Quick Comparison by Return Type

The reason these rules diverge is structural. A Form 1040 is an individual income tax return, so the filer can directly owe federal income tax. A Form 1120-S is generally a pass-through return, but an S corporation can still owe certain entity-level taxes. A Form 1065 is usually an information return for a partnership. Because the underlying tax liability differs, the penalty architecture differs too.

1. Underpayment of Estimated Tax

The underpayment-of-estimated-tax penalty is a timing penalty. It does not depend on whether the return was filed late. It applies when the taxpayer should have paid enough tax during the year through withholding and estimated payments, but did not. The IRS describes the federal income tax system as pay-as-you-go and warns that paying the balance in full with the return in April does not automatically eliminate an estimated-tax penalty.

Form 1040

For individuals, the underpayment penalty is generally analyzed on Form 2210. The IRS says the penalty may apply if the filed return shows at least $1,000 due after subtracting withholding and refundable credits, unless the taxpayer paid enough during the year under one of the safe harbors. The most common safe harbors are paying at least 90% of the current-year tax or 100% of the prior-year tax, with a 110% prior-year safe harbor for many higher-income taxpayers.

This issue most often affects self-employed taxpayers, investors with large non-wage income, clients with large K-1 income, and taxpayers who sell appreciated assets late in the year without having adjusted withholding or made quarterly payments.

Form 1120-S

For S corporations, estimated-tax underpayment is much narrower than many owners expect. Because an S corporation usually does not pay regular federal income tax on its ordinary operating income, the entity usually does not make estimated payments for that pass-through income. The relevant question is whether the S corporation owes one of the limited entity-level taxes that can trigger Form 2220, such as built-in gains tax, excess net passive income tax, and certain recapture-related taxes.

Form 1065

For partnerships filing Form 1065, the ordinary federal underpayment-of-estimated-tax penalty usually is not a partnership-return issue, because partnerships generally do not pay income tax. Instead, income, deductions, and credits pass through to the partners, and the partners themselves may need to make estimated tax payments on their own returns.

2. Failure to Pay

The failure-to-pay penalty applies when tax was due but not paid on time. The IRS states that the standard penalty is 0.5% of the unpaid tax for each month or part of a month, up to a maximum of 25%. The rate can drop to 0.25% per month for an individual who filed on time and is on an approved installment agreement, and can rise to 1% per month after a notice of intent to levy.

Form 1040

On an individual return, this penalty is one of the most common post-filing charges. A taxpayer may file a timely extension and still incur failure-to-pay penalties if the balance due was not paid by the original due date. This is the practical consequence of the IRS rule that an extension to file is not an extension to pay.

Form 1120-S

For an S corporation, the failure-to-pay penalty matters only when the corporation actually owes entity-level tax. The Instructions for Form 1120-S say the penalty is generally one-half of 1% of the unpaid tax for each month or part of a month, up to 25%, unless the corporation can show reasonable cause.

Form 1065

For a standard Form 1065, the classic failure-to-pay penalty is generally less central because a partnership usually does not owe regular federal income tax on the return itself. The real payment issue usually sits with the partners at the individual level.

3. Failure to File

The failure-to-file penalty is usually the most severe of the common IRS return penalties when unpaid tax exists. But its structure depends heavily on the type of return.

Form 1040

The IRS states that the failure-to-file penalty is generally 5% of the tax due for each month or partial month the return is late, up to 25%. If the return is more than 60 days late, a minimum penalty applies — for returns required to be filed in 2026, the minimum is the lesser of the tax due or $525. If both failure to file and failure to pay apply in the same month, the late-filing penalty is reduced by the late-payment penalty for that month.

This is why filing on time, or at least extending on time, is so important for individuals who expect to owe. The late-filing penalty is far more expensive than the base late-payment penalty.

Form 1120-S

The late-filing penalty for Form 1120-S is very different. The 2025 Instructions for Form 1120-S say that when no tax is due, the penalty is $255 for each month or part of a month, up to 12 months, multiplied by the total number of shareholders. If tax is due, additional percentage-based penalties apply. That structure surprises many S corporation owners because the corporation is usually a pass-through.

Form 1065

The partnership late-filing rule is similar in structure. The 2025 Instructions for Form 1065 say the penalty is $255 for each month or part of a month, up to 12 months, multiplied by the total number of partners. This is the central penalty risk for many partnerships — a late-filed 1065 can generate a significant per-partner penalty even when there is no traditional income tax due.

How IRS Interest Works

Interest is separate from penalties, but it often matters just as much because it continues to accrue until the balance is fully paid. The IRS says interest generally accrues on unpaid tax from the due date of the return, without regard to extensions, and compounds daily.

The interest rate is determined quarterly under section 6621. The IRS quarterly-interest page shows that the rate can change from quarter to quarter. For example, the IRS set a 7% underpayment rate for Q1 2026 and a 6% rate for Q2 2026. Because the rate is variable and quarterly, taxpayers should verify the current rate before using a specific percentage in planning.

Extensions: What They Do and What They Do Not Do

Extensions protect filing deadlines, not payment deadlines. For an individual, a timely Form 4868 can avoid the late-filing penalty, but unpaid tax can still draw the failure-to-pay penalty and interest from the original deadline. For a business entity, Form 7004 can extend the filing due date, but the form instructions say the extension is conditioned on properly estimating the tax and paying what is due.

Practical Takeaways by Return Type

• Form 1040: The biggest risks are underpaying during the year, filing after the deadline without a valid extension, and paying the balance too late.
• Form 1120-S: The biggest trap is assuming pass-through status means there is no federal penalty exposure at the entity level. Late-filing penalties can still be large.
• Form 1065: The biggest core risk is usually the per-partner late-filing penalty, not a classic income-tax late-payment penalty.
• Interest compounds daily and can apply to unpaid tax, penalties, and prior interest until the balance is fully paid.
• Reasonable-cause arguments and penalty-relief programs may help in some cases, but they do not erase the basic rule that filing and paying on time are the safest options.

The most useful client message is simple: file or extend on time, pay as much as possible by the original deadline, and never assume a pass-through return is penalty-free. For more details, see the IRS penalties overview.