shortfall in July isn’t ideal, but it’s fixable. Fewer remaining paychecks means a larger per-check hit. Paid biweekly and $2,600 short in July? About 13 pay periods remain, so Line 4c is $200. The same shortfall caught in January, with 26 periods ahead, would’ve been $100.

The IRS Withholding Estimator handles this automatically by factoring in year-to-date withholding.¹² If you’re doing it by hand, count actual remaining paydays on your employer’s calendar rather than estimating. Even withholding added in December is treated as paid evenly across all four quarters for penalty purposes, which makes a late Line 4c change far more forgiving than a late estimated payment.⁹

Submitting the Updated W-4

Give the completed W-4 to payroll or HR, or enter it through your company’s electronic payroll portal. The IRS does not receive the form directly; your employer keeps it on file.¹

Federal rules require your employer to begin applying the new withholding no later than the start of the first payroll period ending on or after 30 days from the date they received the form.⁶ Most employers process it within one or two pay cycles. Check your first pay stub after submitting to confirm the federal income tax line reflects the increase. If it doesn’t, follow up right away.

When to Revisit Line 4c

A Line 4c amount that’s perfect for 2026 could be wrong for 2027. Review your withholding at least once a year, ideally in early January when you have a clean picture of the prior year. Also revisit after anything that reshapes your tax picture: marriage or divorce, a new child or dependent, starting or ending a second job, a spike in investment income, or a large bonus or commission (especially if withheld at the flat 22%).

If you owed more than $1,000 when you filed last year, or you got a refund large enough to notice, your Line 4c amount is probably off. A big refund means you’ve been over-withholding, which is money that could’ve been earning interest in your account all year. A big balance due means you may face an underpayment penalty. Either way, run the numbers again.⁷

1
Internal Revenue Service. Form W-4 – Employee’s Withholding Certificate (2026)
2
Internal Revenue Service. IRS Releases Tax Inflation Adjustments for Tax Year 2026
3
Internal Revenue Service. Self-Employment Tax (Social Security and Medicare Taxes)
4
Social Security Administration. Contribution and Benefit Base
5
Internal Revenue Service. Topic No. 554, Self-Employment Tax
6
Internal Revenue Service. Publication 15 (2026), (Circular E), Employer’s Tax Guide
7
Internal Revenue Service. Underpayment of Estimated Tax by Individuals Penalty
8
Office of the Law Revision Counsel. 26 USC 6654 – Failure by Individual to Pay Estimated Income Tax
9
Internal Revenue Service. Publication 505 (2025), Tax Withholding and Estimated Tax
10
Internal Revenue Service. Pay As You Go, So You Won’t Owe
11
Internal Revenue Service. Estimated Taxes
12
Internal Revenue Service. Tax Withholding Estimator FAQs
13
Internal Revenue Service. IRS Tax Withholding Estimator Helps Taxpayers Get Their Federal Withholding Right
14
Internal Revenue Service. Child Tax Credit