The IRS sets an interest rate each quarter that is applicable to underpayments by individuals. The rate for the third and fourth quarters of 2020 is 3%. You can see all interest rates listed by the National Association of Tax Professionals at https://bit.ly/3bVcCmw.
Gross income less allowable adjustments, such as IRA, alimony, and Keogh deductions. AGI determines whether various tax benefits are phased out, such as personal exemptions, itemized deductions, and the rental loss allowance and modified adjusted gross income (MAGI).