As of Oct 1, 2023, the IRS interest rate is 8% annually for taxes owned and not withheld or paid via estimated payments. That increase from 3% 2 years ago reflects the rising interest rate environment. Taxpayers need to have paid at least 90% of what they owe in the current year, or 100% of what they owed in the previous year (110% if their previous year AGI was >$150k). Remember that estimated tax payment dates for 2024 taxes are April 15, 2024, June 17, 2024, September 16, 2024, and Jan 15, 2025.