In 365 days (12 months), interest grows by $1,812. 50, and the total tax owed grows to $26,812. The numbers are even higher when failure-to-pay penalties are stacked on top of the interest rate. On november 13, 2025, the irs announced that the interest rates charged on tax underpayments (and paid on overpayments) will remain unchanged for the calendar quarter beginning january 1, 2026.