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Form 4868 Calculator

How much to send with Form 4868, and what penalties and interest you'd owe if you pay or file late.

Updated November 4, 2026. Free, no sign-up; everything runs in your browser and nothing you type is stored.

Form 4868 extension payment and penalty calculator

Expected total tax for the year ($)Withholding and estimated payments ($)Payment sent with Form 4868 by April 15 ($)Months after April 15 until you pay the restWill you file by October 15?If not: months late after October 15IRS underpayment interest rate (%)

Total cost of paying late

–

–Unpaid tax after April 15

–Failure-to-pay penalty

–Failure-to-file penalty

–Interest (estimate)

–90% test

What Form 4868 does, and doesn't do

Form 4868 gives individuals an automatic 6-month extension to file Form 1040, to October 15 (for 2026 returns, October 15, 2027). You don't need a reason, and paying any part of your expected balance electronically and marking it as an extension payment also counts as filing the extension. But it is not an extension to pay: tax not paid by April 15 runs up penalties and interest from that date.

The three costs of paying late

Failure-to-pay penalty: 0.5% of the unpaid tax for each month or part of a month, up to 25%. Form 4868's instructions treat you as having reasonable cause, so no penalty, for the extension period if you paid at least 90% of your total tax by April 15 and pay the rest with a return filed by October 15.
Failure-to-file penalty: only if you miss October 15 too. It's 5% of the unpaid tax per month, up to 25%, reduced by the failure-to-pay penalty for the same months; if you're more than 60 days late the minimum is $525 (or 100% of the tax owed, if less) for returns due in 2026.
Interest: the federal short-term rate plus 3 points, compounded daily: 7% a year through December 31, 2026.

Worked example

You expect $12,000 of tax and $9,000 was withheld. That's 75% of the tax, under the 90% safe level, so the $3,000 balance draws the failure-to-pay penalty. Paying it 4 months late costs $60 in penalty and about $71 of interest, around $131 in all, as long as you file by October 15. Sending $1,800 with the extension gets you to 90% and removes the penalty. See also tax extension rules, IRS payment plans and the tax refund estimator.

Common questions

Does Form 4868 extend the time to pay?

No, it only extends the time to file to October 15; tax is still due April 15.

How do I avoid the late payment penalty with an extension?

Pay at least 90% of your total tax by April 15 and the rest with a return filed by the extended date.

What is the failure-to-pay penalty?

0.5% of the unpaid tax per month, up to 25%.

What is the IRS interest rate on unpaid tax?

7% a year for the fourth quarter of 2026, compounded daily.

Sources

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General information, not financial, tax or legal advice. Check important numbers with the official source before acting on them.