Form 4868 Extension Calculator 2026 β Filing Extension and Penalty Simulator
Calculate what to pay with Form 4868 automatic 6-month extension. Estimate failure-to-pay penalty and interest if you pay late, with 90% and safe harbor safe tests.
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Your best estimate of total federal income tax owed for 2026 $
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Total quarterly estimated payments (Form 1040-ES) $
For safe harbor calculation (100%/110% of 2025 tax) $
Determines 100% vs 110% safe harbor threshold ($150K AGI) %
Current IRS underpayment rate (approx 7-8% in 2026) $0
Balance Due to Pay with 4868
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Safe Harbor Amount (to avoid penalty)
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90% Test β Underpayment Penalty
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Interest if Paid Oct 15 (6 mo.)
Extension Payment Analysis
Form 4868: Extension to File, Not to Pay
Form 4868 buys you 6 more months to file (October 15, 2026), but tax owed is still due April 15. Underpaying by April 15 triggers the failure-to-pay penalty plus compounding daily interest.
Safe Harbor Rules
90% Rule: Pay at least 90% of 2026 tax by Apr 15 β no failure-to-pay penalty
100% Rule: Pay 100% of 2025 tax by Apr 15 β safe harbor (if 2025 AGI under $150K)
110% Rule: Pay 110% of 2025 tax by Apr 15 β safe harbor (if 2025 AGI over $150K)
100% Rule: Pay 100% of 2025 tax by Apr 15 β safe harbor (if 2025 AGI under $150K)
110% Rule: Pay 110% of 2025 tax by Apr 15 β safe harbor (if 2025 AGI over $150K)
Penalties if You Miss the April 15 Payment
Failure-to-pay: 0.5%/month on unpaid balance, max 25%. Interest: compounds daily at the federal short-term rate + 3% (approximately 7-8% in 2026). Both accrue from April 15 even with an extension on file.
Sources and References (click to expand)
- IRC Section 6651 β Failure to File Tax Return or to Pay Tax
- IRS Form 4868 Instructions β Application for Automatic Extension of Time to File
- IRS Tax Topic 304 β Extensions of Time to File Your Tax Return
- Treasury Reg 301.6651-1 β Failure to File Tax Return
- IRS β Quarterly Interest Rates on Underpayments and Overpayments
Extended
Penalty and Interest Simulator
Compare penalty and interest costs at different payment dates (Apr 15, Jun 15, Oct 15, Dec 31). SVG line chart of cost over time. Safe harbor checker.
Compare total penalty plus interest cost at different payment dates.
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| Payment Date | Days Late | FTP Penalty (0.5%/mo) | Interest | Total Extra Cost |
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Check all three safe harbor tests with your numbers.
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Cumulative penalty plus interest over time for an unpaid balance.
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Frequently Asked Questions
Does Form 4868 extend the time to pay my tax bill?
No. Form 4868 grants an automatic 6-month extension to FILE your return (moving the deadline from April 15 to October 15), but it does not extend the time to PAY your tax. Any tax owed must be paid by the original April 15 deadline. If you do not pay by April 15, you will be charged the failure-to-pay penalty of 0.5% per month (or fraction thereof) on the unpaid balance, plus interest currently accruing at about 7-8% annually (set quarterly based on the federal short-term rate plus 3 percentage points).
What is the failure-to-file penalty versus failure-to-pay?
These are two separate penalties. Failure-to-file: 5% of unpaid tax per month, maximum 25%, applies if you do not file by April 15 (or October 15 if you filed Form 4868). Failure-to-pay: 0.5% of unpaid tax per month, maximum 25%, applies if tax is not paid by April 15. If both apply in the same month, the combined maximum is 5% (failure-to-file reduced by the failure-to-pay rate). Filing Form 4868 prevents the failure-to-file penalty entirely, but failure-to-pay still accrues on any balance due from April 16 onward.
How much should I pay with Form 4868 to avoid the failure-to-pay penalty?
To avoid the failure-to-pay penalty, you must pay at least 90% of the tax owed for the current year (2026) by April 15. Alternatively, you can use the safe harbor: pay 100% of last year's (2025) tax liability (110% if your 2025 AGI exceeded $150,000). The balance due with Form 4868 should be your estimated total tax for 2026 minus withholding, estimated tax payments already made, and any other credits. You can pay online via IRS Direct Pay, EFTPS, or by check with the Form 4868 voucher.
What are the 2026 IRS interest rates on underpayments?
IRS interest on underpayments is set quarterly at the federal short-term interest rate plus 3 percentage points, compounded daily. The IRS announces the rate each quarter. For 2026 estimation purposes, assume approximately 7-8% annually. Interest accrues from the original due date (April 15) until paid, and is not deductible by individuals. Interest compounds daily, so the effective annual rate is slightly higher than the stated rate. Interest applies even with an extension and even if you pay with Form 4868.
Can I get an extension beyond October 15?
Generally no. The 6-month extension granted by Form 4868 is the maximum for most individual taxpayers. However, some special categories get additional time: US citizens and resident aliens living abroad on April 15 get an automatic 2-month extension to June 15 (with the original deadline), and may request an additional 4-month extension to October 15. Military personnel in combat zones may receive extended deadlines. Taxpayers outside the US at the normal filing deadline may request an additional 2-month extension to December 15, but must attach an explanation. Even these extended filers owe interest on any unpaid tax from April 15.