The estimated tax penalty (and how to avoid it)

If you owe taxes at the end of the year, you might also owe an extra penalty for underpayment during the year. The IRS wants its money in the same year you earned the income, not 12 months later. The penalty is an interest charge on what you should have paid quarterly — and it's charged on top of the tax you owe.

The safe harbor rules

You avoid the penalty if you paid, during the year, at least the smaller of: (1) 90% of this year's actual tax liability, or (2) 100% of last year's total tax liability (110% if you're a higher earner making over $150k). Hit either of those numbers across your withholding + estimated payments and the IRS leaves you alone, even if you owe a lot in April.

Why 'last year's tax' is the hack

Using 100% of last year's tax liability as your target is almost always the easier number to hit. You know it at the start of the year — it's printed on your 1040 from last year. Divide by 4, pay that much each quarter via estimated payments or W-4 withholding, and the penalty can't touch you regardless of how much more you earn this year.

Fixing underpayment mid-year

If you realize partway through the year that you're underpaid, the best fix is usually to increase withholding from your W-2 job rather than making a big estimated payment. Withholding is treated as 'evenly paid' throughout the year for penalty purposes, regardless of when you actually withheld it. A big December W-4 increase can retroactively satisfy underpayment penalties from earlier quarters.