Form 1120-F is the US income tax return a foreign corporation files on its US income. File it more than 18 months after its due date and the corporation loses the right to deduct anything at all, leaving the IRS to compute tax on gross effectively connected income.
A company with $4 million of US revenue and $3.6 million of US costs owes tax on $400,000 when it files on time. Miss the 18-month window and the same company is taxed on the full $4 million.
That deadline sits in the regulations instead of on the form, which is why it goes missing from most summaries.
TL;DR
- Four situations trigger the filing, and the first one applies even with no US source income: being engaged in a US trade or business.
- The due date depends on whether you keep a US office. With one, the 15th day of the 4th month after year-end. Without one, the 15th day of the 6th month.
- The 18-month rule is the expensive one. Deductions and credits survive only if the return is filed within 18 months of its due date.
- A protective return preserves those deductions when you have concluded you owe nothing, in case the IRS later disagrees.
- Two taxes can apply: 21% on effectively connected taxable income, and a 30% branch profits tax on earnings taken out of the US business.
- The minimum late penalty rose for returns filed in 2026 to the smaller of the tax due or $525, on top of the deduction risk.
What Form 1120-F Is
The form's full title is the US Income Tax Return of a Foreign Corporation. A corporation organized outside the United States uses it to report US income, claim deductions against that income, and calculate what it owes.
The form covers two distinct kinds of income, and they are taxed on different bases:
- Effectively connected income (ECI), earned through a US trade or business, taxed on a net basis at the corporate rate after deductions
- Fixed, determinable, annual or periodical income (FDAP), such as certain interest, dividends and royalties, generally taxed on a gross basis at 30% or a lower treaty rate, usually collected by withholding at source
Section II handles ECI and Section III handles the branch profits tax. A company with only FDAP income fully covered by withholding may have no filing obligation at all.
Who Has to File
The instructions set out four situations, and the first catches companies that had no US income whatsoever during the year. A foreign corporation must file if during the tax year it:
- Was engaged in a trade or business in the United States, whether or not it had US source income
- Had income, gains or losses treated as effectively connected with the conduct of a US trade or business
- Was not engaged in a US trade or business, but had income from any US source where the tax was not fully satisfied by withholding at source
- Was, or had a branch that was, a qualified derivatives dealer
"Engaged in a Trade or Business" Has No Revenue Floor
The first condition turns on activity rather than income. A foreign corporation that opened a US office, hired US staff and generated no revenue at all in its first year still meets this test.
No statutory definition and no bright-line threshold exist. Courts and the IRS look at whether the activity is considerable, continuous and regular, so the answer depends on the facts of each case.
Activity carried on through another person can count as your own:
- A dependent agent who habitually concludes contracts on your behalf can create a US trade or business for the foreign corporation
- A genuinely independent agent acting in the ordinary course of its own business generally does not
For a company selling into the US without an office, the agent's status is usually where the analysis starts and often where it ends.
Filing to Claim Deductions or a Treaty Position
Beyond the four triggers, a foreign corporation also files to claim a refund of over-withheld tax, or to disclose a treaty-based return position on Form 8833.
A treaty claim is a filing event in its own right. A treaty that protects you from US tax changes what the return says while leaving the return itself due.
What Goes in Each Section of Form 1120-F
The form is built around the ECI and FDAP split, and knowing which section carries which income makes the rest of the instructions readable.
| Section | Title | What it reports |
|---|---|---|
| I | Income From US Sources Not Effectively Connected With the Conduct of a Trade or Business in the United States | FDAP and similar income, generally taxed gross and collected by withholding |
| II | Income Effectively Connected With the Conduct of a Trade or Business in the United States | Gross receipts, deductions and effectively connected taxable income |
| III | Branch Profits Tax and Tax on Excess Interest | The second-level taxes under section 884 |
Section III carries two charges, not one. Alongside the branch profits tax sits a tax on excess interest, which addresses interest deducted by the US business but not actually paid by it.
Form 1120-F Schedules and What Each One Covers
Most of the computation happens in the attachments, not on the face of the form:
- Schedule H allocates and apportions deductions to effectively connected income
- Schedule I handles the interest expense allocation, which is where a foreign group's worldwide debt enters the US calculation
- Schedule J computes the tax; Schedule L carries the balance sheet
- Schedules M-1 and M-3 reconcile book income to taxable income
- Schedule P covers interests in US partnerships, a common route to unintended ECI
- Schedule S claims the exclusion for international shipping and aircraft income, where a reciprocal exemption applies
Schedules H and I determine how much of a global cost base lands against US income, so they carry most of the planning weight on a return of any size.
E-Filing and Estimated Tax for Form 1120-F
Two obligations run alongside the return and are easy to trip over:
- E-filing is mandatory for a corporation filing 10 or more returns of any type during the calendar year, for returns filed on or after 1 January 2024
- That count aggregates across return types, so a company filing a handful of W-2s and 1099s can cross it without filing ten of anything
- Estimated tax payments apply to a foreign corporation with effectively connected income, on the standard corporate installment schedule
The Deadline Depends on Whether You Keep a US Office
A single due date for Form 1120-F does not exist, and content that gives one is describing a particular company's facts. The rule splits on whether the corporation maintains an office or place of business in the United States.
| Situation | Due date | Example date |
|---|---|---|
| Maintains a US office or place of business | 15th day of the 4th month after year-end | 15 April |
| Does not maintain a US office | 15th day of the 6th month after year-end | 15 June |
| US office, tax year ending 30 June | 15th day of the 3rd month after year-end | 15 September |
Deciding to formalize a US office also chooses an earlier filing deadline, two months earlier, and a 30 June year-end pulls it earlier again.
Extending With Form 7004
Form 7004 gives an automatic extension of time to file, and it is narrower than it looks:
- It has to be filed by the original due date, which means the earlier of the two dates above if you keep a US office
- The extension is automatic, so approval is not discretionary
- It does not extend the time to pay. Tax remains due on the original date, and interest runs from there
An extension moves the filing date only. It does nothing for the 18-month deduction clock, which runs from the return's original due date.
The Extended Due Dates, and Why They Differ
Extending works in two stages for a corporation with a US office, which is where most of the confusion on this point comes from.
| Situation | Original | First stage | Final extended date |
|---|---|---|---|
| US office | 15 April | 15 June, automatic under Reg. 1.6081-5(a)(3), no Form 7004 needed | 15 October, via an additional 4 months on Form 7004 line 4 |
| No US office | 15 June | Not applicable | 15 December, via the standard 6 months on Form 7004 |
The first stage for a US-office filer is granted by regulation rather than by application, so it needs a qualifying statement attached instead of a form. Dates shown are for a calendar-year filer.
The 18-Month Rule Is the One That Costs Money
Section 882(c)(2) conditions a foreign corporation's deductions and credits on filing a true and accurate return. The regulations put a hard outer limit on how late that return can be.
Form 1120-F is generally considered timely filed for this purpose if it is filed no later than 18 months after the due date of that year's return. Past that point the deductions and credits attributable to the ECI are generally gone.
Which Deductions and Credits Survive a Late Form 1120-F
The loss is close to total, with four narrow carve-outs:
| Item | After the 18-month window |
|---|---|
| Business deductions against ECI (salaries, rent, cost of sales) | Lost |
| Most other credits against the ECI tax | Lost |
| Charitable contribution deduction | Preserved |
| Credit for US income tax paid or withheld at source | Preserved |
| Credit from Form 2439 (undistributed capital gains) | Preserved |
| Credit for federal tax on fuels | Preserved |
A distribution business running thin margins can owe more tax than it earned in profit, because the 21% then applies to revenue with no cost of sales subtracted.
The Clock Runs From the Due Date, Not From Discovery
The 18 months are counted from the due date of that year's return, so the window for an old year can already be closed by the time a company works out that it had a filing obligation.
Each year stands on its own. A company discovering a three-year exposure in September 2026, having kept a US office and a calendar year end, is looking at three different answers:
| Tax year | Due date | 18-month window closes | Position in September 2026 |
|---|---|---|---|
| 2023 | 15 April 2024 | 15 October 2025 | Closed. Deductions generally lost |
| 2024 | 15 April 2025 | 15 October 2026 | Weeks left. File now |
| 2025 | 15 April 2026 | 15 October 2027 | Open. Full year of runway |
The order of work follows from the table. The year with weeks left outranks the year with a year left, and the closed year becomes a conversation about exposure instead of about deductions.
The regulations do allow the deductions where the corporation establishes that, on the basis of the facts and circumstances, it acted reasonably and in good faith in failing to file. That relief is discretionary and fact-dependent, so it is not something to plan around.
The Protective Return: Filing When You Believe You Owe Nothing
Some foreign corporations conclude that they owe no US tax, either because the activity does not rise to a US trade or business, or because a treaty exempts it. The protective return exists for exactly that situation.
What a Protective Form 1120-F Protects
You file a protective Form 1120-F to safeguard the right to deductions and credits if the IRS later determines that the income was effectively connected after all. The filing is what preserves the deductions if that happens.
Without it, a company that reasonably concluded it had no US trade or business, and is later found to have had one, faces gross-basis taxation for those years. The good-faith position does not preserve the deductions on its own.
A Protective Return Has the Same Due Date and the Same 18-Month Limit
A protective return runs on the same due dates as an ordinary Form 1120-F, and the same 18-month outer limit applies to it. Filing one late gives up the protection it was meant to provide.
Nothing naturally prompts a company that believes it owes no US tax to file anything by 15 April or 15 June, so the protective return needs a calendar entry of its own.
A Protective Return Reports No Effectively Connected Income
A protective return does not report ECI, because the filer's position is that there is none. It identifies the corporation and states the protective nature of the filing.
Three situations produce most protective filings:
- A foreign company selling into the US through agents or contractors whose activity might constitute a US trade or business
- A company relying on a permanent establishment article to conclude that no US tax applies
- A foreign parent whose US presence is close to the line between preparatory activity and a business
When a Treaty Changes the Answer
A US income tax treaty can change the tax result without changing the filing obligation, and the two questions it raises are separate from the domestic ones.
Permanent Establishment Sits Above US Trade or Business
| Domestic law | Treaty | |
|---|---|---|
| The test | US trade or business | Permanent establishment |
| Threshold | Lower | Generally higher |
| If met | Return required | Business profits taxable in the US |
A company can therefore be engaged in a US trade or business under domestic law while having no permanent establishment under an applicable treaty. Business profits are not taxable in the US in that case, and the return is still filed.
Disclosing a Treaty Position on Form 8833
A treaty-based return position is disclosed on Form 8833 attached to the 1120-F. A treaty on the books does not settle the question on its own:
- The limitation on benefits article, which can deny treaty benefits to a company that does not meet its ownership and activity tests, even where the country has a treaty
- Whether the treaty reduces the branch profits tax, since treaty rates on the dividend equivalent amount vary and some treaties eliminate the charge
What You Actually Pay
Two separate charges can apply to the same US operation: 21% on effectively connected taxable income, and a 30% branch profits tax on earnings treated as taken out of the US business.
21% Corporate Tax on Effectively Connected Income
Effectively connected taxable income is taxed at the 21% corporate rate, computed on a net basis. Ordinary and necessary business expenses attributable to that income are deductible, which is the entitlement the 18-month rule puts at risk.
The 30% Branch Profits Tax
Section 884 imposes a second tax of 30% on the dividend equivalent amount. It exists so a foreign corporation operating through a US branch is not treated more favorably than one operating through a US subsidiary that pays dividends to its parent.
The dividend equivalent amount starts from effectively connected earnings and profits, then adjusts for what happened to the equity in the US business:
- An increase in US net equity during the year is generally treated as reinvestment, reducing the amount subject to the tax
- A decrease is generally treated as a disinvestment of prior years' earnings, which can pull earlier profits into the charge
Income tax treaties commonly reduce the 30% rate and in some cases eliminate the tax, subject to the limitation on benefits article. The reduced rate is claimed on the return.
Penalties, and What Changed for 2026
The late-filing penalty runs separately from the deduction risk. It is charged at 5% of the unpaid tax for each month or part month the return is late, capped at 25%.
The penalties and the deduction rule are independent of each other, and they bite on different timescales:
| Consequence | When it starts | Size |
|---|---|---|
| Failure-to-file penalty | Day after the due date | 5% of unpaid tax per month, capped at 25% |
| Minimum penalty, returns over 60 days late | 61 days after the due date | Smaller of the tax due or $525 for 2026 filings |
| Failure-to-pay penalty | Original due date | 0.5% of unpaid tax per month |
| Loss of deductions and credits | 18 months after the due date | Tax computed on gross ECI |
A return filed late but inside the 18-month window carries a capped, calculable cost. The same return filed after it carries a cost driven by revenue, which is where the numbers stop being proportionate to the delay.
The Minimum Penalty Rose for 2026 Filings
Where a return is more than 60 days late, the minimum penalty is now the smaller of the tax due or $525, for returns required to be filed in 2026.
A separate failure-to-pay penalty of 0.5% per month can apply on top, which is why Form 7004 extending only the filing date matters in practice.
Mistakes That Create Exposure
Treating No Income as No Filing
Being engaged in a US trade or business triggers the return whether or not there was US source income. A first-year US operation that spent money and earned nothing still files, and filing is what protects the deductions that will offset later profits.
Assuming a Treaty Removes the Return
A treaty can eliminate the tax while leaving the filing obligation intact. The treaty-based position is disclosed on Form 8833 attached to the return, so the exemption is claimed through a filing.
Using the Wrong Due Date
Applying the 4th-month rule without a US office gives up two months of runway. Applying the 6th-month rule with a US office puts the return two months late, which starts the penalty clock running.
Forgetting Form 5472
A foreign corporation engaged in a US trade or business that had reportable transactions with a related party has a Form 5472 obligation alongside the 1120-F. Our guide to who needs to file Form 5472 covers the thresholds and the separate penalty that attaches to it.
How FinStackk Helps With Foreign Corporation Filings
FinStackk is an accounting and tax compliance platform for US businesses, built for companies operating in the United States from outside it.
Fin-Tax covers federal and state filings with a consolidated deadline calendar and proactive alerts, and handles extensions and amendments. Our team includes CPAs, CAs and EAs, and every client gets a dedicated SPOC, a single point of contact in-house.
Get in touch to talk through your US filing position.
Frequently Asked Questions
When is Form 1120-F due?
It depends on whether the corporation maintains a US office or place of business. With one, the return is due the 15th day of the 4th month after year-end, or 15 April for a calendar-year filer.
Without a US office, it is the 15th day of the 6th month, or 15 June. Extended, those become 15 October and 15 December.
A tax year ending 30 June moves the with-office date to the 15th day of the 3rd month. Form 7004 extends the filing date but not the time to pay.
Does a foreign corporation with no US income file Form 1120-F?
If it was engaged in a US trade or business during the year, yes. That trigger applies whether or not there was US source income, and filing is what preserves deductions against future effectively connected income.
What happens if I file Form 1120-F more than 18 months late?
Deductions and credits attributable to effectively connected income are generally lost, and tax is computed on gross ECI. Charitable contributions and the credit for tax withheld at source are the main survivors.
Because the consequence is this severe and the facts vary, a late filing is a situation to put in front of a US tax professional, not one to work through from a checklist.
Is a protective return the same as filing a normal return?
No. A protective return does not report effectively connected income, because the filer's position is that there is none. It preserves the right to claim deductions and credits if that position is later successfully challenged.
This article is general information on US federal tax rules for foreign corporations and is not tax or legal advice. Rules change and individual facts matter, so confirm your position with a qualified professional before acting.