Underpayment penalty
Also: underpayment penalty, Form 2210 penalty, estimated tax penalty
A federal (and state) charge imposed when a taxpayer's withholding plus estimated payments falls short of the safe harbor. Computed as interest on each quarter's shortfall, currently 8% annualized federal.
The underpayment penalty is effectively interest on unpaid tax for the period between the quarterly due date and the later date the shortfall is funded. The IRS computes it quarterly on Form 2210 at the short-term AFR plus 3%. As of 2025 the federal rate is 8% annualized. States apply their own rates, often 4% to 8%. The penalty is not a fine: it is not deductible, and it accrues on each quarter’s shortfall independently, so a January 15 catch-up payment does not cure a Q1 underpayment retroactively.
Example: a filer owes $600,000 in 2025 tax and has $400,000 of withholding. She makes no estimated payments during the year. Even if she pays the $200,000 balance with her return in April, the IRS charges underpayment penalty from each missed quarterly date. At 8% annualized, with the shortfall spread across four quarters, the penalty totals roughly $11,000.
Common mistake: using the “annualized income installment” method on Form 2210 for equity without keeping workpapers. The method reduces the penalty for unevenly earned income but requires quarterly income tracking.
Underpayment penalties matter any year with large uneven equity income, out-of-state income, or a significant rise above prior-year tax.