Learn
Underpayment interest (§6621 / §6622)
Interest on underpayments compounds daily at quarterly rates published by the IRS.
IRC §6621 sets the underpayment interest rate each quarter (federal short-term rate plus 3 percentage points for most underpayments).
IRC §6622 requires daily compounding. Noticemath uses the published annual underpayment percentages with a 365-day divisor and versioned quarterly tables.
If your date range falls outside the bundled rate table, those days are omitted and flagged — update the JSON table from IRS.gov before relying on the figure.