IRS interest calculator
Estimates the interest the IRS charges on late/underpaid tax — or pays on your refund — using the real method: the quarterly §6621 rate in force each day, compounded daily under §6622, across every rate change on record (back to 2017).
Estimate for reference only — courts, agencies and creditors may apply different rounding or conventions. Not legal, tax, or financial advice.
How it works
IRS interest rates reset every calendar quarter — the underpayment rate is currently 7% — and interest compounds daily: each day's interest is added to the balance before the next day's is computed. This calculator walks day-by-day through your date range, applying the correct quarter's rate (365- or 366-day years handled), which matters whenever a balance spans a rate change.
Not covered here: penalties (failure-to-file, failure-to-pay — separate from interest), and the estimated-tax additions under §§6654/6655, which use the underpayment rate but as simple interest. The IRS computes official amounts with its Rev. Proc. 95-17 factor tables; small rounding differences are possible.