What is the underpayment penalty and how does the safe harbor protect me?

If you don't pay enough tax during the year through withholding and estimates, the IRS charges an underpayment penalty — essentially interest on what you owed but didn't pay on time. You avoid it by meeting a safe harbor: pay at least 90% of this year's tax, or 100% of last year's total tax (110% if your prior-year AGI exceeded $150,000), whichever is smaller. Hitting a safe harbor means no penalty even if you owe a big balance at filing, as long as it's settled by the deadline. For W-2 earners, the easiest path is setting withholding to cover 100%/110% of last year's tax. Self-employed and variable-income folks often use the prior-year safe harbor because this year's number is hard to predict. Map out your quarterly targets so you stay covered.