CT-1120I: Computation of Interest Due on Underpayment of Estimated Tax (2024)
Connecticut · Connecticut DRS · Used for tax years 2024
Connecticut Form CT-1120I calculates interest due on underpayments of estimated tax for 2024. It includes calculations for the required annual payment, required installments, annualized income installments, and interest based on payments and credits received.
Who files this form
Use Form CT-1120 I to calculate interest on underpaid estimated Connecticut Corporation Business Tax or unrelated business income tax for filers of:
- 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.
If your 2024 tax is $1,000 or less, you do not need to complete this form.
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 tax credits.
When it's due
For the 2024 income year, required estimated tax installments are due on or before:
- The fifteenth day of the third month.
- The fifteenth day of the sixth month.
- The fifteenth day of the ninth month.
- The fifteenth day of the twelfth month.
The instructions say 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.
What you'll need
Have these returns and records available to complete the form:
- Your applicable 2024 return: Form CT-1120, CT-1120CU, or CT-990T.
- Your applicable 2023 return, if filed, for the prior-year tax calculation.
- Records of estimated tax payments and credits, including amounts and dates.
- Records of any prior-year overpayment applied to 2024, along with the prior-year return filing date and extension information, if applicable.
- Your Connecticut Tax Registration Number, corporation name, and income-year beginning and ending dates.
If using the annualized income method, you also need net income for the first 2, 5, 8, and 11 months and estimated allowable Corporation Business Tax credits and any available PE credit.
How to file
Electronic filing: File Form CT-1120 I using myconneCT, which allows taxpayers to file, pay, and manage state tax responsibilities electronically.
Mail filing: Complete the form in blue or black ink only. Do not use staples. Submit the correct year's form to the Department of Revenue Services. 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 a penalty.
Common mistakes
- Using the wrong year's form. Each form is year specific. Use the correct year to avoid processing delays.
- Using staples or the wrong ink when mailing. Use blue or black ink only and no staples.
- Including recaptured tax credits in Part I. Lines 1 and 3 should not include recapture of tax credits.
- Using prior-year tax when it does not qualify. 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.
- Making non-electronic payments without a waiver. These payments are subject to a penalty.
- Assuming every private delivery service option qualifies. Not all service types meet the timely filing and payment rules. The instructions refer to Policy Statement 2016(4) for qualified services.
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.
How is the required annual payment divided into installments?
Part II uses 30% for the first installment, 40% for the second, 10% for the third, and 20% for the fourth.
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. Use Part III for the annualized income installment calculation.
How are payments applied to unpaid installments?
Payments first cover remaining unpaid installments that are due, then the next required installment.
Can a prior-year overpayment count toward estimated payments?
Yes. Include prior-year overpayments applied to the current year. If the prior-year return was filed on time, or within the extension period after a timely extension request, the overpayment is treated as paid on the fifteenth day of the third month. Otherwise, it is treated as paid on the date the prior-year return was filed.