The short answer
Usually because less tax was taken out during the year than you actually owed. Common causes are a second job, side or gig income, a new W-4, retirement withdrawals, or losing a credit you used to get.
Last reviewed: October 2026
The most common reasons
- Two jobs, or both spouses working. Each employer withholds as if it's your only job.
- Gig or side income. Apps don't withhold tax, and there's self-employment tax on top.
- A new W-4, or one that was never updated after a life change.
- Unemployment or retirement withdrawals with little or no tax withheld.
- Lost credits. A child turned 17, moved out or aged out of a credit.
- More investment income, like interest, dividends or a sale.
How to keep it from happening next year
Check your withholding now, not in April. See How do I fix my withholding so I don't owe? If you have gig income, see how much to set aside.
Owe more than you can pay right now? See I can't pay what I owe.
Official source: IRS: Tax Withholding Estimator.
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Disclaimer: This page is general information, not tax advice. Every situation is different, and tax rules change. Reading this page doesn't make you a client.