The tax withheld from your paycheck is not your tax. It is your employer's estimate of your tax, computed from a form you filled out, using tables that know nothing about the rest of your financial life.
Your actual liability is determined once a year, on your return, from your total income, deductions, and credits. The refund or balance due is simply the difference between the estimate and the answer.
How the estimate gets made
The United States runs a pay-as-you-go tax system, so tax is collected throughout the year rather than in one payment. For employees that happens through withholding.
Your employer takes the information on your Form W-4 — filing status, dependents, other income, deductions — and applies the IRS withholding tables to each paycheck as though that paycheck's annualized rate were your whole year.
That last assumption is the source of most mismatches. Payroll has no idea that you have a second job, that your spouse works, that you sold stock in March, or that you will get a large bonus in December.
The multiple-income problem
This is the single most common reason people owe unexpectedly.
Each employer withholds as if its wages were your only income, so each one applies the low brackets and the full standard deduction to its own portion. Two jobs at $50,000 each produce withholding appropriate to two $50,000 earners, when your actual situation is one $100,000 earner facing higher marginal rates.
The same applies to a married couple filing jointly with two incomes. Each employer sees only its half.
The current Form W-4 addresses this directly with a multiple-jobs step, and the IRS Tax Withholding Estimator handles it well. Ignoring it reliably produces a bill.
Income with no withholding at all
Freelance and contract income arrives gross, with nothing withheld and self-employment tax owed on top. Investment income — dividends, interest, capital gains — typically has no withholding. Rental income, none. Retirement account withdrawals may have a default rate that is far from your actual bracket.
For these, the system expects quarterly estimated tax payments. Skipping them means owing the full amount at filing plus an underpayment penalty.
The bonus withholding trap
Supplemental wages — bonuses, commissions, severance — are commonly withheld using the flat percentage method: 22% federal on amounts up to $1 million in a year, 37% on the portion above.
That 22% is a withholding convention, not a tax rate. For someone in the 12% bracket it over-withholds, and the excess comes back as refund. For someone in the 24%, 32%, or 35% bracket it under-withholds, and the shortfall appears as a balance due.
A $20,000 bonus for someone whose marginal rate is 32% is under-withheld by roughly $2,000 federally on that bonus alone, before state tax.
The penalty, and the safe harbors
The IRS can charge an underpayment penalty when too little was paid during the year. It is computed like interest on the shortfall for each period it was outstanding, so paying in full by the April deadline does not erase it.
There are safe harbors. You generally avoid the penalty if your withholding and estimated payments totaled at least 90% of the current year's tax, or at least 100% of the prior year's total tax — 110% if your prior-year adjusted gross income exceeded $150,000.
The prior-year safe harbor is the practical one for anyone with volatile income: it is a known number, available in January, and satisfying it removes the penalty risk regardless of how the current year turns out.
What to aim for
A small refund or a small balance due means your withholding was well calibrated. A very large refund means you routed money to the government interest-free all year. A large balance due means you kept money you owed, possibly with a penalty attached.
The reasonable target is landing within a few hundred dollars in either direction, which is achievable with one W-4 review a year.
That said, there is an honest argument for over-withholding: if a refund is the only way a household reliably saves, the forgone interest is a small price for a mechanism that works. The arithmetic argument and the behavioral argument point different directions, and both are legitimate.