CT-1120I: Computation of Interest Due on Underpayment of Estimated Tax (2025)
Connecticut · Connecticut DRS · Used for tax years 2025
Form CT-1120I calculates interest due on underpaid Connecticut estimated tax for 2025. It includes calculations for the required annual payment, installment amounts, annualized income installments, and interest on underpayment balances.
Who files this form
Use Form CT-1120I to determine interest due on underpaid estimated Connecticut Corporation Business Tax or unrelated business income tax for:
- Form CT-1120, Corporation Business Tax Return.
- Form CT-1120CU, Combined Unitary Corporation Business Tax Return.
- Form CT-990T, Connecticut Unrelated Business Income Tax Return.
You do not need to complete this form if your current-year tax is $1,000 or less.
You are not subject to interest on underpaid installments if you made timely installment payments at the required installment rates and paid at least 90% of the current-year tax or 100% of the prior-year tax before applying any tax credits.
When it's due
For the 2025 income year, required estimated tax installments are due on or before:
- The 15th day of the third month of the income year.
- The 15th day of the sixth month.
- The 15th day of the ninth month.
- The 15th day of the twelfth month.
The instructions state that if a due date falls on a Saturday, Sunday, or legal holiday, the return is considered timely if filed on the next business day.
The supplied text does not give a separate filing deadline for Form CT-1120I.
What you'll need
Have these returns and details available to complete the calculation:
- Your corporation name, Connecticut Tax Registration Number, and income-year beginning and ending dates.
- Your 2025 Form CT-1120, CT-1120CU, or CT-990T for the current-year tax amount.
- Your 2024 return, if filed, for the prior-year tax amount. Special rules apply if no prior-year return was filed or it covered less than 12 months.
- Amounts and dates of estimated tax payments and credits, including prior-year overpayments applied to the current year.
If you use the annualized income method, you also need net income for the first 2, 5, 8, and 11 months, estimated allowable business tax credits, and any available PE credit. Complete Form CT-1120K to determine the business tax credits available.
How to file
Electronically: File Form CT-1120I using myconneCT, which allows taxpayers to file, pay, and manage state tax responsibilities electronically.
By mail: Complete the form in blue or black ink only. Do not use staples, and use the correct year's form. The supplied text does not provide a mailing address.
Enter the total interest from Part IV, Line 24, on the appropriate Connecticut tax form as well.
Non-electronic payments made without an electronic payment waiver are subject to penalty. If a waiver has been granted, payments accompanying the listed estimated installment forms are credited as of the U.S. Postal Service cancellation mark.
Certain designated private delivery services also qualify for timely filing and payment rules, but not all service types qualify. The instructions refer to Policy Statement 2016(4) for the list.
Common mistakes
- Using the wrong year's form: Submit the correct year's form to avoid processing delays.
- Using staples or the wrong ink when mailing: Use blue or black ink only and no staples.
- Including excluded credits in Part I: Line 1 must not include recapture of tax credits or refundable tax credits. Line 3 must not include recapture of tax credits.
- Using an ineligible prior-year tax amount: If no prior-year return was filed, or it covered less than 12 months, leave Part I, Line 3 blank and enter Line 2 on Line 5.
- Completing the annualized schedule out of order: Complete Part III, Column A entirely before moving to the next column. Leave Line 11, Column A blank.
- Paying non-electronically without a waiver: These payments are subject to penalty.
- Assuming every private delivery service qualifies: Only designated services and service types meet the timely filing and payment rules.
Questions people ask
How is interest calculated?
Interest is calculated monthly on the accumulated unpaid required installment balance at 1% per month or fraction of a month. Add Part IV, Lines 10 through 23, to find the total on Line 24.
Are the four required installments equal?
No. Part II divides the required annual payment into installments of 30%, 40%, 10%, and 20%.
Can I base estimated payments on annualized income?
Yes. Estimated payments may be based on the applicable percentage of current-year tax determined by annualizing net income. Part III contains the annualized income installment schedule.
Can a prior-year overpayment count as an estimated payment?
Yes. Include prior-year overpayments applied to the current year in Part III, Line 19. If the prior-year return was filed on time, or within an extension period requested on time, the overpayment is treated as paid on the 15th day of the third month. Otherwise, it is treated as paid on the date the prior-year return was filed.
How are payments applied when an earlier installment remains unpaid?
Payments first cover remaining unpaid installment balances that are due. They are then applied to the next required installment.