How to Fill Out New York Form CT-300: Mandatory First Installment (MFI)

New York Form CT-300 is how certain corporations prepay a portion of their estimated tax at the start of the tax year. The form is one page. You take your tax after credits from two years ago, multiply by either 25% or 40%, and send the payment to the Department of Taxation and Finance by the 15th day of the third month of your tax year. For calendar-year filers in 2026, that deadline is March 16, 2026.¹

Does Your Corporation Have to File

The mandatory first installment is triggered by what you owed two years ago, not last year. For a 2026 filing, look at your 2024 tax after credits.

Article 9-A C corporations must file if the second preceding year’s franchise tax after credits exceeded $5,000. That $5,000 threshold applies to tax years beginning on or after January 1, 2026.²
Corporations taxed under Article 9 (Sections 184, 186-a, and 186-e) and Article 33 must file if the second preceding year’s tax after credits exceeded $1,000.¹
Authorized life insurance companies file if the second preceding year’s tax after credits exceeded $1,000.¹

New York S corporations do not file Form CT-300. Their estimated tax process under Section 213-b uses the preceding year’s tax rather than two years back.³ And if your corporation didn’t exist two years ago, meaning it wasn’t required to file a return for the second preceding tax year, you owe no MFI at all.¹

How to Calculate the Payment

Two tiers, set by the size of your second preceding year’s tax after credits:

If that tax was above your applicable threshold but did not exceed $100,000, multiply it by 0.25.
If that tax exceeded $100,000, multiply by 0.40.

Authorized life insurance companies always use 40%, regardless of amount.¹

MTA Surcharge

If your corporation is subject to the metropolitan transportation business tax (MTA surcharge) and meets the thresholds above, you also owe an MFI on the surcharge at the same percentage you used for the primary tax. A 25% filer pays 25% of the second preceding year’s surcharge; a 40% filer pays 40%.¹

Filling Out the Form Line by Line

Enter the corporation’s name, address, Employer Identification Number, and the beginning and ending dates of the current tax year in mm-dd-yyyy format.³ Then work through the numbered lines:

Line 1: total franchise, excise, or gross receipts tax after credits from your second preceding tax year.
Line 2: Line 1 multiplied by 0.25 or 0.40. This is the MFI of your primary tax.
Line 3: the MTA surcharge from the second preceding tax year. Leave blank if you aren’t subject to the surcharge.
Line 4: Line 3 multiplied by the same percentage used on Line 2.
Line 5: any prior-year overpayment you want to apply (optional, with the caveat below).
Line 8: the estimated overpayment remaining after partial application to the MFI.

Lines 1 and 3 come straight from your 2024 corporate return, so pull that return before you start.¹

Using a Prior-Year Overpayment on Line 5

Line 5 lets you cover part or all of the MFI with an overpayment you expect from the preceding year. A 2026 filer can apply an anticipated overpayment from the 2025 return even if that return hasn’t been filed yet by March 16.¹

One point catches filers out. If you use Line 5, you must enter the total anticipated overpayment from the preceding year, not just the amount you want applied to the MFI. The Department applies payments to the earliest liability first. If your 2025 return ends up owing more than you expected, the overpayment you earmarked for the MFI can be pulled to cover the 2025 balance instead, leaving the MFI short and exposing you to penalties and interest. The official instructions put it plainly: if you aren’t sure how much overpayment will be available, pay the MFI with new funds and leave Line 5 blank.¹

When the Payment Is Due

The MFI is due on or before the 15th day of the third month of your tax year. Calendar-year corporations in 2026 pay by March 16, 2026, because March 15 falls on a Sunday.¹ Fiscal-year filers count from the start of their own tax year. The MFI is separate from the remaining installments, which are made later in the year on Form CT-400.

How to File and Pay

File online through a Business Online Services account at tax.ny.gov. Log in or create an account, go to the Corporation tax menu, and select “Make estimated or MFI payment.” You get an instant confirmation.⁴

If you file by mail, send the completed form and payment to:

NYS ESTIMATED CORPORATION TAX
PO BOX 15200
ALBANY NY 12212-5200¹

Private delivery services listed in Publication 55 are also accepted. Keep your mailing receipt as proof of timely filing.

E-File Mandate

Many corporations are required to file electronically. The mandate applies if you prepare your own tax documents without a tax professional, use approved e-file software or a computer to prepare the form, and have broadband internet access. If all three are true, paper filing triggers a $50 penalty per document, plus a separate $50 penalty for not paying electronically.⁵ Any claimed overpayment also won’t earn interest until the return is filed electronically. Electronic payment options include ACH debit (through Online Services or your software) and ACH credit.

Penalties for Underpayment

A shortfall at the deadline is treated as an underpayment of estimated tax under New York Tax Law Section 1085. The penalty is calculated as interest on the underpayment amount at the rate set by the Tax Commissioner, with a floor of 7.5% per year. For the first quarter of 2026, the corporate underpayment rate is 11%.⁶ Interest accrues from the missed payment date through the 15th day of the fourth month after the close of the tax year, which is April 15 for calendar-year filers. A missed March MFI payment runs up roughly 13 months of interest before the clock stops.

Larger corporations face a stricter standard. Under Section 1085(e), corporations with New York business income of $1 million or more in any of the three preceding years cannot use the 91% safe harbor that smaller corporations rely on; they must cover 100% of the current year’s actual tax liability through estimated payments.

1
New York State Department of Taxation and Finance. Instructions for Form CT-300, Mandatory First Installment (MFI) of Estimated Tax for Corporations
2
New York State Senate. New York Tax Law Section 213-B – Payments on Account of Estimated Tax
3
New York State Department of Taxation and Finance. Instructions for Form CT-300, Mandatory First Installment (MFI) of Estimated Tax for Corporations
4
New York State Department of Taxation and Finance. Form CT-300, Mandatory First Installment (MFI) of Estimated Tax for Corporations
5
New York State Department of Taxation and Finance. Electronic Filing Mandate for Business Taxpayers
6
New York State Department of Taxation and Finance. Interest Rates: 1/01/2026 – 3/31/2026