Tax Extension Underpayment Calculator 2026

Q: Does filing a tax extension stop interest from accruing?

No. Filing Form 4868 extends the deadline to file your return to October 15, but it does not extend the deadline to pay. IRS interest begins accruing on any unpaid balance from April 15, the original due date, regardless of extension status.

Q: Does the 90% safe harbor eliminate all penalties?

The 90% safe harbor eliminates only the failure-to-pay penalty, and only if two conditions are both met: you paid at least 90% of your total 2025 tax liability by April 15, and you pay the remaining balance by October 15, 2026 (the extended filing deadline). If the remaining balance is not paid by October 15, the FTP penalty applies to the entire period. IRS interest at the current rate (6% for Q2 2026) always accrues on the unpaid balance from April 15 until you pay -- safe harbor does not eliminate interest.

Q: How is IRS underpayment interest calculated?

IRS underpayment interest is the federal short-term rate plus 3 percentage points, compounded daily. For Q2 2026, the rate is 6% annually (3% federal short-term rate + 3%), per IRB 2026-08. For Q1 2026, the rate was 7%. The daily rate is the annual rate divided by 365, compounded each day the balance remains unpaid.

Item	Amount
Unpaid balance as of April 15	-
Days from April 15 to payment date	-
Daily interest rate (6% / 365)	0.01644%
IRS interest accrued	-
Whole months late (FTP)	-
Failure-to-pay penalty (0.5%/month)	-
Total extra cost (interest + FTP)	-
Total owed on payment date	-

Direct Answer

If you filed Form 4868 but did not pay all tax by April 15, you owe interest (currently 6% annually for Q2 2026) plus a 0.5%/month failure-to-pay penalty on the unpaid balance. Both accrue from April 15. The FTP penalty is waived only if two conditions are both met: you paid at least 90% of your total tax by April 15, AND you pay the remaining balance by October 15, 2026. Interest always runs regardless of safe harbor status. Interest and the FTP penalty stop when you pay in full.

Key Takeaways

Form 4868 extends the filing deadline to October 15 - it does not extend the payment deadline. April 15 remains the payment due date.
IRS interest accrues at 6% annually (Q2 2026, compounded daily) on any unpaid balance from April 15 regardless of extension status.
The failure-to-pay penalty is 0.5% per month on the unpaid balance, up to a maximum of 25%, but is waived if you paid at least 90% of your total tax by April 15 AND pay the remaining balance by October 15, 2026. Both conditions are required.
Paying the full balance as soon as possible stops both interest and FTP penalty from continuing to accrue - the cost grows with every passing month.

Written by Munib Ur Rehman · Reviewed by Nausheen Shahid (LMN Tax Inc.) · Updated April 17, 2026

Practitioner Insight

Extension buys time to FILE, not time to PAY. This is the most common extension misconception. Interest runs from April 15 regardless of extension status. If you filed Form 4868 expecting to pay later without cost, the meter started April 15. At 6% annually, a $5,000 unpaid balance costs roughly $250 in interest over six months - plus the FTP penalty if the 90% threshold was not met. Pay as much as possible before or on April 15 to reduce the accruing balance.

Quick Facts - Extension Underpayment Costs

Item	Rate / Deadline	Source
Underpayment interest rate - Q2 2026	6% annually (compounded daily)	IRB 2026-08
Failure-to-pay (FTP) penalty rate	0.5% per month, max 25%	IRC § 6651(a)(2)
Extension filing deadline (individuals)	October 15, 2026	Form 4868 Instructions

Real-World Scenario: $12,000 Total Tax, $9,500 Paid by April 15

A taxpayer owes $12,000 for tax year 2025. They had $9,500 withheld and paid in estimated payments by April 15. They filed Form 4868 and plan to pay the remaining $2,500 on October 15, 2026 - exactly 183 days after April 15.

Step 1 - Check the 90% safe harbor: $9,500 / $12,000 = 79.2%. This is below 90% ($10,800 threshold), so the FTP penalty applies.

Unpaid balance (April 15)$2,500.00
Days unpaid (Apr 15 to Oct 15)183 days
Interest: $2,500 x (6% / 365) x 183$75.25
Whole months late: 6 complete months6
FTP penalty: $2,500 x 0.5% x 6$75.00
Total extra cost (interest + FTP)$150.25
Total owed on October 15$2,650.25

If this taxpayer had paid $10,800 (90%) by April 15, the FTP penalty would be eliminated, reducing extra cost to $75.25 in interest only.

How the Calculation Works

The IRS charges two separate costs when a tax balance is unpaid after April 15 - even with a filed extension.

Underpayment interest - The federal short-term rate plus 3 percentage points, compounded daily. For Q2 2026 (April 1 - June 30), the rate is 6%. For Q1 2026 (January 1 - March 31), it was 7%. Interest accrues every day until the balance is paid in full.
Failure-to-pay (FTP) penalty - 0.5% of the unpaid amount for each month or part of a month the balance remains unpaid after the original due date (April 15). This is waived if two conditions are both met: you paid at least 90% of your total tax by April 15 (90% safe harbor), AND you pay the remaining balance by October 15, 2026. If the remaining balance is not paid by October 15, the FTP penalty applies. The FTP penalty cannot exceed 25% of the unpaid tax.
Failure-to-file (FTF) penalty - This 5%/month penalty does NOT apply if you filed Form 4868 on time. It is $0 in this scenario.

The calculator uses 6% flat for the entire period as an approximation. If your payment date spans both Q1 and Q2 2026, the actual blended interest will be slightly higher than shown, because Q1 used the 7% rate.

When This Calculator Is Not Exact

Safe harbor eliminates the FTP penalty - but interest still accrues regardless. Even a $0 FTP result does not mean zero cost for paying late.
This calculator uses 6% (Q2 2026) as a flat rate across the full accrual period. Actual IRS daily compounding produces a slightly higher result than simple multiplication, and the Q1 2026 rate was 7% for any days before April 1.
If the IRS has assessed additional penalties (such as a levy, installment agreement failure, or fraud penalty), your actual balance will be higher. This calculator covers only standard extension underpayment costs.
State-level late payment interest and penalties are not included. Most states impose their own underpayment charges independent of IRS charges.

What to Do If You Owe After Extension

Decision Step

You filed Form 4868 and you owe a balance. You have three main paths. Choose based on your ability to pay now.

Path A

Pay in Full Now

The fastest way to stop interest and FTP from growing. Pay online at IRS Direct Pay (irs.gov/directpay) or by check with a payment voucher. Interest and FTP stop accruing on the date the IRS receives payment. This is the lowest-cost option if funds are available.

Path B

Set Up an Installment Plan

If you cannot pay in full, apply for an IRS installment agreement. The FTP penalty rate drops from 0.5% to 0.25% per month while your plan is active. Interest continues at the standard rate. See what happens if you miss a tax deadline for installment plan details and how to apply.

Path C

Request Penalty Abatement

After paying in full, first-time penalty abatement (FTA) may eliminate the FTP penalty if you had no penalties in the prior three years. Submit Form 843 or call the IRS after payment. Also compare with estimated tax penalty calculator if you also had quarterly underpayments during the year.

Frequently Asked Questions

Does filing a tax extension stop interest from accruing?

No. Form 4868 gives you extra time to file your return - it does not extend the time to pay. IRS interest begins accruing on any unpaid balance from April 15, the original due date, regardless of whether an extension was filed.

What is the penalty for not paying by April 15 if I filed an extension?

If you filed a tax extension but did not pay at least 90% of your total tax by April 15, the IRS charges a failure-to-pay (FTP) penalty of 0.5% per month on the unpaid balance, up to 25%. In addition, underpayment interest at 6% annually (Q2 2026) accrues from April 15 until the balance is paid in full.

Does the 90% safe harbor eliminate all penalties?

No - only the failure-to-pay penalty, and only if both conditions are met: (1) you paid at least 90% of your total 2025 tax liability by April 15, and (2) you pay the remaining balance by October 15, 2026. If the remaining balance is not paid by October 15, the FTP penalty applies to the full period. IRS interest at 6% (Q2 2026) always accrues on the unpaid balance from April 15 until you pay in full - safe harbor does not eliminate interest.

How is IRS underpayment interest calculated?

IRS underpayment interest equals the federal short-term rate plus 3 percentage points, compounded daily. For Q2 2026, the rate is 6% (3% federal short-term rate + 3%), per IRB 2026-08. The daily rate is 6% divided by 365. For Q1 2026, the rate was 7% per IRB 2025-48. The IRS adjusts the rate quarterly based on federal short-term rate changes.

What if I can't pay my full balance by October 15?

If you cannot pay in full by the October 15 extension deadline, you can apply for an IRS installment agreement (payment plan). The FTP penalty continues at 0.5% per month until paid, but drops to 0.25% per month while an active installment agreement is in place. Interest at the standard quarterly rate continues to accrue until the full balance is paid.

Is there a failure-to-file penalty if I filed an extension?

No. The failure-to-file (FTF) penalty of 5% per month does not apply if you filed Form 4868 by April 15 and submit your return by October 15. Only the failure-to-pay penalty (if applicable) and interest apply to any unpaid balance when an extension is filed on time.

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