If your employer isn’t withholding enough federal taxes, the cause is almost always the Form W-4 on file. Your employer doesn’t decide the amount on its own; it runs the entries from your W-4 through the IRS withholding tables and deducts whatever the tables produce. When the inputs on that form don’t match your real income, filing status, or credits, the paycheck math comes out short, and the gap shows up as a bill (and possibly a penalty) at tax time.
The four steps on the current W-4 each drive a piece of the calculation. Step 1 sets your filing status and the standard deduction the payroll system applies. Step 2 accounts for multiple jobs in the household. Step 3 reduces withholding for credits like the Child Tax Credit. Step 4 handles non-wage income, itemized deductions, and any extra flat dollar amount you want added to each check.1Internal Revenue Service. Form W-4 (2026) A wrong entry in any of those steps pushes withholding in the wrong direction.
The Dual-Income Trap
The single most common reason paychecks come up short is a married couple filing jointly where both spouses work, one or both W-4s show “Married Filing Jointly” in Step 1, and Step 2 was skipped.
Here’s what happens inside payroll. Each employer’s system looks at your W-4, sees the married status, and applies the full $32,200 married standard deduction and the wider married tax brackets to that job alone.1Internal Revenue Service. Form W-4 (2026) Both employers do this independently. Neither knows the other exists. The result is that your combined household income is taxed as if it were two separate, smaller incomes, and the higher marginal rate that actually applies to the combined total is never captured. For couples with two solid salaries, the shortfall at filing time easily runs into the thousands.
The same underlying problem shows up when a single person holds two jobs at once. Each employer’s system assumes its paycheck is your entire income and gives it the full benefit of the standard deduction and the lower brackets. Step 2 exists specifically to correct for this.
An Outdated W-4 or Overstated Entries
Your employer keeps using the last W-4 you submitted until you send a new one.2Internal Revenue Service. Tax Withholding for Individuals Life events don’t update it automatically. A spouse starting work, a divorce, a child aging out of the Child Tax Credit—each of those changes the correct answer on the form, and if the form doesn’t get refiled, withholding stays calibrated to a household that no longer exists.
Overstated entries in Step 3 or Step 4(b) also directly shrink withholding. If you projected charitable contributions or itemized deductions at the start of the year that never materialized, or claimed credits you no longer qualify for, the payroll system spread that reduction across every paycheck. Your employer isn’t checking whether the estimates are realistic. It applies the numbers you gave it.
Bonuses and RSUs Withheld at 22%
Bonuses, commissions, and equity like restricted stock units are treated as supplemental wages. Employers are allowed to withhold federal income tax on them at a flat 22% rate, regardless of what your W-4 says.3Internal Revenue Service. Publication 15 (2026), (Circular E), Employers Tax Guide – Section: 7. Supplemental Wages That rate is fine if your marginal bracket is close to 22%. It creates a real shortfall if you’re higher.
For 2026, single filers hit the 24% bracket at $105,701 of taxable income and married-filing-jointly filers hit it at $211,401. Anything above those points means every bonus dollar is under-withheld by at least two percentage points. In the 35% bracket, a $20,000 bonus has roughly $2,600 less withheld than the tax actually owed on it.
RSU vesting is where this hits hardest. When RSUs vest, the fair market value on the vesting date is treated as supplemental wage income, and most employers apply the 22% flat rate. Add substantial RSU income to a solid base salary and the gap between 22% withholding and your true marginal rate can produce a five-figure shortfall by December. The correction is to use line 4(c) on your W-4 to add extra withholding from regular paychecks, or to set aside cash from each vesting event to cover the difference.
If a single employer pays you more than $1 million in supplemental wages in a calendar year, withholding on the excess jumps to 37%, and that mandatory rate applies regardless of what your W-4 says.4eCFR. 26 CFR 31.3402(g)-1 – Supplemental Wage Payments
Starting a Job Mid-Year
Starting partway through the year can under-withhold in a way that surprises people. Payroll systems annualize a single paycheck to estimate your full-year income. Start in October at $8,000 a month and the system treats you as a $96,000 earner, spreads that income across the brackets, and withholds accordingly. It has no idea you also earned wages at a previous job earlier in the year, and the two incomes stacked together may push you higher than either job alone would suggest.
If your combined employment across all jobs will be 245 days or fewer for the calendar year, you can ask in writing for your employer to use the “part-year employment method,” which adjusts the calculation for your shorter employment period.5Internal Revenue Service. Publication 15-T (2026), Federal Income Tax Withholding Methods Employers aren’t required to agree, but many will.
Payroll software errors are uncommon but possible. If your pay stub shows withholding that doesn’t line up with what your W-4 should produce, ask payroll to verify how the system is reading your entries.
How to Fix It
Start with the IRS Tax Withholding Estimator at irs.gov. It asks for your income, deductions, credits, and year-to-date withholding, then produces the exact numbers to put on a new W-4.6Internal Revenue Service. Tax Withholding Estimator You’ll want your most recent pay stub and last year’s return handy. The tool can generate a completed W-4 you download and hand to payroll.
Submit the new form through your employer’s HR or payroll portal. Changes usually take effect with the next pay cycle, and there’s no limit on how often you can refile the W-4 during the year.2Internal Revenue Service. Tax Withholding for Individuals
If disclosing a spouse’s income or a second job on Step 2 concerns you, the Estimator solves that too. It can calculate a single dollar figure to enter on line 4(c) instead. Your employer sees only “withhold an extra $X per paycheck” with no indication of where the number came from.7Internal Revenue Service. FAQs on the 2020 Form W-4 The same trick works for non-wage income you’d rather not itemize on line 4(a).
If you’re already late in the year and the shortfall is big enough that a W-4 change alone won’t close it, 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, 2027.8Internal Revenue Service. 2026 Form 1040-ES The January payment can be skipped if you file the full return and pay the balance by February 1, 2027. Payments can go through online, by phone, or by mail.
The Underpayment Penalty and the Safe Harbors
The IRS charges a penalty when you owe too much at filing. It’s calculated as interest on the shortfall for each quarter you were behind, not as a flat fee, and the rate adjusts quarterly. For 2026 it’s 7% in the first quarter and 6% in the second.9Internal Revenue Service. Quarterly Interest Rates10Internal Revenue Service. Internal Revenue Bulletin: 2026-08
No penalty applies if the total tax due after withholding and credits is less than $1,000.11Office of the Law Revision Counsel. 26 USC 6654 – Failure by Individual to Pay Estimated Income Tax For larger shortfalls, there are two safe harbors, and you only have to meet one:
- Current-year test: Your withholding plus estimated payments equal at least 90% of the tax you owe for 2026.
- Prior-year test: Your payments equal at least 100% of the tax on your 2025 return. If your 2025 adjusted gross income was over $150,000 ($75,000 if married filing separately), the threshold rises to 110%.11Office of the Law Revision Counsel. 26 USC 6654 – Failure by Individual to Pay Estimated Income Tax
The prior-year test is the more useful of the two when income spikes unexpectedly, because it lets you base payments on a known number instead of a projection.
Waivers exist for specific circumstances. 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 covers casualties, disasters, and other unusual circumstances.12Internal Revenue Service. Instructions for Form 2210 Taxpayers in federally declared disaster areas generally get automatic penalty relief without filing the form.