Connecticut tax deadlines follow the federal calendar closely: individual returns (Form CT-1040) are due April 15, calendar-year pass-through entity returns are due March 15, and calendar-year corporation returns are due May 15. Miss a payment deadline and the Department of Revenue Services (DRS) charges a flat 10% penalty plus 1% interest per month until you pay. The exact date that applies to you depends on whether you file as an individual, a corporation, a pass-through entity, or an employer remitting withholding.
Individual Income Tax
Connecticut residents file Form CT-1040 by April 15 for the prior calendar year. Part-year residents and nonresidents with Connecticut income file Form CT-1040NR/PY by the same date.1Connecticut State Department of Revenue Services. Connecticut Resident Income Tax Information When April 15 lands on a weekend or legal holiday, the deadline moves to the next business day.
If you expect to owe $1,000 or more after withholding and any pass-through entity tax credit, you owe quarterly estimated payments on Form CT-1040ES. For calendar-year taxpayers, those installments are due April 15, June 15, September 15, and January 15 of the following year.2Department of Revenue Services. Form CT-1040ES – Estimated Connecticut Income Tax Payment Coupon for Individuals A missed installment carries an underpayment penalty of 1% per month from the missed date until you pay.3Justia Law. Connecticut General Statutes Title 12 Chapter 229 Section 12-722 – Underpayment and Nonpayment of Estimated Income Tax by Individuals
Corporations
Form CT-1120 is due on the fifteenth day of the month after the federal corporate return’s due date. For calendar-year corporations, the federal Form 1120 falls on April 15, which puts the Connecticut return on May 15.4Connecticut State Department of Revenue Services. Corporation Business Tax – FAQs Fiscal-year filers use the same rule: find the federal due date, add one month.
Pass-Through Entities
Every partnership, S corporation, and LLC treated as a partnership must file Form CT-1065/CT-1120SI, the Connecticut Composite Income Tax Return, on the fifteenth day of the third month after the close of the taxable year. That’s March 15 for calendar-year entities.5Connecticut State Department of Revenue Services. Composite Income Tax Information Estimated payments are not required for this return.6Connecticut Department of Revenue Services. Connecticut Composite Income Tax – General Instructions
A pass-through entity may separately elect the Connecticut Pass-Through Entity Tax (PET) by filing Form CT-PET, also due March 15 for calendar-year filers. The election is made by checking a box on the CT-1065/CT-1120SI. If you elect the PET and expect $1,000 or more in liability, quarterly estimated payments are required on April 15, June 15, September 15, and January 15 for calendar-year entities.7Connecticut State Department of Revenue Services. Connecticut Pass-Through Entity Tax Information
Sales and Use Tax
Sales and use tax is reported on Form OS-114. The DRS assigns a monthly, quarterly, or annual filing frequency based on your taxable sales volume. Whatever frequency applies, the return and payment are due by the last day of the month following the reporting period.8Connecticut State Department of Revenue Services. Sales and Use Tax Information
Employer Withholding
Withholding deposits on Form CT-WH follow a tiered schedule based on payroll liability. Weekly remitters whose payday falls Wednesday through Friday must deposit by the following Wednesday; if payday falls Saturday through Tuesday, the deposit is due the second Wednesday after payday. Monthly remitters pay by the fifteenth of the following month. Quarterly remitters pay by April 30, July 31, October 31, and January 31.
Every employer, regardless of remitter frequency, files a quarterly reconciliation on Form CT-941 by the last day of the month after each quarter ends. Annual wage reporting on Form CT-W3, with copies of federal W-2s, is due January 31.9Connecticut State Department of Revenue Services. Withholding Tax Information
Extensions
Connecticut offers a six-month filing extension, but you must request it by the original due date. Individuals file Form CT-1040 EXT, corporations file Form CT-1120 EXT, and pass-through entities file Form CT-1065/CT-1120SI EXT.10Department of Revenue Services. Form CT-1040 EXT – Application for Extension of Time to File Connecticut Income Tax Return for Individuals For calendar-year filers, the extended deadlines are:
- Individuals: October 15
- Pass-through entities: September 15
- Corporations: November 15
There’s a shortcut worth knowing. If you’ve already requested a federal extension and expect to owe no additional Connecticut tax after withholding, estimated payments, and any PET credit, the DRS grants the six-month extension automatically. You don’t need to file a separate state form.10Department of Revenue Services. Form CT-1040 EXT – Application for Extension of Time to File Connecticut Income Tax Return for Individuals If you do owe additional tax, you must file the CT extension form and pay the tentative amount by the original due date.11Cornell Law Institute. Connecticut Agencies Regulations 12-723-1 – Extension of Time for Filing Returns
An extension only extends the time to file. Any tax owed after the original deadline accrues interest and may trigger penalties.
Amended Returns After a Federal Change
If an IRS audit or a voluntary federal amendment changes your federal tax and affects your Connecticut liability, you have 90 days from the final federal determination to file an amended Connecticut return.12Connecticut State Department of Revenue Services. Other Helpful Pass-Through Entity Tax Information The federal determination letter can arrive well after an audit closes, so the 90-day window often starts running before people realize it.
Penalties and Interest
Connecticut imposes a flat 10% penalty on any tax that remains unpaid after the due date. The penalty applies even if you obtained a filing extension, because the extension does not move the payment deadline.13Connecticut State Department of Revenue Services. Corporation Business Tax Information
If you file a required return late but owe no tax, the DRS can impose a separate $50 late-filing penalty. You won’t face both the late-payment penalty and the late-filing penalty for the same tax period; the DRS applies only one.14Justia Law. Connecticut General Statutes Title 12 Chapter 201 Section 12-30 – Penalty for Failure to File Return or Report
Interest accrues at 1% per month, or any fraction of a month, on unpaid tax, running from the original due date until the balance is paid.1Connecticut State Department of Revenue Services. Connecticut Resident Income Tax Information Interest cannot be waived. Because the 1% applies to any fraction of a month, even a payment a few days late is charged a full month of interest.
Electronic Payment Requirements
Any taxpayer who files a return electronically must also pay the associated tax electronically. For withholding taxes, mandatory electronic payment kicks in when the prior-year liability exceeds $2,000, and weekly withholding remitters must pay electronically regardless of amount.9Connecticut State Department of Revenue Services. Withholding Tax Information Mailed payments must arrive by the due date, not merely be postmarked, so timing matters if you’re cutting close on a paper check.