Revenue Ruling 2021-17 provides interest rates for underpayments and overpayments. The rates for interest determined under Section 6621 of the code for the calendar quarter beginning October 1, 2021, will be 3 percent for overpayments (2 percent in the case of a corporation), 3 percent for underpayments, and 5 percent for large corporate underpayments. The rate of interest paid on the portion of a corporate overpayment exceeding $10,000 will be 0.5 percent.

Revenue Ruling 2021-17 will be in IRB:  2021-37, dated September 13, 2021.