If your employer isn’t withholding enough federal taxes, the cause is almost always the Form W-4 you have on file. Your employer doesn’t decide the amount on its own: it plugs your W-4 entries into the IRS withholding tables, and whatever the form says is what the paycheck reflects. When the form’s assumptions don’t match your real income, marital situation, or side income, too little tax comes out. The fix takes about 15 minutes with the IRS Tax Withholding Estimator and a new W-4.
Below are the specific reasons this happens, in rough order of how often they’re the culprit.
A Second Income the W-4 Doesn’t Know About
This is the single most common cause of a surprise tax bill. In a dual-income household, both spouses check “Married Filing Jointly” in Step 1 and then skip Step 2. Each employer’s payroll system then applies the full $32,200 married standard deduction and the generous married brackets to its own paycheck, as if that job were the household’s only income.1Internal Revenue Service. Form W-4 (2026) The combined income actually falls into higher brackets, but neither employer knows that. The shortfall at filing time regularly runs into the thousands.
The same math breaks the same way if you personally hold two jobs. Each employer treats its paycheck as if it were your entire income, and both apply the lower brackets that should only cover the first slice of your earnings.
Step 2 of the W-4 exists to correct this. You either use the IRS estimator, check the box in 2(c) if both jobs pay similarly, or work through the multiple-jobs worksheet.
An Outdated W-4 After a Life Change
Your employer keeps applying whatever W-4 you last submitted, indefinitely, until you file a new one.2Internal Revenue Service. Tax Withholding for Individuals Nothing on the form updates automatically. If your spouse started a job, if you got divorced, if a child aged out of the Child Tax Credit, or if you picked up significant investment income, the old form’s assumptions no longer describe your life. Withholding continues at the old, now-too-low level until you file a replacement.
Overstated Credits or Deductions
Step 3 of the W-4 lets you reduce withholding for tax credits you expect to claim, such as the Child Tax Credit, entered as a dollar amount.1Internal Revenue Service. Form W-4 (2026) Line 4(b) reduces withholding for itemized deductions above the standard deduction. Both are projections, and both cause under-withholding when the projections turn out too optimistic.
Common examples: charitable contributions you planned but never made, medical expenses that didn’t reach the threshold, or a dependent who no longer qualifies. Your employer doesn’t verify these figures. It applies whatever number you wrote.
Bonuses and RSUs Withheld at the Flat 22%
Bonuses, commissions, and equity compensation like restricted stock units are treated as supplemental wages, and your employer is permitted to withhold federal income tax on them at a flat 22%, regardless of your W-4 entries.3Internal Revenue Service. Publication 15 (2026), (Circular E), Employers Tax Guide – Section: 7. Supplemental Wages That’s fine when 22% is close to your actual marginal rate. It’s a real problem when your income puts you in the 24%, 32%, 35%, or 37% brackets.
For 2026, a single filer enters the 24% bracket at $105,701 of taxable income, and married-filing-jointly enters it at $211,401. Above those thresholds, every bonus dollar is under-withheld by at least two percentage points, sometimes far more. On a $20,000 bonus, someone in the 35% bracket sees roughly $2,600 less withheld than the actual tax owed.
RSU vesting amplifies this. The fair market value on the vesting date counts as supplemental wage income, most employers apply the flat 22%, and the gap between 22% and a high earner’s true marginal rate can produce a five-figure shortfall over the year. If supplemental wages from one employer exceed $1 million in a year, withholding on the excess jumps to a mandatory 37%.4eCFR. 26 CFR 31.3402(g)-1 – Supplemental Wage Payments Below that threshold, the 22% default sticks unless you add extra withholding on line 4(c).
Starting a Job Mid-Year
When you start a job partway through the year, payroll annualizes your paycheck. If you begin in October at $8,000 a month, the system treats you as earning $96,000 annually and picks brackets accordingly, even though you’ll only receive $24,000 from that employer this year. That’s usually fine on its own, but it under-withholds when you also had earlier income from another job the new employer doesn’t know about.
If you expect to work for all employers combined no more than 245 days in the calendar year, you can request in writing that your employer use the part-year employment method, which adjusts the calculation for a shorter work period.5Internal Revenue Service. Publication 15-T (2026), Federal Income Tax Withholding Methods The employer isn’t required to agree, but many will.
How to Fix It
Go to irs.gov and open the Tax Withholding Estimator. It asks for your income, deductions, credits, and year-to-date withholding, then tells you exactly what to put on a new W-4 to hit your target for the year.6Internal Revenue Service. Tax Withholding Estimator You’ll need your most recent pay stub and last year’s return. The tool can generate a completed W-4 you download and hand to payroll.
Submit the new W-4 through your employer’s HR or payroll portal. Changes usually take effect on the next pay cycle, and there’s no limit on how many times you can update the form during the year.2Internal Revenue Service. Tax Withholding for Individuals
Line 4(c) is the simplest lever. You enter a flat dollar amount to add to each paycheck’s withholding, and payroll withholds that extra amount on top of the normal calculation. No explanation of where the number came from is required. This also solves a privacy concern with Step 2: if you don’t want to disclose a spouse’s income or a second job to your employer, use the Estimator to calculate the right total extra amount and put only that dollar figure in 4(c).7Internal Revenue Service. FAQs on the 2020 Form W-4
If you’re already deep into the year and adjusting the W-4 alone can’t close the gap, make estimated tax payments directly to the IRS using Form 1040-ES. The 2026 quarterly due dates are April 15, June 15, September 15, and January 15, 2027. You can skip the January payment if you file your full return and pay the balance by February 1, 2027.8Internal Revenue Service. 2026 Form 1040-ES Payments can be made online, by phone, or by mail.
Whether You’ll Owe a Penalty
Under-withholding doesn’t automatically trigger a penalty. You owe nothing extra if the tax due after subtracting withholding and credits is less than $1,000.9Office of the Law Revision Counsel. 26 USC 6654 – Failure by Individual to Pay Estimated Income Tax Beyond that, you avoid the penalty by meeting one of two safe harbor tests:
- Your total payments (withholding plus estimated tax) come to at least 90% of what you actually owe for 2026.
- Your total payments come to at least 100% of the tax shown on your 2025 return, or 110% if your 2025 adjusted gross income was over $150,000 ($75,000 if married filing separately).9Office of the Law Revision Counsel. 26 USC 6654 – Failure by Individual to Pay Estimated Income Tax
You only need to meet one. The prior-year test is the more predictable one when your income jumps unexpectedly, because you’re aiming at a known number from last year’s return.
When the penalty does apply, it’s calculated as interest on the shortfall for each quarter you were short, not a flat fee. The rate resets quarterly: for 2026 it’s 7% in the first quarter and 6% in the second.10Internal Revenue Service. Quarterly Interest Rates11Internal Revenue Service. Internal Revenue Bulletin: 2026-08
The IRS can waive the penalty in specific situations. If you retired after age 62 or became disabled during the tax year or the year before, and the underpayment was due to reasonable cause rather than neglect, you can request a waiver on Form 2210. The same form handles waivers for casualties, disasters, and other unusual circumstances.12Internal Revenue Service. Instructions for Form 2210 Taxpayers in federally declared disaster areas generally receive automatic penalty relief without filing anything.