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 standard calculation, and W-4 Line 4c extra withholding is the cleanest fix whenever your situation is more complicated than a single job with no outside income. The usual triggers are a second job, a working spouse, freelance or investment income, and bonuses withheld at the flat 22% rate when your real bracket is higher. You pick the dollar amount, your employer adds it to each check, and the shortfall closes.

How Line 4c Works on Your Paycheck

The line is labeled “Extra withholding” and asks for any additional tax you want withheld each pay period.¹ Payroll runs the normal withholding calculation from your wages and other W-4 entries, then adds the flat amount you entered. That combined figure is what leaves the check.

The amount is fixed. It doesn’t scale with overtime or fluctuating hours. So your pay schedule matters when you back into a number: $50 on Line 4c produces $1,300 a year if you’re paid biweekly (26 checks) and $1,200 if you’re paid semimonthly (24 checks).

Line 4c vs. Line 4a

Step 4 of the W-4 gives you two different tools, and they behave differently. Line 4a takes an annual total of other income you expect that won’t have its own withholding, such as interest, dividends, or retirement distributions.¹ Payroll adds that figure to your wages before running the tables, so the extra tax gets calculated proportionally and spread across your checks.

Line 4c doesn’t touch the calculation. It bolts a flat dollar amount onto whatever payroll already computed. Use 4a when you can round-estimate non-job income and want the system to do the math. Use 4c when you need precise control, when the gap comes from something 4a doesn’t cover (like two jobs each using the lower brackets), or when you’ve already done the math and just want to plug in a number.

When You Need Extra Withholding

Two Jobs or a Working Spouse

Each employer withholds as if its wages are your only income. Both apply the low brackets and full standard deduction separately, and the combined result is almost always under-withheld. Step 2 tries to fix this, and its Multiple Jobs Worksheet can generate a Line 4c number, but the worksheet uses rough estimates. If household income pushes well past the 22% bracket ($105,700 for single filers, $211,400 for married filing jointly in 2026), the standard corrections often fall short.²

Calculate the actual shortfall and put a per-paycheck amount on Line 4c that covers it. Only one W-4 needs the adjustment. Most people load it onto the higher-paying job so the hit to any single check is proportionally smaller.

Investment, Rental, and Other Non-Wage Income

Interest, dividends, capital gains, and rental income arrive without withholding. If the amounts are small and predictable, Line 4a handles them fine. When they’re large or hard to pin down, Line 4c gives you more room to maneuver. You pick a dollar figure that covers your best estimate and revise it mid-year if the actual income drifts.

Self-Employment and Gig Income

Freelance income triggers both income tax and the 15.3% self-employment tax: 12.4% for Social Security on net earnings up to $184,500 in 2026, plus 2.9% for Medicare on all net earnings.³⁴ Line 4a only covers the income tax side. Line 4c can cover both pieces at once. You can also deduct half of the self-employment tax when calculating adjusted gross income, which trims the income tax portion slightly.⁵

If you have a W-2 job next to your freelance work, routing everything through Line 4c is usually simpler than juggling quarterly payments.

Bonuses Withheld at the Flat Rate

Employers typically withhold federal tax on bonuses and commissions at a flat 22%, and 37% on the portion above $1 million in a calendar year.⁶ If your marginal rate is 24%, 32%, or higher, 22% leaves a gap. A $10,000 bonus withheld at 22% shorts you by $200 if your real rate is 24%, and by $1,000 at 32%. Line 4c lets you raise withholding on regular paychecks around the payout to make up the difference.

Why Line 4c Usually Beats Estimated Payments

Paycheck withholding has a timing advantage the 1040-ES route doesn’t. Federal income tax withheld from wages is treated as paid in equal amounts across the four quarterly due dates, no matter when in the year the withholding actually happened.⁷ Estimated payments have to hit their specific due dates of April 15, June 15, September 15, and January 15, and a miss racks up a penalty for that quarter even if you catch up later.⁸

That rule makes Line 4c especially useful if you realize mid-year you’re behind. Bumping withholding up in September or October effectively spreads the credit back across the whole year for penalty purposes. Catching up on estimated payments that late doesn’t buy you the same retroactive relief. If you don’t have a regular paycheck, or your non-wage income dwarfs your salary, estimated payments still make sense.⁹ Otherwise Line 4c is the simpler path.

How to Calculate the Right Amount

The IRS Tax Withholding Estimator

The IRS offers a free Tax Withholding Estimator that takes your income, deductions, and credits and outputs a specific Line 4c amount. It factors in what’s already been withheld year-to-date, so the recommendation adjusts based on how far into the year you are.¹⁰

Have your recent pay stubs (with year-to-date withholding), last year’s return, and your expected non-wage income ready before you start. If you’re married filing jointly, you’ll also need your spouse’s income and withholding.¹¹ The tool asks for none of your identifying information.

One boundary: the IRS warns that if your return involves alternative minimum tax, long-term capital gains, or qualified dividends, you should use the Publication 505 worksheets instead of the estimator.¹¹ Nonresident aliens should follow Notice 1392 instead.¹²

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 single, $32,200 married filing jointly, $24,150 head of household. Use itemized if higher.²
Apply the tax brackets to what’s left. For a single filer in 2026, the first $12,400 is taxed at 10%, the next chunk up to $50,400 at 12%, and so on through 37% above $640,600.
Add self-employment tax if it applies: 15.3% of net self-employment earnings after the 92.35% adjustment. Half of that comes back as an above-the-line deduction.⁵
Subtract credits, such as the Child Tax Credit or Credit for Other Dependents.¹³
Compare to current withholding. Pull year-to-date withholding from a recent stub and project it forward through year-end. The gap between projected tax and projected withholding is your annual shortfall.
Divide the shortfall by the number of paychecks you have left. That’s your Line 4c number.

A quick sanity check on the total: to avoid the underpayment penalty, your withholding needs to cover at least 90% of this year’s tax, or 100% of last year’s tax (110% if your prior-year AGI exceeded $150,000).¹⁴¹⁵ The prior-year figure is the easiest safe harbor to target because you already know it. Pull line 24 from last year’s Form 1040.

Catching Up Mid-Year

Finding a shortfall in July isn’t ideal, but it’s fixable. You have fewer checks left, so the per-check number is larger. If you’re paid biweekly and you’re $2,600 short in July, roughly 13 pay periods remain, so Line 4c needs to be $200. Caught the same gap in January, it would have been $100 across 26 checks.

The IRS estimator handles the math automatically once you plug in year-to-date withholding.¹⁰ Doing it by hand, count actual remaining paydays on the employer’s calendar rather than guessing. And remember the timing rule: even a December catch-up through Line 4c is treated as paid evenly across all four quarters for penalty purposes.⁷

Submitting the Updated W-4

Hand the completed W-4 to payroll or HR, or enter it in your company’s payroll portal. The IRS doesn’t receive the form; your employer keeps it on file.¹

Federal rules require the employer to begin applying the new withholding no later than the start of the first payroll period ending on or after 30 days from receipt.⁶ Most process it within one or two pay cycles. Look at the federal income tax line on your first stub after submitting. If the change isn’t there, follow up rather than assuming payroll will catch up on its own.

When to Revisit the Number

A Line 4c amount that’s right for this year can be badly wrong for next. Review it once a year, ideally in early January when you have a clean picture of the prior year. Also revisit after any event that changes the tax picture:

Marriage or divorce, which changes filing status and bracket thresholds.
A new child or dependent, which adds credits.
Starting or ending a second job.
A spike in investment income, such as capital gains from a property sale.
A large bonus or commission, especially when withheld at the flat 22%.

If you owed more than $1,000 last April, or got a refund large enough to notice, your Line 4c number is probably off. A big refund means you’ve been over-withholding money that could have been in your account earning interest. A big balance due can trigger an underpayment penalty. Either way, run the numbers again.¹⁴

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. Publication 505 (2025), Tax Withholding and Estimated Tax
8
Internal Revenue Service. Pay As You Go, So You Won’t Owe
9
Internal Revenue Service. Estimated Taxes
10
Internal Revenue Service. Tax Withholding Estimator FAQs
11
Internal Revenue Service. IRS Tax Withholding Estimator Helps Taxpayers Get Their Federal Withholding Right
12
Internal Revenue Service. Supplemental Form W-4 Instructions for Nonresident Aliens
13
Internal Revenue Service. Child Tax Credit
14
Internal Revenue Service. Underpayment of Estimated Tax by Individuals Penalty
15
Office of the Law Revision Counsel. 26 USC 6654 – Failure by Individual to Pay Estimated Income Tax