Extra Withholding on W-4: Line 4(c), Safe Harbors, and How to Set It

Extra withholding on a W-4 is a flat dollar amount you tell your employer to pull from every paycheck on top of the federal income tax already being withheld. You enter it on Line 4(c) of Form W-4, and payroll adds it to whatever the normal calculation produces based on your filing status and pay.¹ People use it to cover tax on income that doesn’t have its own withholding, to make up for under-withholding on bonuses, or to avoid an underpayment penalty at filing time.

How Line 4(c) Works

The entry has to be a specific dollar figure, not a percentage. Write $75, and $75 comes out of every paycheck in addition to the regular federal withholding. Write $200, and $200 comes out. That amount reduces your take-home pay and increases either your refund or the credit against what you owe when you file.

Line 4(c) sits apart from the rest of the form. Steps 1 through 3 set your filing status, account for multiple jobs or a working spouse, and handle dependents. Lines 4(a) and 4(b) adjust for other income and deductions by changing the formula your employer runs. Line 4(c) doesn’t change the formula at all; it just tacks a fixed amount onto the result.

When to Use It

You Hold More Than One Job

Each employer’s payroll system calculates withholding as if its job were your only income. Neither knows about the other paycheck, so each assumes a lower total income and withholds at a lower effective rate. The combined withholding almost always falls short of what you actually owe, especially if the second job pushes you into a higher bracket. The same problem hits married couples who both work and file jointly. Step 2 of the W-4 addresses this, but adding a precise dollar amount on Line 4(c) is often the simplest fix.

You Have Investment or Rental Income

Interest, dividends, capital gains, and rental income are taxable but don’t come with automatic withholding the way wages do. The IRS expects tax on this income to be paid throughout the year, not settled up in one lump in April. If you also have a W-2 job, adding extra withholding through Line 4(c) covers the tax on these income streams without the hassle of quarterly estimated payments.

You Have Self-Employment Income on the Side

Freelance or small business income reported on Schedule C carries both regular income tax and the 15.3% self-employment tax (12.4% for Social Security plus 2.9% for Medicare) on net earnings.² Quarterly estimated payments on Form 1040-ES are the standard way to handle this, but many people find it easier to raise withholding at a day job instead. The IRS doesn’t care which method you use as long as enough tax gets paid on time.

You Receive Bonuses or RSUs

Employers typically withhold a flat 22% on supplemental wages like bonuses, commissions, and restricted stock units, as long as total supplemental wages for the year stay at or below $1 million. Above that, the rate jumps to 37%.³ If your marginal rate is higher than 22%, the flat withholding doesn’t cover the full tax on that income. Someone in the 32% or 35% bracket who receives a $50,000 bonus will be under-withheld by thousands of dollars on that payment alone. Raising Line 4(c) on your regular paycheck closes the gap gradually instead of leaving a surprise at tax time.

Why Extra Withholding Beats Estimated Payments

Here’s the single best reason to use Line 4(c) instead of mailing quarterly estimated payments: the IRS treats federal income tax withheld from wages as if it were paid in four equal installments across the full year, no matter when the money was actually withheld.⁴ Estimated tax payments are date-sensitive. Miss the June 15 deadline, and you owe a penalty on that quarter even if you overpay later.

This matters if you realize in October that you’ve under-paid all year. Cranking up W-4 withholding for the last few months effectively spreads the credit back to January in the IRS’s calculations. You can’t do that with an estimated payment mailed in December.

Safe Harbors to Aim For

The IRS charges an underpayment penalty when you don’t pay enough tax through the year, but three safe harbors let you avoid it:⁵

You owe less than $1,000 after subtracting withholding and credits.
Your total withholding and estimated payments covered at least 90% of the tax shown on your current-year return.
Your total payments equaled or exceeded 100% of the tax on last year’s return. This jumps to 110% if your prior-year adjusted gross income exceeded $150,000 ($75,000 if married filing separately).⁴

The prior-year safe harbor is the easiest target for most people setting Line 4(c). Pull up last year’s Form 1040, find the total tax line, and make sure your current-year withholding will at least match it (or 110% of it for higher earners). That guarantees no penalty even if your income jumps. The 90% current-year rule is harder in practice because it requires predicting your income accurately.

How to Figure the Amount

The IRS Tax Withholding Estimator

The fastest and most accurate route is the IRS Tax Withholding Estimator.⁶ You’ll need a recent pay stub from each job (showing year-to-date income and withholding), estimates of non-wage income you expect, and last year’s return. The tool runs the numbers and tells you what to enter on Line 4(c). For multiple jobs, it typically directs you to add the extra amount on the W-4 for only one job, usually the highest-paying.⁷

Doing It by Hand

Project your total income for the year from every source: wages, freelance work, investments, rental properties. Apply the 2026 federal brackets to that total. For reference, single filers run from 10% on the first $12,400 up to 37% on income above $640,600; for married filing jointly, the 37% rate kicks in above $768,700.⁸ Subtract credits you expect to claim. The result is your projected tax liability.

Then check your latest pay stub for the current per-paycheck withholding and multiply by the pay periods in the year to get projected withholding. Subtract projected withholding from projected liability. That gap is your annual shortfall.

Divide the shortfall by the pay periods remaining in the year. A $2,600 gap identified in July with 13 biweekly periods left means $200 on Line 4(c). Rerun the math whenever your income picture shifts; a figure that fit in March can be far off by September if you sold property, picked up a new client, or lost a job.

Submitting and Changing the Form

Hand the completed W-4 to payroll or HR, or submit it through your employer’s self-service portal. The employer must apply the new withholding no later than the start of the first payroll period ending on or after 30 days from when they received it.³ Most payroll departments process the change within one or two cycles.

To change the amount later, submit a new W-4. To stop extra withholding entirely, enter $0 on Line 4(c) of the replacement form. There’s no limit on how often you can update it, and your employer can’t refuse a valid W-4.¹

State Withholding Is Separate

Line 4(c) only affects federal withholding. If you live in a state with an income tax, a separate adjustment may be needed for state purposes. Some states accept the federal W-4 for state calculations; others require their own form. Check your state’s department of taxation or ask payroll whether a separate state form exists and whether it offers a similar extra-withholding line.

When to Revisit the Number

Check Line 4(c) at least once a year, ideally in January when new brackets take effect. Recalculate whenever something significant changes: a new job, a lost job, a marriage or divorce, the birth of a child, a large investment sale, or a jump in self-employment income. The worst outcome is setting a figure in January and ignoring it after your finances shift. The second-worst is over-withholding by a wide margin, which hands the government an interest-free loan and returns your own money as a refund.⁹

1
Internal Revenue Service. Form W-4 (2026) Employee’s Withholding Certificate
2
Internal Revenue Service. Self-Employment Tax (Social Security and Medicare Taxes)
3
Internal Revenue Service. Publication 15 (2026), (Circular E), Employer’s Tax Guide
4
Office of the Law Revision Counsel. 26 USC 6654 – Failure by Individual to Pay Estimated Income Tax
5
Internal Revenue Service. Underpayment of Estimated Tax by Individuals Penalty
6
Internal Revenue Service. Tax Withholding Estimator
7
Internal Revenue Service. FAQs on the 2020 Form W-4
8
Internal Revenue Service. IRS Releases Tax Inflation Adjustments for Tax Year 2026
9
Internal Revenue Service. About Form W-4, Employee’s Withholding Certificate