CT-1040ES Estimated Income Tax Form: Due Dates and Underpayment

The CT-1040ES estimated income tax form is the quarterly payment coupon Connecticut uses to collect income tax on earnings that aren’t covered by paycheck withholding. Residents, part-year residents, and nonresidents file it when they expect to owe $1,000 or more in Connecticut income tax after withholding and any Pass-Through Entity Tax Credit. The packet contains an Estimated Tax Worksheet and four payment vouchers, one for each installment of the year.¹

Do You Actually Need to File It?

Two conditions must both be true. Your Connecticut income tax for the year, after subtracting Connecticut withholding and any PE Tax Credit, must come to $1,000 or more. And you must expect your withholding plus PE Tax Credit to fall short of your required annual payment for 2026.¹ If withholding already covers the full bill, you can skip estimated payments regardless of how large the total tax is.

To avoid underpayment interest, your estimated payments need to hit one of two safe harbors: 90% of the tax shown on your 2026 return, or 100% of the tax shown on your 2025 return if that return covered a full twelve months.² The prior-year number is usually the easier target because it’s already known. If your 2025 return covered a shorter period, only the 90% test is available.

Calculating What You Owe Each Quarter

The Estimated Tax Worksheet sits on Page 3 of the form. Pull your prior-year Connecticut return and your 2026 federal Form 1040-ES worksheet before you start. The worksheet works through these steps:¹

Line 1 asks for your expected 2026 federal adjusted gross income, which carries over from the federal Form 1040-ES worksheet.
Line 2 adjusts that figure for Connecticut-specific modifications. One common subtraction is federally taxable Social Security benefits: single and married-filing-separately filers can subtract the full amount if expected federal AGI is under $75,000, and the threshold rises to $100,000 for married-filing-jointly, qualifying surviving spouse, and head of household filers. Above those thresholds, the Social Security Benefit Adjustment Worksheet on Page 4 produces a partial subtraction.
Line 3 combines Lines 1 and 2 into Connecticut adjusted gross income. Nonresidents and part-year residents use the greater of Connecticut-sourced income or Connecticut AGI.
Lines 4 through 6 apply the Tax Calculation Schedule printed on the form and then the apportionment factor. Residents enter 1.0000; nonresidents and part-year residents enter the ratio of Connecticut-sourced income to Connecticut AGI.
Line 7 is the credit for income taxes paid to other qualifying states.
Lines 8 through 11 add any Connecticut alternative minimum tax and subtract credits from Schedule CT-IT Credit.

The remaining lines subtract your expected withholding, PE Tax Credit, and other credits to produce the net estimated tax. Divide that by four. Each voucher carries 25% of your required annual payment. You can also pay the whole year with the first installment if that’s simpler.

Filling Out the Payment Voucher

Each voucher is a single page. Print your first name, middle initial, and last name on the top line. If you’re filing jointly, add your spouse’s name on the second line. Enter the full mailing address, including apartment number or PO Box. Fill in your Social Security Number and your spouse’s SSN if applicable. Write the exact dollar amount of this installment in the payment field.

Vouchers are pre-labeled by installment, so match the voucher to the quarter you’re paying. Use the voucher for the correct year as well; the 2026 form is a separate document from prior years.¹

Submitting the Payment

You can file and pay electronically through DRS’s myconneCT portal. Payment comes directly from a checking or savings account, or by credit or debit card (American Express, Discover, Mastercard, or Visa). Card payments carry a convenience fee charged by the card service provider; the portal displays the fee before you commit, so you can back out if you’d rather pay another way.³ The portal also shows payment history, which is useful for confirming an installment posted correctly.⁴

To pay by mail, send the voucher and a check to:

Department of Revenue Services
PO Box 2932
Hartford, CT 06104-2932

Make the check payable to “Commissioner of Revenue Services” and write “2026 Form CT-1040ES” on the front. Adding your SSN to the check is optional but helps DRS match the payment if the voucher and check get separated. Paper clip the check to the voucher. Don’t staple, and don’t send cash.¹ Keep a copy of the voucher and note the date you mailed it.

Due Dates

For calendar-year taxpayers, the 2026 installments are due:

April 15, 2026 (covers January 1 through March 31)
June 15, 2026 (covers April 1 through May 31)
September 15, 2026 (covers June 1 through August 31)
January 15, 2027 (covers September 1 through December 31)

If a due date falls on a Saturday, Sunday, or legal holiday, the deadline moves to the next business day.⁵ Fiscal-year filers follow the same pattern tied to their own year: the fifteenth day of the fourth, sixth, and ninth months, plus the fifteenth day of the first month of the following fiscal year.²

If Your Income Comes in Unevenly

Equal quarterly installments assume steady income. If yours arrives in bursts (seasonal work, a one-time capital gain, a large commission in a single quarter) the standard split can push you into overpaying early or underpaying late. Connecticut allows the annualized income installment method, which recalculates each installment based on income actually earned through that point in the year.⁶

To use it, complete the annualized income installment schedule on Form CT-2210 and attach it to your Connecticut income tax return. If you elect the annualized method for any installment, you must use it for all four. When an annualized installment comes out below the standard 25%, the difference gets recaptured by raising a later installment. Your total for the year is the same; the timing shifts to match your cash flow.⁶

Joint Payments for Married Couples

Married couples can make joint estimated payments even without deciding yet whether they’ll file their annual returns jointly or separately. If you pay jointly and later file separate Connecticut returns, you and your spouse divide the payments between the two returns however you agree.⁷

If the two separate returns together claim more than 100% of the joint payments, DRS treats that as no agreement and allocates the payments proportionally based on the tax shown on each return relative to the combined total. Once DRS allocates, the split is final. Neither spouse can override it later by showing they personally funded all or part of the payments.⁷

What Happens If You Underpay

Connecticut charges interest at 1% per month (or fraction of a month) on the underpaid amount of any installment. Interest runs separately for each installment from its due date until the earlier of April 15 of the following year or the date you cover the shortfall.⁵ Payments apply to unpaid installments in the order they came due, so a double payment in June first clears any April shortage before covering June.

There’s a way to skip the fourth installment entirely: file your annual Connecticut income tax return by January 31 and pay the full balance due at that time. Farmers and fishermen have a parallel rule allowing them to file by March 1 and pay the full amount, with no interest on the single required January 15 installment.⁸

One boundary worth noting: a decedent’s estate is exempt from estimated payments for any tax year ending within two years of the date of death. The same two-year grace period extends to certain grantor trusts that receive the residue of the decedent’s estate under a will, or that are primarily responsible for paying the decedent’s debts, taxes, and administration expenses when no will is admitted to probate.⁸

1
Department of Revenue Services. 2026 Estimated Connecticut Income Tax Payment Coupon for Individuals
2
Connecticut State Department of Revenue Services. Estimated Connecticut Income Taxes
3
Connecticut State Department of Revenue Services. Filing and Paying
4
Connecticut State Department of Revenue Services. myconneCT
5
Connecticut State Department of Revenue Services. Estimated Connecticut Income Taxes
6
Legal Information Institute (Cornell Law School). Conn. Agencies Regs. 12-722(d)(2)-1 – Annualized Income Installments
7
Legal Information Institute (LII). Conn. Agencies Regs. 12-722-1 – Estimated Tax Payments by Husband and Wife. Change of Status. Death of a Spouse
8
Connecticut Department of Revenue Services. Form CT-2210 Underpayment of Estimated Income Tax by Individuals, Trusts, and Estates