The IRS has announced (Rev. Rul. 2020-18) the interest rates for overpayments and underpayments for the last three months of 2020. These rates are unchanged for the third quarter.
For most taxpayers, the rate for underpayments and overpayments will be 3%. This will also be the rate charged in computing estimated tax underpayments for the quarter.
For "large corporate underpayments," the IRS will charge a 5% rate for the quarter. These are C corporation underpayments in excess of $100,000.
Past IRS interest rates may be found here.