Interest on unpaid inheritance tax begins the day after the payment deadline and keeps running until the balance is paid in full. At the federal level, the IRS charges the federal short-term rate plus three percentage points, compounded daily; for the second quarter of 2026, that works out to 6% a year.1Internal Revenue Service. Quarterly Interest Rates The five states that impose a true inheritance tax set their own rates, most around 10% and climbing as high as 18% once collection escalates.
Which Tax Are You Actually Paying
The federal government imposes an estate tax, paid by the estate before assets pass to heirs. A true inheritance tax is paid by the person receiving the assets, and only five states impose one: Kentucky, Maryland, Nebraska, New Jersey, and Pennsylvania. If your bill is federal, the rules below on the 6% rate and daily compounding apply. If it is a state inheritance tax, skip to the state section, because the deadlines and rates are different in every one of those five states.
When the Federal Interest Clock Starts
The federal estate tax return (Form 706) and the full tax payment are both due nine months after the date of death.2Office of the Law Revision Counsel. 26 U.S. Code 6075 – Time for Filing Estate and Gift Tax Returns Interest starts accruing on that original due date. The IRS doesn’t have to send a bill or a notice first. The clock is automatic, there is no grace period, and it runs until the balance is zero.3Internal Revenue Service. Interest
Here’s the trap most executors fall into. Filing Form 4768 gets you an automatic six-month extension to file the return, but it does not extend the time to pay.4Internal Revenue Service. Instructions for Form 4768 So an executor who files late with an extension and pays late still owes interest from the original nine-month deadline, not from the extended filing date. A separate extension of time to pay is possible, but you have to explain why paying on time is impossible or impractical, and the IRS grants it for no more than 12 months at a time.
The Federal Rate and Daily Compounding
The IRS underpayment rate is the federal short-term rate plus three percentage points, recalculated every quarter using the short-term rate from the first month of the prior quarter.5Office of the Law Revision Counsel. 26 U.S. Code 6621 – Determination of Rate of Interest For Q2 2026 the rate is 6%. A debt that spans several quarters will move up or down with those quarterly resets, and the IRS publishes the new rate before each quarter starts.
Federal tax interest compounds daily.6Office of the Law Revision Counsel. 26 U.S. Code 6622 – Interest Compounded Daily Each day the prior day’s interest is added to the balance, and the next day’s interest is computed on that slightly larger number. Over a few months the effect is modest. Over years it is not. At 6% compounded daily, a $500,000 unpaid balance grows to roughly $530,900 after one year and about $563,600 after two. For large estates that spend years negotiating with the IRS, daily compounding adds tens of thousands to the final bill.
Penalties Run Alongside Interest
Interest isn’t the only cost. Two separate penalties can stack on top:
- Failure to file: 5% of the unpaid tax for each month the return is late, capped at 25%. If the return is more than 60 days late, the minimum penalty is the lesser of $525 (for returns due in 2026) or 100% of the tax owed.7Internal Revenue Service. Topic No. 653, IRS Notices and Bills, Penalties and Interest Charges
- Failure to pay: 0.5% of the unpaid tax per month, capped at 25%. If the IRS issues a notice of intent to levy and the tax is still unpaid 10 days later, the rate doubles to 1% per month.8Office of the Law Revision Counsel. 26 USC 6651 – Failure to File Tax Return or to Pay Tax
When both apply in the same month, the failure-to-file penalty is reduced by the failure-to-pay amount, so the combined rate that month is 5% rather than 5.5%. Once the return is filed, failure-to-pay continues alone. Interest, meanwhile, accrues on both the unpaid tax and on accumulated penalties, which is how a manageable shortfall becomes a much bigger problem after a few years of silence.
Reduced Interest for Closely Held Business Estates
Estates where a closely held business makes up more than 35% of the adjusted gross estate can elect to defer the estate tax attributable to that business, postponing the first payment for up to five years and spreading the remainder over up to ten annual installments.9Office of the Law Revision Counsel. 26 USC 6166 – Extension of Time for Payment of Estate Tax Where Estate Consists Largely of Interest in Closely Held Business Interest still runs on the deferred amount, but at a sharply reduced rate.
The law splits the deferred tax in two. The portion attributable to the first $1,000,000 of taxable business value (inflation-adjusted, and above $1,900,000 in recent years) carries a fixed 2% annual interest rate. Anything above that threshold accrues interest at 45% of the normal underpayment rate, which comes out to 2.7% when the underpayment rate is 6%.10Office of the Law Revision Counsel. 26 USC 6601 – Interest on Underpayment, Nonpayment, or Extensions of Time for Payment of Tax One warning: if an installment payment is more than six months late, the estate loses the favorable rate for that payment.
Can the IRS Abate the Interest
Rarely. The IRS can reduce or eliminate interest when an employee’s unreasonable error or delay in a ministerial or managerial act caused it to pile up, or when the agency gave erroneous written advice that led to the underpayment.11Office of the Law Revision Counsel. 26 U.S. Code 6404 – Abatements What does not qualify: the estate ran out of cash, the executor didn’t know the deadline, probate dragged on, or the beneficiaries argued about who would fund the payment. General financial hardship is not a basis for interest relief.
Reasonable cause can support penalty abatement even when it cannot touch interest. Natural disasters, the death or serious illness of the executor, and an inability to obtain records needed to file are the kinds of facts the IRS will consider.12Internal Revenue Service. Penalty Relief for Reasonable Cause Getting a 25% failure-to-file penalty removed matters for interest too, because interest accrues on penalties.
Interest on State Inheritance Tax
Five states impose a true inheritance tax, and each sets its own rules. Payment deadlines run from nine months after death in Pennsylvania and New Jersey to twelve months in Nebraska. Interest rates on late payments generally sit around 10% per year, with escalated collection situations reaching 18%. Those rates can run well above the federal underpayment rate, which makes state inheritance tax debt especially expensive to carry.
Maryland uses an escalating structure worth flagging. An unpaid invoice picks up a 10% penalty plus interest after 30 days, and after 90 days the debt transfers to a central collection unit that can charge interest up to 18%. Some states also add flat penalty fees on top of percentage accrual. Pennsylvania offers a 5% discount for payments made within three months of death, a real reason to pay early rather than wait out the nine-month window.
Maryland is also the only state that imposes both an estate tax and an inheritance tax, which means two separate deadlines and two separate interest calculations on top of the federal return. Treating those obligations as one bucket is a common and quietly expensive mistake.