Filed a Tax Extension but Underpaid? What to Do Next

If you filed a tax extension, submitted a payment with it, and later recalculated and found that the payment was short, you are in a common and recoverable position. The next step is to complete your return rather than to estimate a penalty, because the finished return is what establishes the amount you actually owe.

An Extension Did Not Extend Your Payment Deadline

Form 4868 is the IRS extension form for individuals. Filing it grants an automatic six-month extension, which moves the tax filing deadline from April 15 to October 15 on the standard calendar-year schedule. If either deadline falls on a Saturday, Sunday, or legal holiday, the deadline generally moves to the next business day. That extension applies only to filing. Your payment deadline remains the original tax payment deadline, and the extension does not give you additional time to pay taxes owed.

If the estimated payment you submitted with your extension was lower than the amount you actually owed, the unpaid portion begins accruing interest from the original due date. Penalties may apply as well, depending on your circumstances.

Your state return is a separate filing with its own rules. Some states extend it automatically once your federal extension is on file. Others require their own extension form, and several require an estimated payment alongside it. Confirm whether your state accepts a federal extension before assuming that your federal filing covers your state return.

What to Do as Soon as You Realize the Payment Was Short

Acting promptly limits the amount of interest that accrues and may prevent penalties from increasing further. You do not need a final figure from the IRS before you begin.

1. Finish your tax return

Complete the return using your actual income, deductions, credits, withholding, and estimated tax payments. A finished income tax return is the only document that establishes the correct tax for the tax year. Anything calculated before that point remains an estimate.

2. Account for what you already paid

Include the payment you submitted with your extension form, along with any withholding and any estimated tax payments you made during the year. Once all of those amounts are applied, you will be able to determine whether you owe a remaining balance or are due a refund.

3. Pay what you can

If a balance remains, submit a payment even if you cannot pay the full amount. Every business day the balance stays outstanding is another day the IRS assesses interest on it. The IRS advises taxpayers who cannot pay taxes in full to pay what they are able to pay and to consider a payment plan for the remainder. There is no requirement to wait for a penalty calculation first.

Work Out the Tax You Owe Before You Estimate Penalties

The calculator is the wrong starting point. Your completed return already holds every number this calculation needs: total tax liability, federal withholding, estimated tax payments, whatever you sent with the extension, and any refundable credits. Take the payments and credits away from the total tax liability. What is left is the tax you still owe, and at this stage it is the only figure worth acting on.

Keep that figure separate from penalties and interest. The IRS calculates those from the amount left unpaid, the date it became due, the length of time it remained unpaid, and the quarterly rate in effect during that period. Our penalty and interest calculator provides an estimate of those amounts. The IRS produces your final tax bill.

You should not delay a payment you are able to make while you refine the figure. If your return shows a balance due, submitting part of that balance now is more useful than waiting until every amount has been settled precisely.

Pay in Full, Pay Part, or Consider a Payment Plan

Once you know approximately what remains, you generally have three options.

Paying the balance in full is the most direct of them, because clearing the balance stops underpayment interest from accruing on it. If you cannot cover the full amount, a partial payment is still worthwhile. It reduces the balance on which those charges are calculated. Penalties and interest already assessed remain in place. If paying in full would create genuine financial strain, you can apply for a payment plan instead. The IRS advises taxpayers who cannot pay their full tax liability on time to pay what they can and to apply for a plan. Our guide to tax payment plans and penalties explains how that application works. A payment plan allows you to pay the balance in manageable installments. Penalties and interest continue to accrue until the balance is resolved.

What Interest and Penalties May Apply?

Interest and penalties are assessed separately, and the difference between them is worth understanding before you estimate what you owe.

Interest accrues on unpaid tax from the original due date until the tax is paid. Filing an extension has no effect on it. The IRS sets underpayment interest rates quarterly and compounds them daily. For individual taxpayers, the rate is 7% for the third quarter of 2026 and 7% for the fourth quarter of 2026. If your unpaid balance spans more than one quarter, more than one rate may apply to it.

Penalties are less predictable, because submitting a short extension payment does not automatically trigger any particular penalty. A late payment penalty may be assessed when tax is not paid by the original due date. A separate late filing penalty, also called the failure to file penalty, may be assessed when the income tax return itself is filed late. Which penalties apply, and whether you face penalties at all, depends on your individual circumstances. For that reason, this page does not attempt to calculate your final tax bill.

Why Filing the Completed Return Still Matters

The extension gave you additional time to prepare your return. Your tax liability continued to run throughout. If your return is not yet finished, continue working toward the extended filing deadline.

Filing your tax return gives you an accurate picture of what you owe. It also establishes the tax shown on the return, which is the figure the IRS uses when determining certain penalties. Rather than waiting for the IRS to tell you where you stand, file the return and determine it for yourself.

When an Updated Payment Estimate Needs Tax-Professional Review

A straightforward balance due is usually something you can identify on your own.

You should consider consulting a tax professional when your updated estimate differs substantially from what you expected. The same applies when your year included self-employment or business income, a significant change in income, substantial investment or non-wage income, large estimated tax payments, or payments submitted across multiple dates. Consider it as well if you may be subject to an estimated-tax underpayment penalty, if there is an accuracy-related issue, or if you have received prior IRS notices or carry unpaid balances from an earlier tax year. A tax professional can separate the correct tax you owe from the penalties and interest assessed on it, and can advise you on whether any special rules apply to your situation.

Related Situations

Looking for the next step?

• Tax payment plans and penalties: what the IRS charges once a balance runs past April 15, and which installment options exist.
• How long a tax extension lasts: why the extended deadline lands on October 15, and what happens if you miss that one too.
• State tax extensions: which states extend automatically and which want their own form.
• Tax Extension Help: the rest of the extension process, from Form 4868 through confirming the IRS accepted it.

Ready to finish?

Interest is running on whatever you still owe, and it keeps running until the balance clears. Finish the return, establish the number, and pay down as much of it as you can.

Complete your return and explore your payment options with FileTax.com. If you still need to request additional time on a return you have not yet filed, you can e-file Form 4868 with FileTax.com.