How Does the IRS Calculate Penalties and Interest?

The IRS calculates penalties and interest as separate charges layered on top of your unpaid tax. Penalties are set percentages: 5% per month for filing late, 0.5% per month for paying late, 20% for inaccurate returns, and 75% for fraud. Interest is the federal short-term rate plus three percentage points, adjusted each quarter and compounded daily. For the first quarter of 2026, that rate is 7% for individual taxpayers.¹ The charges begin the day after your deadline and keep running until the balance is paid.

The Failure-to-File Penalty

If you don’t file your return by the deadline (including any extension), the IRS charges 5% of your unpaid tax for each month or partial month the return is late.² Partial month is literal: one day into a new month triggers the full 5% for that period. The penalty caps at 25% of the unpaid balance, which it reaches after five months.

If your return is more than 60 days late, a minimum penalty applies. For returns due after December 31, 2025, that minimum is the lesser of $525 or 100% of the tax you owe.³ The dollar amount adjusts periodically for inflation.² If your return shows no tax due or you’re owed a refund, this penalty does not apply.

The Failure-to-Pay Penalty

When you file on time but don’t pay in full, the IRS charges 0.5% of the unpaid tax for each month or partial month the balance sits.² Like the filing penalty, it caps at 25% of the unpaid amount, but reaching that ceiling takes roughly 50 months.

The rate drops to 0.25% per month if you filed on time and have an approved installment agreement in place.⁴ That reduction matters if you need years to clear the balance.

When Both Penalties Hit the Same Month

If you owe the filing and payment penalties for the same month, the IRS reduces the 5% filing charge by the 0.5% payment charge. The combined rate stays at 5% rather than climbing to 5.5%.² Once the failure-to-file penalty tops out at 25% after five months, the 0.5% failure-to-pay charge keeps growing on its own until it reaches its own 25% cap. In a worst case where you never file and never pay, combined penalties can reach 47.5% of the tax you owed: 22.5% for late filing (the 25% cap minus the offset) plus 25% for late payment.

What an Extension Actually Changes

Filing Form 4868 for an automatic extension gives individuals until October 15 to submit the return and eliminates the failure-to-file penalty during the extension window. It does not extend your time to pay.⁵ If you owe, the failure-to-pay penalty and interest still start accruing on the original April deadline.

The distinction matters because the filing penalty is ten times the payment penalty. An extension knocks out the larger charge while you gather documents, but paying as much as you can by the original due date still saves you money on the smaller penalty and on interest.

The Estimated Tax Underpayment Penalty

If you have income that isn’t withheld (self-employment earnings, investment gains, rental income), you’re expected to make quarterly estimated payments on April 15, June 15, September 15, and January 15 of the following year.⁶ The IRS looks at each quarter on its own, so a large fourth-quarter payment does not cure a shortfall from the first quarter.⁷

Two safe harbors avoid the penalty: pay at least 90% of the current year’s tax, or 100% of the tax shown on last year’s return. If your prior-year adjusted gross income exceeded $150,000 ($75,000 if married filing separately), the prior-year safe harbor rises to 110%.⁸

This penalty is not a flat monthly rate. The IRS applies the underpayment interest rate to each quarter’s shortfall for the time it stayed unpaid.⁷ And the daily compounding that governs other tax interest does not apply here. Estimated tax underpayment uses simple interest.⁹

Accuracy Penalties and Fraud

Beyond timing, the IRS penalizes errors in what you reported. The accuracy-related penalty is 20% of the underpayment attributable to negligence, careless disregard of tax rules, or a substantial understatement of income tax.¹⁰ A substantial understatement means your reported tax fell short by the greater of 10% of the correct tax or $5,000. If you claimed the qualified business income deduction under Section 199A, the 10% threshold drops to 5%.

The rate doubles to 40% for gross valuation misstatements, where deductions or income tied to property values are significantly off.¹⁰

If any part of the underpayment is due to fraud, the penalty jumps to 75% of the fraudulent portion.¹¹ The IRS carries the burden of proving fraud. When it does, the 75% penalty replaces the 20% accuracy penalty on that portion rather than stacking on top of it.

How Interest Runs

Interest is separate from penalties and follows its own rules. The IRS charges interest on any unpaid tax from the original due date until the balance is paid.¹² Interest also runs on assessed penalties, so the penalties themselves generate more interest as they age.¹³

For individual underpayments, the rate is the federal short-term rate plus three percentage points.¹⁴ The IRS resets it every calendar quarter. In the first quarter of 2026, individuals are charged 7%.¹

Penalties cap at 25%. Interest does not. It also compounds daily, meaning each day’s interest is added to the running balance so the next day’s interest is calculated on that larger number.⁹ The effective annual cost runs slightly above the stated quarterly rate. Over months or years, the layering of interest on the tax, interest on the penalties, and interest on earlier interest can push a balance well past what you originally owed.

Getting Penalties Reduced or Removed

The IRS can waive failure-to-file and failure-to-pay penalties through two main routes. Neither route removes interest, which continues to accrue regardless of whether penalties are abated.

First Time Abate

If you have a clean compliance record, the First Time Abate waiver removes the penalty for a single tax period. You qualify if you filed the same type of return on time (or on valid extension) for the three years before the penalty year and received no penalties in those three years, or any penalty you did receive was removed for a reason other than First Time Abate.¹⁵ You can request it before you’ve paid the tax in full, though the failure-to-pay penalty keeps growing until the balance is cleared.

Reasonable Cause

If First Time Abate isn’t available, you can ask the IRS to remove a penalty by showing reasonable cause: you exercised ordinary care and prudence but still couldn’t meet the deadline. Accepted circumstances include fire or natural disaster, serious illness or death of the taxpayer or an immediate family member, inability to obtain necessary records, and system failures that blocked timely electronic filing.¹⁶

Some common arguments generally fail. Relying on a tax professional who erred, not knowing about a filing requirement, and simple lack of funds are not typically accepted as reasonable cause on their own.¹⁶

1
Internal Revenue Service. Quarterly Interest Rates
2
Office of the Law Revision Counsel. 26 U.S. Code 6651 – Failure to File Tax Return or to Pay Tax
3
Internal Revenue Service. Failure to File Penalty
4
Internal Revenue Service. Failure to Pay Penalty
5
Internal Revenue Service. IRS Reminds Taxpayers an Extension to File Is Not an Extension to Pay Taxes
6
Internal Revenue Service. Estimated Tax
7
Office of the Law Revision Counsel. 26 U.S. Code 6654 – Failure by Individual to Pay Estimated Income Tax
8
Internal Revenue Service. 2026 Form 1040-ES – Estimated Tax for Individuals
9
Office of the Law Revision Counsel. 26 U.S. Code 6622 – Interest Compounded Daily
10
Office of the Law Revision Counsel. 26 U.S. Code 6662 – Imposition of Accuracy-Related Penalty on Underpayments
11
Office of the Law Revision Counsel. 26 U.S. Code 6663 – Imposition of Fraud Penalty
12
Office of the Law Revision Counsel. 26 U.S. Code 6601 – Interest on Underpayment, Nonpayment, or Extensions of Time for Payment, of Tax
13
Internal Revenue Service. Interest
14
Office of the Law Revision Counsel. 26 U.S. Code 6621 – Determination of Rate of Interest
15
Internal Revenue Service. Administrative Penalty Relief
16
Internal Revenue Service. Penalty Relief for Reasonable Cause