When it comes to filing a U.S. corporate tax return, one of the most common questions business owners face is Form 1120 vs 1120-S vs 1120-F: which return do I file? Each form serves a different purpose, depending on whether your business is a C corporation, an S corporation, or a foreign corporation operating in the United States. Choosing the right form is critical, since filing the wrong one can lead to penalties, processing delays, or lost tax benefits.

This guide breaks down the differences between Form 1120, Form 1120-S, and Form 1120-F, who needs to file each, and how they impact your corporation’s tax obligations. By the end, you’ll have a clearer understanding of which IRS return applies to your situation and the steps to take next.

TL;DR: Key Differences at a Glance

Form 1120 (C-Corporations)

  • Filed by: Domestic C corporations
  • Tax treatment: Pays a flat 21% U.S. federal corporate income tax
  • Election requirement: None – C corp is the default status
  • Deadline: April 15 (calendar year) or the 15th day of the 4th month after year-end

Form 1120-S (S-Corporations)

  • Filed by: S corporations (domestic corporations that elect S status)
  • Tax treatment: Pass-through taxation – income, deductions, and credits flow through to shareholders
  • Election requirement: Must file Form 2553 to elect S corp status and meet eligibility rules
  • Deadline: March 15 (calendar year) or the 15th day of the 3rd month after year-end

Form 1120-F (Foreign Corporations)

  • Filed by: Foreign corporations with U.S. effectively connected income (ECI) or other triggers (e.g., deductions, treaty benefits)
  • Tax treatment: Taxed only on U.S.-source ECI, FDAP income, and effectively connected gains
  • Deadline: There are two deadlines depending on your situation:
    • Foreign Corporation with an Office or Place of Business in the United States – 15th day of the 4th month after the end of its tax year.
    • Foreign Corporation with No Office or Place of Business in the United States – 15th day of the 6th month after the end of its tax year (extensions available with Form 7004).

Form 1120 — C-Corporations

Who files:
 Form 1120 is required for all domestic C corporations, regardless of whether the business reports profits, losses, or even has no activity during the year. Unlike other entities that may only file when they have taxable income, C corporations must submit this return every year as long as the corporation exists.

Tax rate:
 C corporations are taxed at a flat 21% federal corporate income tax rate. This structure can be advantageous for businesses that plan to reinvest profits back into the company rather than distribute them as dividends.

Key schedules:
 Several schedules are commonly attached to Form 1120, depending on the corporation’s size and structure:

  • Schedule J – Calculates tax liability and records payments.
  • Schedule M-1 or M-3 – Reconciles book income with taxable income (M-3 is mandatory for larger corporations).
  • Schedule G – Provides details about shareholders and ownership.

Deadlines:
 The filing deadline for calendar-year C corporations is April 15. Fiscal-year filers must submit Form 1120 by the 15th day of the fourth month after the end of their tax year. Extensions may be requested using Form 7004, but payment of tax due is still required by the original deadline.

Best suited for:
 Form 1120 is often the best fit for:

  • Startups planning to raise venture capital funding, since investors usually prefer the C corporation structure.
  • Companies that intend to reinvest profits into growth rather than pay out dividends.
  • Entities that do not qualify for, or do not wish to elect, S corporation status.

Form 1120-S — S-Corporations

Who files:
 Form 1120-S is used by domestic corporations that have elected S-corporation status by submitting Form 2553 to the IRS. Unlike C corporations, which pay federal corporate income tax directly, S corporations are designed to allow profits and losses to pass through to shareholders.

Tax treatment:
 S corporations are treated as a pass-through entity. The business itself generally does not pay federal corporate income tax. Instead, income, deductions, and credits flow directly to shareholders, who report them on their individual tax returns using Schedule K-1 (Form 1120-S). This helps owners avoid double taxation that often applies to C corporations.

Eligibility rules:
 To qualify for S corporation status, the business must meet specific IRS requirements:

  • No more than 100 shareholders
  • All shareholders must be U.S. individuals or certain qualifying trusts and estates
  • Only one class of stock is allowed (voting and non-voting shares permitted, but no preferred stock)
  • Must be a domestic corporation and cannot be an ineligible entity, such as certain banks or insurance companies

Deadlines:
 For calendar-year S corporations, the filing deadline is March 15. Fiscal-year filers must submit by the 15th day of the third month after the end of the tax year. An extension may be requested using Form 7004.

Best suited for:
 Form 1120-S is often best for:

  • Small to mid-sized businesses where owners want to avoid double taxation
  • Companies that need corporate liability protection but prefer pass-through income treatment

Form 1120-F — Foreign Corporations

Who files:
Form 1120-F applies to foreign corporations that are engaged in a U.S. trade or business or that earn effectively connected income (ECI) from U.S. sources. Even if no tax is ultimately owed, a foreign company may still be required to file if it wants to claim deductions, refunds, or benefits under an applicable tax treaty.

Tax treatment:
 Foreign corporations are generally subject to U.S. corporate income tax only on their effectively connected income. They may also face additional obligations, such as the branch profits tax or withholding on certain U.S.-source income. In many cases, Form 5472 must also be filed to disclose transactions with related parties, adding another layer of reporting.

Deadlines:
 The filing deadline for Form 1120-F depends on whether the foreign corporation has an office or place of business in the United States:

  1. Foreign Corporation with an Office or Place of Business in the United States – 15th day of the 4th month after the end of its tax year. For example, a company with a December 31 year-end must file by April 15.
  2. Foreign Corporation with No Office or Place of Business in the United States – 15th day of the 6th month after the end of its tax year. For example, a company with a December 31 year-end must file by June 15.

Best suited for:
 Form 1120-F is typically required for non-U.S. companies with:

  • U.S. branches or offices
  • U.S.-source property income, such as rental income from real estate
  • U.S. subsidiaries that have reportable transactions

Comparison table

Feature Form 1120 Form 1120-S Form 1120-F
Applies to Domestic C-corps Domestic S-corps (with election) Foreign corps with U.S. income
Taxed at entity level? Yes, 21% flat No (pass-through to shareholders) Yes (on ECI)
Deadline Apr 15 Mar 15 6th month after YE
Key schedules J, M-1/M-3, G K-1, L, M-2 H, I, M
Common use VC-backed, large corps Closely held businesses Foreign companies with U.S. nexus

Which Form Should I File?

Choosing between Form 1120, Form 1120-S, or Form 1120-F ultimately comes down to your entity type, your tax elections, and where your business operates.

Step 1: Check your entity type

  • If you are a C corporation, you will file Form 1120.
  • If you are an S corporation, you will file Form 1120-S.
  • If you are a foreign corporation with U.S. income, you may need to file Form 1120-F.

Step 2: Review your tax elections
 If your corporation never filed Form 2553 to elect S corporation status, the IRS defaults your business to a C corporation. In that case, your return will be filed using Form 1120.

Step 3: Consider your operations
 The source of your income determines whether additional filings are needed. For example:

  • A foreign corporation with only foreign-source income likely does not need to file Form 1120-F.
  • If that same corporation has effectively connected income (ECI) in the United States, Form 1120-F becomes necessary.

Common Mistakes to Avoid When Filing

Even experienced business owners can run into problems when choosing and submitting the right IRS return. Some of the most frequent mistakes include:

  • Filing Form 1120-S without first submitting Form 2553
     Many corporations assume they qualify as an S corporation without realizing that the election requires a timely Form 2553 filing. Without this election, the IRS will treat the business as a C corporation, meaning the correct return is Form 1120 instead.
  • Missing the deadline and facing late-filing penalties
     Each form has a unique due date, and missing it can result in penalties, interest charges, and possible loss of elections. Knowing whether your deadline is March 15, April 15, or June 15 depending on your entity type is essential.
  • Foreign corporations failing to file Form 1120-F even when no U.S. tax is due
     Some foreign companies believe they do not need to file if their U.S. income is offset by deductions or if treaty benefits apply. However, Form 1120-F is still required to preserve those treaty protections and claim deductions.
  • Forgetting required schedules
     Certain forms require additional schedules that are easy to overlook. For example, S corporations must issue Schedule K-1 to shareholders, while foreign corporations often need to submit Form 5472 to report related-party transactions. Omitting these schedules can delay processing and increase audit risk.

By being aware of these pitfalls, corporations can reduce errors and avoid unnecessary penalties or complications with the IRS.

What Happens If You File the Wrong Form?

Filing the wrong corporate tax return can create more than just confusion — it can trigger costly consequences with the IRS. Here are some of the most common outcomes:

  • IRS rejection or processing delays
     If the IRS determines you submitted the wrong form, your return may be rejected outright. This can delay processing, create backlogs, and push you closer to late-filing penalties if not corrected quickly.
  • Penalties and interest
     Filing the wrong form can mean that your true return is considered late. This may result in IRS penalties and interest charges on any unpaid tax balance.
  • Loss of S corporation pass-through treatment
     If a corporation intends to file as an S corporation but fails to make or maintain the election, the IRS may disregard the S status. This means the business will default to a C corporation and be taxed under Form 1120 rules, potentially leading to unexpected double taxation.
  • Foreign corporations losing treaty protections
     Non-U.S. corporations that fail to file Form 1120-F correctly risk losing valuable treaty protections, including the ability to claim deductions or avoid higher withholding tax rates.

The IRS allows certain corrections, but mistakes can be time-consuming and expensive to fix. Filing the correct form from the start is always the best strategy.

Form 1120 vs 1120-S vs 1120-F corporate tax filing in the U.S.
Comparison of IRS corporate tax returns, including Forms 1120, 1120-S, and 1120-F.

Conclusion

Understanding the differences between Form 1120, Form 1120-S, and Form 1120-F is essential for corporations doing business in the United States. Each form comes with unique eligibility rules, tax treatments, schedules, and deadlines — and filing the wrong one can mean penalties, lost tax benefits, or missed treaty protections.

If you are unsure which IRS return applies to your business, working with a qualified advisor can help you avoid costly mistakes and stay compliant.

Need help determining the right corporate tax return for your business? Our team at TMP specializes in U.S. corporate tax filing, S corporation elections, and foreign corporation reporting. We’ll guide you through the right form, the correct schedules, and all filing requirements so you can focus on running your business.