How does the IRS calculate interest on unpaid taxes?

The IRS calculates interest based on the federal short-term rate plus 3 percent. Interest is compounded daily and applies to any unpaid tax from the due date of the return until the date of payment.

What is the penalty for filing my taxes late?

The failure-to-file penalty is usually 5% of the unpaid taxes for each month or part of a month that a tax return is late. This penalty caps at 25% of your unpaid taxes.

Is there a penalty if I file on time but cannot pay?

Yes, the failure-to-pay penalty is 0.5% of the unpaid taxes for each month or part of a month the tax remains unpaid, up to a maximum of 25%.

Can I use an IRS penalty calculator for state taxes?

This calculator is specifically designed for federal IRS penalties and interest. State penalty rates vary significantly by jurisdiction and require a state-specific calculation.

2025 IRS Penalty & Interest Calculator | Karla CPA

⚖️ 2025 Tax Year

IRS Penalty & Interest Calculator

Calculate your exact failure-to-file & failure-to-pay penalties and daily-compounded interest if you filed a Form 4868 extension and are paying your 2025 tax bill after April 15, 2026.

Tax Information

Total Tax Liability (Line 24) ?Your total 2025 federal income tax from Line 24 of Form 1040.

Amount Paid by April 15, 2026 ?All withholding + estimated payments + any extension payment made on or before 4/15/2026.

✓ ≥90% paid by April 15 — FTP penalty waived under extension safe harbor

90% safe-harbor only applies with a timely extension.

Final Payment Date

Extension deadline: October 15, 2026

Filed a timely Form 4868 extension

Extension grants extra time to file, not to pay. Unchecking activates the failure-to-file penalty (5%/mo).

IRS Quarterly Rates §6621

Editable

Q2 confirmed at 6% (Rev. Rul. 2026-5). Q3/Q4 assumed same until announced.

Q2 2026 (Apr–Jun)

Q3 2026 (Jul–Sep)

Q4 2026 (Oct–Dec)

Payments After April 15

Each payment reduces the principal on that date, cutting future interest.

No additional payments added.

Total Due on Payment Date

$0.00

Extra Cost (P&I)

Unpaid at 4/15

Failure-to-File Penalty (IRC §6651)

$0.00

Failure-to-Pay Penalty (IRC §6651)

$0.00

Interest on Tax (IRC §6622)

$0.00

Interest on Penalties (IRC §6601)

$0.00

Total Penalties & Interest

$0.00

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Quarter-by-Quarter Interest Breakdown

Period	Days	Rate	Opening	Interest	Closing

Month-by-Month Penalty Interest Detail

Month	Assessed	Amount	Type	Days	Interest

About This IRS Penalty Calculator

This free tool calculates the IRS failure-to-file penalty, failure-to-pay penalty under IRC §6651, and daily-compounded underpayment interest under IRC §6622 for individuals paying their 2025 Form 1040 balance after April 15, 2026.

The Q2 2026 individual underpayment rate of 6% is confirmed per Rev. Rul. 2026-5, published in IRS Internal Revenue Bulletin 2026-8 on February 17, 2026. Q3 and Q4 2026 IRS underpayment rates are assumed at 6%; official rates are announced quarterly. Results may differ from IRS notices due to rounding, additional penalties, or rate changes.

Have questions about your specific tax situation? Contact us for personalized guidance.

DISCLAIMER: This calculator is provided for estimation purposes only and does not constitute professional tax advice.

IRS Penalty & Interest Calculator

About This IRS Penalty Calculator

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