Connecticut Computation of Interest Due on Underpayment of Estimated Tax

Computation of Interest Due on Underpayment of Estimated Tax
Form CT-1120I
Department of Revenue Services
State of Connecticut
(Rev. 12/23)

2023

Form CT-1120I

Computation of Interest Due on Underpayment of Estimated Tax

Enter Income Year Beginning_____________________________ ,	 2023, and Ending__________________________ ,_________
Corporation name

Connecticut Tax Registration Number

Part I – Computation of Required Annual Payment
	 1.	 Tax due from 2023 Form CT‑1120, Schedule C, Line 8, minus Schedule C, Line 1c; Form CT‑1120CU, Part III,
Line 5, minus Part I, Line 8, Combined Group Total column; or Form CT‑990T, Computation of Amount Payable,
Line 5. See instructions............................................................................................................................................ 1.
	 2.	 Multiply Line 1 by 90% (.90)......................................................................................................................... 2.
	 3.	 Tax from 2022 Form CT‑1120, Schedule C, Line 1, minus Schedule C, Line 1c;
Form CT‑1120CU, Part III, Line 1, minus Part I, Line 8, Combined Group Total column; or
Form CT‑990T, Computation of Amount Payable, Line 3. See instructions................................................. 3.
	 4.	 Multiply Line 3 by 100% (1.00)..................................................................................................................... 4.
	 5.	 Required annual payment: Enter the lesser of Line 2 or Line 4. .................................................................. 5.

Part II – Computation of Required Installments
	 6.	 First required installment: Multiply Line 5 by 30% (.30). Enter here and on Part III,
Line 13, Column A, or Part IV, Line 10a. ......................................................................................................
	 7.	 Second required installment: Multiply Line 5 by 40% (.40). Enter here and on Part III,
Line 13, Column B, or Part IV, Line 13c. ......................................................................................................
	 8.	 Third required installment: Multiply Line 5 by 10% (.10). Enter here and on Part III,
Line 13, Column C, or Part IV, Line 16c. ......................................................................................................
	 9.	 Fourth required installment: Multiply Line 5 by 20% (.20). Enter here and on Part III,
Line 13, Column D, or Part IV, Line 19c. ......................................................................................................

6.
7.
8.
9.

Part III – Annualized Income Installment Schedule

	

Estimated Payment Calculation

	 1.	Enter your Connecticut corporation business income
for each period. See instructions.
	 2.	Annualization factor
	 3.	Annualized Connecticut corporation business
income: Multiply Line 1 by Line 2.

A

First 2
Months

B

First 5
Months

C

First 8
Months

D

First 11
Months

6

2.4

1.5

1.09091

.27

.63

.72

.90

	 4.	Multiply Line 3 by 7.5% (.075).
	 5.	Enter surtax, if applicable. See instructions.
	 6.	Add Line 4 and Line 5.
	 7.	Corporation Business Tax credits and PE credit:
See instructions.
	 8.	Total annualized Corporation Business Tax: Subtract
Line 7 from Line 6.
	 9.	Applicable percentages
	10.	Multiply Line 8 by Line 9.
	11.	Add the amounts in all preceding columns of Line 17.
See instructions.
	12.	Annualized income installment using net income:
Subtract Line 11 from Line 10. If zero or less, enter “0.”
	13.	Enter your required installment for the period.
See instructions.
	14.	Enter the amount from Line 16 of the preceding
column of this worksheet.
	15.	Add Line 13 and Line 14.
	16.	If Line 15 is more than Line 12, subtract Line 12
from Line 15 (otherwise enter “0”).
	17.	Enter the lesser of Line 12 or Line 15.
	18.	Total required installment for the period: Add Line 11
and Line 17.
	19.	Estimated tax payments made through the due date
for the period.
	20.	 Estimated tax payment required by the next due
date: Subtract Line 19 from Line 18 and enter the
result, but not less than “0.”

Visit us at portal.ct.gov/DRS for more information.

Part IV – Computation of Interest
	10a.	First installment: Enter the required installment amount due on the fifteenth day
			 of the third month. See instructions. ............................................................................................................10a.
	10b.	Enter payments made or credits received on or before the fifteenth day of the third month. ......................10b.
	10c.	First installment underpayment balance: Subtract Line 10b from Line 10a.................................................10c.
	 10.	 Interest due - Sixteenth day of the third month through the fifteenth day of the
		 fourth month. Multiply Line 10c by .01 if greater than zero. ...................................................................... 10.
	11a.	Enter payments made or credits received on or before the fifteenth day of the fourth month. ....................11a.
	11b.	First installment underpayment balance: Subtract Line 11a from Line 10c. ................................................11b.
	 11.	 Interest due - Sixteenth day of the fourth month through the fifteenth day of the
	 	 fifth month. Multiply Line 11b by .01 if greater than zero. .......................................................................... 11.
12a.	Enter payments made or credits received on or before the fifteenth day of the fifth month. .......................12a.
	12b.	First installment underpayment balance: Subtract Line 12a from Line 11b. ................................................12b.
	 12.	 Interest due - Sixteenth day of the fifth month through the fifteenth day of the
		 sixth month. Multiply Line 12b by .01 if greater than zero. ........................................................................ 12.
13a.	Second installment: Enter payments made or credits received on or before the fifteenth day
			 of the sixth month. .......................................................................................................................................13a.
13b.	First installment underpayment balance: Subtract Line 13a from Line 12b.................................................13b.
	13c.	Enter the second required installment amount due on the fifteenth day of the sixth month.
			 See instructions............................................................................................................................................13c.
	13d.	Second installment underpayment balance: Add Line 13b and Line 13c. ...................................................13d.
	 13.	 Interest due - Sixteenth day of the sixth month through the fifteenth day of the
		 seventh month. Multiply Line 13d by .01 if greater than zero. .................................................................. 13.
	14a.	Enter payments made or credits received on or before the fifteenth day of the seventh month..................14a.
	14b.	Second installment underpayment balance: Subtract Line 14a from Line 13d............................................14b.
	 14.	 Interest due - Sixteenth day of the seventh month through the fifteenth day of the
		 eighth month. Multiply Line 14b by .01 if greater than zero. ..................................................................... 14.
15a.	Enter payments made or credits received on or before the fifteenth day of the eighth month. ...................15a.
	15b.	Second installment underpayment balance: Subtract Line 15a from Line 14b. ..........................................15b.
	 15.	 Interest due - Sixteenth day of the eighth month through the fifteenth day of the
		 ninth month. Multiply Line 15b by .01 if greater than zero. ....................................................................... 15.
16a.	Third installment: Enter payments made or credits received on or before the fifteenth day
			 of the ninth month. .......................................................................................................................................16a.
	16b.	Second installment underpayment balance: Subtract Line 16a from Line 15b............................................16b.
	16c.	Enter the third required installment amount due on the fifteenth day of the ninth month.
			 See instructions. ..........................................................................................................................................16c.
	16d.	Third installment underpayment balance: Add Line 16b and Line 16c. .......................................................16d.
	 16.	 Interest due - Sixteenth day of the ninth month through the fifteenth day of the
		 tenth month. Multiply Line 16d by .01 if greater than zero. ....................................................................... 16.
Continued Part IV – Computation of Interest on Page 3.

Form CT-1120I (Rev. 12/23)

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Part IV – Computation of Interest, continued
	17a.	Enter payments made or credits received on or before the fifteenth day of the tenth month. .................... 17a.
	17b.	Third installment underpayment balance: Subtract Line 17a from Line 16d............................................... 17b.
	 17.	 Interest due - Sixteenth day of the tenth month through the fifteenth day of the
			eleventh month. Multiply Line 17b by .01 if greater than zero. ......................................................... 17.
18a.	Enter payments made or credits received on or before the fifteenth day of the eleventh month................ 18a.
	18b.	Third installment underpayment balance: Subtract Line 18a from Line 17b............................................... 18b.
	 18.	 Interest due - Sixteenth day of the eleventh month through the fifteenth day of the
			twelfth month. Multiply Line 18b by .01 if greater than zero. ............................................................ 18.
19a.	Fourth installment: Enter payments made or credits received on or before the fifteenth day
			 of the twelfth month. ................................................................................................................................... 19a.
19b.	Third installment underpayment balance: Subtract Line 19a from Line 18b............................................... 19b.
	19c.	Enter the fourth required installment amount due on the fifteenth day of the twelfth month.
			 See instructions. ......................................................................................................................................... 19c.
	19d.	Fourth installment underpayment balance: Add Line 19b and Line 19c. .................................................... 19d.
	 19.	 Interest due - Sixteenth day of the twelfth month through the fifteenth day of the
			thirteenth month. Multiply Line 19d by .01 if greater than zero. ....................................................... 19.
20a.	Enter payments made or credits received on or before the fifteenth day of the thirteenth month. ............. 20a.
20b.	Fourth installment underpayment balance: Subtract Line 20a from Line 19d. ........................................... 20b.
	 20.	 Interest due - Sixteenth day of the thirteenth month through the fifteenth day of the
			fourteenth month. Multiply Line 20b by .01 if greater than zero. ...................................................... 20.
21a.	Enter payments made or credits received on or before the fifteenth day of the fourteenth month. ............ 21a.
	21b.	Fourth installment underpayment balance: Subtract Line 21a from Line 20b............................................. 21b.
	 21.	 Interest due - Sixteenth day of the fourteenth month through the fifteenth day of the
	 	 fi
	 fteenth month. Multiply Line 21b by .01 if greater than zero. .......................................................... 21.
	22a.	Enter payments made or credits received on or before the fifteenth day of the fifteenth month. ............... 22a.
22b.	Fourth installment underpayment balance: Subtract Line 22a from Line 21b............................................. 22b.
	 22.	 Interest due - Sixteenth day of the fifteenth month to the fifteenth day of the
		 sixteenth month. Multiply Line 22b by .01 if greater than zero. ........................................................ 22.
	23a.	Enter payments made or credits received on or before the fifteenth day of the sixteenth month. ............. 23a.
23b.	Fourth installment underpayment balance: Subtract Line 23a from Line 22b............................................. 23b.
	 23.	 Interest due - Sixteenth day of the sixteenth month to the fifteenth day of the
		 seventeenth month. Multiply Line 23b by .01 if greater than zero. ................................................... 23.
	
	 24.	Total interest due: Add Lines 10 through 23. Enter here and on the appropriate
			Connecticut tax form. ......................................................................................................................................... 24.

Form CT-1120I (Rev. 12/23)

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General Instructions

Use Form CT‑1120 I to determine interest due on
underpayments of 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; or Form CT‑990T, Connecticut Unrelated Business
Income Tax Return.

Filing Electronically

File Form CT-1120I electronically using myconneCT.
DRS myconneCT allows taxpayers
to electronically file, pay and manage
state tax responsibilities.

Part II – Computation of Required Installments

The required installment payments are due on or before the
fifteenth day of the third, sixth, ninth, and twelfth months of
the income year. If the due date falls on a Saturday, Sunday,
or legal holiday, the return is considered timely if filed on the
next business day.
Lines 6 through 9 - Calculate the required minimum installment
payments due. Multiply the required annual payment from
Part I, Line 5, by the required installment rate and enter the
result on the appropriate line.

Part III – Annualized Income Installment Schedule

Complete Column A entirely before proceeding to the
next column.
Line 1 - Enter the corporation’s total net income for the period.

Filing by Mail

If you file by mail, complete this form in blue or black ink only.
Do not use staples.
Please note that each form is year specific. To prevent any
delay in processing your return, the correct year’s form must
be submitted to the Department of Revenue Services (DRS).
If the taxpayer made timely installment tax payments (Forms
CT‑1120 ESA, ESB, ESC, and ESD or Forms CT‑990T ESA,
ESB, ESC, and ESD) and paid at least 90% of the current year
tax or 100% of the prior year tax before the application of any
tax credits at the required installment rate, the taxpayer is not
subject to interest on underpaid installments. If the current year
tax is $1,000 or less, it is not necessary to complete this form.
Estimated tax payments may be based on the applicable
percentage of the current year tax determined by annualizing
net income. For more information, see portal.ct.gov/DRS/
Corporation-Tax/Tax-Information.

Line Instructions
Part I – Computation of Required Annual Payment

Line 1 - Enter the tax liability for the current income year
from Form CT‑1120, Schedule C, Line 8 minus Schedule C,
Line 1c; or Form CT‑1120CU, Part III, Line 5, minus Part I,
Line 8, Combined Group Total column; or Form CT‑990T,
Computation of Amount Payable, Line 5.
The amount entered on Line 1 should not include any
recapture of tax credits.
Line 3 - Enter the total tax from the prior income year before
the application of any tax credits. The amount entered on
this line should not include any recapture of tax credits. If no
return has been filed for the prior income year or a return was
filed for less than a twelve-month period, leave Line 3 blank
and enter the amount from Line 2 on Line 5.
Combined unitary filers included in a 2023 Form CT‑1120CU,
Combined Unitary Corporation Business Tax Return, should
enter the total tax due from all of the taxable members of
the combined group that filed with the designated taxable
member in the prior year minus any recapture of tax credits.

Form CT-1120I Instructions (Rev. 12/23)

Line 4 - The Connecticut Corporation Business Tax rate is
7.5%. Multiply Line 3 by 7.5% (.075).
Line 5 - If the corporation is subject to the surtax, multiply
Line 4 by 10% (.10).
Line 7 - Enter the corporation’s estimated allowable
Connecticut Corporation Business Tax credits for the year.
The amount of tax credits allowable against the Corporation
Business Tax generally may not exceed 50.01% of the
amount of tax due prior to the application of the tax credits.
Research and Development and Research and Experimental
Expenditures tax credits that remain after the application of
the 50.01% limitation may be further used, provided that the
total of all tax credits used shall not exceed 70% of the tax
liability.
In addition, no tax credit other than the PE credit shall reduce
an included corporation’s tax to an amount less than $250.
Also, include any PE credit available to the corporation. The
PE credit is not subject to limitation and may reduce the tax
liability to zero.
Line 11 - Do not enter an amount in Column A. Enter in
Column B the amount from Line 17, Column A. Enter in
Column C the sum of Line 17, Column A, plus Line 17
Column B. Enter in Column D the sum of Line 17, Column A,
plus Line 17, Column B, plus Line 17, Column C.
Line 13 - Enter the appropriate amount of your required
installment as calculated in Part II, Lines 6 through 9, above.
Line 19 - Enter all estimated tax payments made through the
payment due date for the period. Include overpayments of the
prior year tax applied to the current year. If the tax return for
the prior year was filed on time or within the extension period
if a timely request for an extension was filed, the overpayment
will be treated as estimated tax paid on the fifteenth day of
the third month (March 15 for calendar year filers). Otherwise,
the overpayment will be treated as estimated tax paid on the
date the prior year tax return was filed.

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Part IV – Computation of Interest

Lines 10a, 13c, 16c, and 19c - Enter on the appropriate line
the required installment amount due. The required installment
amounts are computed as follows:

Regular Filers

Enter the amount from Part II, Lines 6 through 9; or

Annualized Filers

Enter the amount from Part III, Line 17, Column A, on Line 10a.
Enter the amount from Part III, Line 17, Column B, on Line 13c.
Enter the amount from Part III, Line 17, Column C, on Line 16c.
Enter the amount from Part III, Line 17, Column D, on Line 19c.
Lines 10b, 11a, 12a, 13a, 14a, 15a, 16a, 17a, 18a, 19a,
20a, 21a, 22a, and 23a - Enter the payment amounts on the
appropriate line according to the date of payment. Payments
are first applied to any remaining unpaid installment balances
that are due, and then are applied to the next required
installment due.
Lines 10 through 23 - Interest is calculated monthly on the
accumulated unpaid required installment balance due at 1%
per month or fraction of a month.
Line 24 - Enter the total interest due on unpaid required
installment balances by adding the amounts on Lines 10
through 23. This amount is also entered on the appropriate
Connecticut tax form.

Form CT-1120I Instructions (Rev. 12/23)

Payments if a Waiver From Electronic Filing has
been Obtained

If you have been granted a waiver from electronic filing,
payments accompanying estimated installment Forms
CT‑1120 ESA, ESB, ESC, and ESD or Forms CT‑990T ESA,
ESB, ESC, and ESD will be credited as of the U.S. Postal
Service cancellation mark on the payment. The payment must
be made or credit established on or before the due date of the
required installment. Payments of estimated tax are credited
first against underpaid installments in the order in which such
installments are required to be paid. The overpayment from
the filing of a Corporation Business Tax return is established
as of the U.S. Postal Service cancellation mark on the
completed return.
Non-electronic payments made without an electronic
payment waiver will be subject to penalty.
Taxpayers can use certain private delivery services, in
addition to the U.S. Postal Service, for delivering returns,
claims, statements or other documents, or payments, and
meet the timely filing/payment rules. The timely filed and
timely payment rules will be met if the U.S. Postal Service
cancellation date, or the date recorded or marked by a
designated private delivery service (PDS) using a designated
type of service, is on or before the due date. Not all types of
service provided by these designated PDSs qualify.
See Policy Statement 2016(4), Designated Private Delivery
Services and Designated Types of Service, for a current list
of qualified PDSs.

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