Five percent of unpaid tax for each month or part of a month a return is late, up to 25 percent, with a minimum after 60 days.
Section 6651(a)(1). Reduced by the failure-to-pay penalty in months where both apply. The minimum penalty for returns due in 2026 is the smaller of $525 or 100 percent of the unpaid tax. Stopped by filing; removable by first-time abatement or reasonable cause. Full page: failure to file.