How to Fill Out and File Form CT-1120: Connecticut Corporation Business Tax Return

To file Form CT-1120, Connecticut’s Corporation Business Tax return, you calculate your tax on two bases — net income and capital — pay whichever is higher (or the $250 minimum if both fall below that), and submit the return and payment electronically through the DRS myconneCT portal by the 15th day of the month after your federal Form 1120 is due. For calendar-year corporations, that deadline is May 15.

Confirm You Need to File CT-1120

Every corporation that carries on or has the right to carry on business in Connecticut must register with the Department of Revenue Services and file Form CT-1120.¹ That covers domestic corporations formed under Connecticut law and foreign corporations organized elsewhere. Under Conn. Gen. Stat. § 12-214, “carrying on business” is triggered by any of the following in-state activities:

Owning or leasing real property in the state
Maintaining an office
Selling tangible personal property or soliciting orders for it
Performing or soliciting orders for services
Keeping inventory in a public warehouse or delivering inventory on consignment to in-state distributors
Owning or leasing personal property unrelated to solicitation of orders

A foreign corporation also has the right to carry on business simply by holding a certificate of authority from the Connecticut Secretary of the State, even without active in-state income.¹ If that describes you, you still owe the $250 minimum tax.

A few entities do not use CT-1120. S corporations file Form CT-1065/CT-1120SI, the Connecticut pass-through entity return, instead.² Insurance companies file under separate statutes. Partnerships and disregarded entities do not file CT-1120 either.

One federal exception matters. Under Public Law 86-272, Connecticut cannot impose an income tax on a corporation whose only in-state activity is soliciting orders for sales of tangible personal property, provided the orders are approved outside Connecticut and filled by shipment from outside the state.³ This protection does not cover services, intangible property, or leased goods.

What to Gather Before You Start

CT-1120 pulls directly from your federal return, so finish Federal Form 1120 first. Then pull together:

Your Tax Registration Number (CT REG), assigned by DRS when you registered the business
Your Federal Employer Identification Number (FEIN) for the identification block on page one
Connecticut and total receipts figures if you operate in more than one state
Supporting schedules for any credits you plan to claim, reported on Form CT-1120K (Business Tax Credit Summary)

How the Tax Is Calculated

Connecticut’s Corporation Business Tax uses a dual-base structure. You compute a net income base tax and a capital base tax, then pay whichever is higher. If both come in below $250, you pay the $250 minimum.⁴

Net Income Base

Start with your federal taxable income from Federal Form 1120. Connecticut then requires specific modifications: certain items deducted federally must be added back to arrive at Connecticut net income. Under Conn. Gen. Stat. § 12-217, add-backs include state and local income taxes deducted on the federal return and expenses tied to dividends that received a federal deduction.⁵ Apply the 7.5% rate to your modified Connecticut net income to produce the net income base tax.⁴

Capital Base

The capital base starts with the average value of your corporation’s capital, adds surplus reserves, and subtracts holdings of stock in other corporations. Multi-state companies apportion the capital base to Connecticut. For income year 2025, the capital base tax rate is 0.021%, down from 0.026% the year before, continuing a gradual phaseout.⁴ Check the current year’s instructions to confirm the rate for your filing year.

Surcharge

Connecticut imposes a 10% surcharge on the Corporation Business Tax for companies with more than $250 in tax liability that either report at least $100 million in annual gross income or are taxable members of a combined unitary group. The surcharge applies through the 2025 income year and is scheduled to expire for income years beginning in 2026.⁶ It does not apply to the $250 minimum tax.

Apportioning Income if You Operate in Multiple States

If your corporation does business in Connecticut and elsewhere, you do not pay Connecticut tax on your entire income. Complete Form CT-1120A to apportion. Most corporations use a single receipts factor with market-based sourcing rules for services and intangible property.⁷ The older three-factor formula using payroll, property, and receipts is available only as an election for manufacturers in NAICS Sectors 31, 32, or 33 that derive 75% or more of gross receipts from sales of tangible personal property to the U.S. government, directly or as subcontractors.

Your Connecticut-sourced sales, as a fraction of total sales everywhere, determine how much of your income Connecticut can tax. An error in this fraction ripples through the whole return.

Working Through the Return

The form walks through the net income base first, then the capital base, then compares the two figures. Enter the higher amount, or $250 if that is greater, as your tax. Attach the appropriate credit schedule for anything claimed on Form CT-1120K, such as the Research and Development credit or the Urban and Industrial Site Reinvestment credit. Indicate on the return whether you are filing a combined unitary return or an initial return for a newly formed entity.

Filing and Paying Through myconneCT

All Corporation Business Tax returns must be filed electronically unless you obtain a specific waiver from DRS.⁴ The same rule covers every payment: estimated payments, extension payments, and balance-due payments must all be submitted electronically. Non-electronic payments without an approved waiver draw a penalty.

File through myconneCT at myconneCT.ct.gov.⁸ The portal lets you enter data, upload files, review your entries, and authorize payment in one session. After you submit, it generates a confirmation number. Save it. That number is your proof of filing if DRS later says the return never arrived.

If you receive a waiver from the electronic filing requirement, mail your paper return to:

Department of Revenue Services
PO Box 150420
Hartford, CT 06115-0420

Deadlines and Extensions

Form CT-1120 is due on the 15th day of the month following the due date of your federal return. Calendar-year corporations file by May 15, since Federal Form 1120 is due April 15.⁹ Fiscal-year filers work from their own year-end. A June 30 fiscal year, for example, has a federal deadline of October 15 and a Connecticut deadline of November 15.

To request more time, file Form CT-1120 EXT. Most filers receive a six-month extension; corporations with a June 30 year-end may request seven months.¹⁰ An extension delays filing only. It does not extend time to pay. Under Conn. Gen. Stat. § 12-222, you must file a tentative return and pay the estimated tax by the original deadline to avoid interest.¹¹ Additional tax owed with the final return accrues interest at 1% per month from the original due date.

Estimated Tax Payments During the Year

If your estimated Connecticut Corporation Business Tax for the current year exceeds $1,000, you must make quarterly estimated payments.⁹ The required annual payment is the lesser of 90% of your current-year tax or 100% of the prior-year tax (including surcharge, without regard to credits), assuming you filed a 12-month return the prior year. The installments:

First (ESA): 15th day of the 3rd month of the income year, at 30% of prior-year tax or 27% of current-year tax
Second (ESB): 15th day of the 6th month, cumulative 70% or 63%
Third (ESC): 15th day of the 9th month, cumulative 80% or 72%
Fourth (ESD): 15th day of the 12th month, cumulative 100% or 90%

For a calendar-year corporation, those dates fall on March 15, June 15, September 15, and December 15. Each installment builds on the earlier ones, so the later payments account for what you have already paid.

Penalties and Interest

The penalty for late payment or underpayment of Corporation Business Tax is 10% of the tax due or $50, whichever is greater. If no tax is due, DRS can still charge a $50 penalty for late filing.⁴ Interest on any unpaid balance runs at 1% per month or fraction of a month from the original due date until payment.¹¹

Corporations that file electronically without a payment, or submit a non-electronic payment without a waiver, face an additional penalty. The cleanest way to avoid all of this: pay your best estimate of the tax by the original due date, even when filing for an extension.

Keeping Records After You File

Connecticut regulations require you to maintain all records needed to determine your correct tax liability: books of account, invoices, sales receipts, exemption certificates, and every schedule and working paper used to prepare the return.¹² Failing to keep these records counts as evidence of negligence and can trigger penalties on its own. Keep your filed return and supporting documents for at least three years after filing, and longer if any position on the return could be questioned.

1
Connecticut Department of Revenue Services. 2023 Corporation Business Tax General Instructions
2
Connecticut State Department of Revenue Services. Composite Income Tax Information
3
Connecticut State Department of Revenue Services. IP 2010(29.1), Q and A on Economic Nexus
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Connecticut State Department of Revenue Services. Corporation Business Tax Information
5
Justia. Connecticut Code 12-217 – Deductions From Gross Income
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Connecticut General Assembly. Corporation Business Tax Surcharge
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Connecticut State Department of Revenue Services. Form CT-1120A
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Connecticut State Department of Revenue Services. myconneCT
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Connecticut State Department of Revenue Services. Corporation Business Tax – FAQs
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Connecticut Department of Revenue Services. Form CT-1120 EXT
11
Justia. Connecticut Code 12-222 – Annual Return
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Connecticut eRegulations. Connecticut Code Section 12-2-12 – Recordkeeping and Record Retention