When you start a job, your employer hands you a W-4 form. It looks boring. It kind of is. But what it does is really important: it tells your employer exactly how much federal income tax to pull out of each paycheck before the money ever hits your bank account.
That process is called withholding. The idea is that instead of owing the government a giant lump sum every April, a little bit gets taken out of each check automatically throughout the year. The W-4 is what controls how much gets taken.
Note: If you earn a very small amount — for example, from a part-time or seasonal job — you may not owe any federal income tax at all. If that’s your situation, you can check the IRS Tax Withholding Estimator to confirm whether withholding applies to you.
Here’s where people get into trouble. According to the IRS, if you work multiple jobs at the same time — or you and a spouse both work — your withholding needs to reflect that. If it doesn’t, you’ll very likely owe additional tax when you file your return. You might even owe penalties on top of that.
Same thing if you have income coming in from somewhere other than a regular job — like freelance work or investment income. That stuff usually doesn’t have taxes withheld automatically, so if you don’t adjust your W-4 to account for it, you’re setting yourself up for a surprise bill in April.
On the flip side, the IRS also notes that entering a value higher than what the estimator recommends in Step 3 (the credits step) of your W-4 can also cause you to owe taxes when you file. It works both ways — too little withheld and you owe, but overclaiming credits can trip you up too.
The fix is pretty simple. The IRS has a free Tax Withholding Estimator tool that walks you through your situation and tells you what to put on your W-4. You can also update your W-4 at any time — you’re not locked in forever just because you filled it out on day one of a job. If your life changes (new job, second job, big income change), it’s worth revisiting.