If you marked “Single” on your W-4 and expected that to cover your taxes, the shortfall usually comes down to two things: the old “claim 0” system no longer exists, and checking “Single” by itself tells your employer to assume you have one job and no other income. When that assumption is wrong, withholding falls short. That is why you still owe taxes even though you claimed 0 and single. The current W-4 dropped withholding allowances years ago, and the boxes that replaced them only work if you complete them.
Allowances Are Gone From the W-4
After the Tax Cuts and Jobs Act of 2017 overhauled the federal tax code, the IRS redesigned the W-4 and removed withholding allowances entirely.1Internal Revenue Service. IRS, Treasury Unveil Proposed W-4 Design for 2020 Allowances were tied to the personal exemption, which the law eliminated. The current form asks about specific dollar amounts instead: dependents, other income, deductions, and any extra withholding you want taken out.
If you check “Single” in Step 1 and leave the rest blank, your employer withholds based on the standard deduction for a single filer with one job and no other income. Nothing else is baked in. Any assumption behind that setting that does not match your life — a second job, a working spouse, freelance income, dividends, a bonus — becomes a gap on your return.
A Second Job or a Working Spouse
Federal income tax is progressive, so higher layers of income are taxed at higher rates. The trouble with two paychecks is that each employer runs its withholding as if theirs is your only income. Each one applies the full standard deduction and starts you over at the bottom of the brackets.2Internal Revenue Service. IRS Releases Tax Inflation Adjustments for Tax Year 2026
Say you earn $40,000 at one job and $35,000 at another. Both employers withhold as if you’re a modest earner in the 12% bracket. Your combined $75,000, though, pushes part of your taxable income into the 22% bracket. Two employers under-withhold on every check, and the shortfall lands on your return.
The same thing happens to married couples filing jointly when both spouses work. Each spouse’s payroll assumes theirs is the household’s only paycheck, and the combined income lands in a higher bracket than either employer planned for.
The fix lives in Step 2 of the W-4. The simplest option is the checkbox in Step 2(c), which you check on the W-4 for both jobs. When that box is checked, the employer cuts the standard deduction and bracket ranges in half for its calculation, which raises withholding per paycheck.3Internal Revenue Service. Form W-4, Employees Withholding Certificate That works best when the two jobs pay similar amounts. If one job pays far more than the other, the Multiple Jobs Worksheet in Step 2(b) or the IRS Tax Withholding Estimator at irs.gov/W4App gives a more accurate answer.
Freelance, Gig, and Investment Income
Money you earn outside a traditional job rarely has any tax withheld. A client who pays you $600 or more sends a Form 1099-NEC, but that form only reports the income; nothing is taken out.4Internal Revenue Service. About Form 1099-NEC, Nonemployee Compensation Even a maxed-out W-2 withholding rarely covers what you owe on side earnings.
Self-employment income also carries a 15.3% self-employment tax: 12.4% for Social Security on net earnings up to $184,500 in 2026, and 2.9% for Medicare with no cap.5Office of the Law Revision Counsel. 26 USC 1401 – Rate of Tax As a W-2 employee, your employer pays half of the payroll taxes. On self-employment income you pay both halves. You can deduct half of the self-employment tax as an adjustment to income, which lowers the income subject to regular tax.6Office of the Law Revision Counsel. 26 USC 164 – Taxes
Investment income has the same problem. Interest, dividends, and capital gains generally have nothing withheld unless you failed to provide a valid taxpayer identification number, in which case a 24% backup withholding rate applies.7Internal Revenue Service. Instructions for the Requester of Form W-9 A strong year in the market or a large interest payment lifts your total taxable income, and your paycheck withholding has no way to catch up.
Bonuses Are Often Withheld at 22%
Bonuses, commissions, and other supplemental wages are commonly withheld at a flat 22% (or 37% once your supplemental wages pass $1 million in a year).8Internal Revenue Service. Publication 15 (2026), (Circular E), Employers Tax Guide That flat rate is convenient for payroll, but it creates a shortfall if your regular income already puts you in the 24% or 32% bracket. A $10,000 bonus withheld at 22% when your marginal rate is 32% is roughly $1,000 short on that one payment.
A Life Change That Raised Your Bill
Personal changes can move your tax picture without touching your paycheck. A common one is the Child Tax Credit, worth up to $2,200 per qualifying child under 17 for 2026.9Internal Revenue Service. Child Tax Credit When a child turns 17 or stops qualifying, that credit disappears, but withholding does not automatically rise to replace it. A family that loses a $2,200 credit without updating the W-4 can owe that much or more.
Other events do the same thing quietly: getting divorced (you lose the wider joint brackets), paying off a mortgage (you may drop below the itemizing threshold), or a spouse starting or stopping work. None of these prompt a change to your payroll.
Retirement Distributions and Social Security
Withdrawals from a 401(k), 403(b), or traditional IRA are taxed as ordinary income, but the default withholding varies. For eligible rollover distributions taken as cash, the default is 20% and you cannot elect lower.10Internal Revenue Service. 2026 Form W-4R, Withholding Certificate for Nonperiodic Payments and Eligible Rollover Distributions For other nonperiodic distributions the default is often 10%, which can be well below your actual rate.
Social Security benefits can push a bill higher too. Depending on your combined income (roughly your adjusted gross income plus half of your benefits), up to 85% of your benefits can be subject to federal income tax. For single filers, the taxation begins at relatively low combined-income thresholds. Unless you elect voluntary withholding on Form W-4V, none of it is taken out during the year.
The Additional Medicare Tax at Higher Incomes
Single filers owe a 0.9% Additional Medicare Tax on wages, self-employment income, or both above $200,000 in a calendar year.11Internal Revenue Service. Topic No. 560, Additional Medicare Tax Your employer starts withholding it once your wages there cross $200,000, but that threshold does not account for self-employment income or wages from another job. Combined income above $200,000 from multiple sources can trigger the tax even when no single employer began withholding for it.
How to Fix Your Withholding
The most direct fix is a new W-4. Line 4(c) lets you request a specific dollar amount of extra withholding per pay period.3Internal Revenue Service. Form W-4, Employees Withholding Certificate If last year’s return showed you were $2,400 short and you’re paid biweekly, entering $100 on line 4(c) closes that gap. This works well for a predictable shortfall like a working spouse or a known amount of side income.
For a more precise number, the IRS Tax Withholding Estimator walks you through your income, deductions, credits, and current withholding and recommends a specific adjustment. It accounts for multiple jobs, self-employment income, and other factors the basic W-4 instructions do not fully capture. Running it once or twice a year, especially after a life change, is the most reliable way to avoid both a large tax bill and an unnecessarily large refund.
If a big share of your income has no withholding at all (freelance work, investments, retirement distributions), quarterly estimated payments are the right tool. You can pay through IRS Direct Pay, the Electronic Federal Tax Payment System, or a mailed voucher from Form 1040-ES. The deadlines fall on April 15, June 15, September 15, and January 15 of the following year.12Internal Revenue Service. Individuals 2
Avoiding the Underpayment Penalty
When the gap between what you paid during the year and what you actually owe grows large enough, the IRS adds an underpayment penalty on top of the tax. You avoid the penalty by meeting one of two safe harbor tests:13Internal Revenue Service. 2026 Form 1040-ES, Estimated Tax for Individuals
- Your total withholding and estimated payments equal at least 90% of the tax on your current-year return.
- Your total payments equal at least 100% of the tax on your prior-year return (110% if your prior-year adjusted gross income exceeded $150,000, or $75,000 if married filing separately).
You also avoid the penalty if your total balance due after withholding and credits is under $1,000.14Internal Revenue Service. Underpayment of Estimated Tax by Individuals Penalty Splitting an estimated tax obligation across all four quarterly deadlines, rather than waiting until the end of the year, is what keeps you inside a safe harbor.