Missing a tax deadline can cost your startup thousands in penalties, unwanted IRS attention, and administrative headaches. Whether you're a bootstrapped founder or backed by venture capital, staying on top of your tax obligations is non-negotiable. This guide walks through every deadline that falls in calendar year 2026, what tax year each one belongs to, and which dates move because they land on a weekend.
The 2026 Tax Deadlines Every Startup Needs
Here's your quick reference for the dates that fall in 2026. Four of them are not the dates you would expect, because the statutory day lands on a weekend and shifts to the next business day.
February 2: W-2s, 1099-NECs, Form 940, and the Q4 2025 Form 941. The statutory date is January 31, which is a Saturday in 2026.
March 1: Delaware franchise tax and annual report. Delaware does not move this for weekends even though March 1, 2026 is a Sunday.
March 16: S-Corp and Partnership returns for 2025 (Form 1120-S, Form 1065). The statutory date is March 15, which is a Sunday in 2026.
April 15: C-Corp return for 2025 (Form 1120), individual returns (Form 1040), and the first estimated tax installment for 2026.
December 15: Fourth and final estimated tax installment for calendar-year corporations. There is no individual equivalent, so it does not appear on a personal tax calendar at all.
For startups with employees, add the quarterly Form 941 filings: April 30, July 31, November 2 (October 31 is a Saturday), and February 1, 2027 (January 31 is a Sunday). State payroll deposits and annual state unemployment filings sit alongside them.
What Are Startup Tax Deadlines and Why They Matter
Startup tax deadlines are the fixed dates when you must file tax returns, pay estimated taxes, report employee withholdings, and submit compliance forms to the IRS and state authorities. Missing these deadlines triggers penalties, interest charges, and potential audit risk. For founders, understanding the tax calendar is as important as understanding your burn rate.
The IRS doesn't care if you're pre-revenue or generating millions in ARR. The deadline applies equally to all business structures. A single missed deadline can cascade into other compliance failures, making it harder to raise capital, pass due diligence, and sleep at night.
One thing worth getting straight before the calendar itself: a date falling in 2026 does not mean it belongs to the 2026 tax year. The returns you file in the first half of 2026 report your 2025 activity. The estimated payments you make across 2026 are prepayments against your 2026 liability. Both appear below, labelled.
The Complete 2026 Tax Calendar by Month and Deadline Type
January and February 2026
February 2, 2026: W-2 and 1099 Deadlines (for 2025)
File Form W-2 with the Social Security Administration for all employees paid in 2025 and distribute copies to each employee. If your startup paid independent contractors $600 or more in 2025, file Form 1099-NEC or Form 1099-MISC with the IRS and send copies to the contractors.
The statutory deadline is January 31, but January 31, 2026 is a Saturday, so the date moves to Monday, February 2. Late information returns due in 2026 cost $60 per form if you file within 30 days, $130 if you file by August 1, and $340 after that. Intentional disregard is $680 per return, with no maximum. These amounts are adjusted every year, so the $330 and $660 figures still in circulation belong to returns that were due in 2025.
Note that the $600 threshold applies to the 2025 forms you file now. Under the One Big Beautiful Bill Act, the reporting threshold rises to $2,000 for payments made in 2026, so the forms you file in early 2027 follow a different rule. The $600 threshold survives for gross proceeds paid to an attorney, reported in box 10 of Form 1099-MISC, which is the carve-out a startup is most likely to run into.
February 2, 2026: Q4 2025 Payroll Tax (Form 941) and Form 940
Employers file Form 941 covering October through December 2025, which reconciles federal income tax and Social Security and Medicare withholdings along with employer contributions. Form 940, the annual federal unemployment return, is due the same day. FUTA runs at 6.0% on the first $7,000 of each employee's wages, with a credit of up to 5.4% for state unemployment tax, which leaves 0.6% in most states.
February 2, 2026: Form 3921 for ISO Exercises
If any employee exercised an incentive stock option during 2025, you owe them a Form 3921 by this date. The IRS copy is due March 2 on paper or March 31 electronically. You file one Form 3921 for each transfer, not one per employee, so three separate exercises in 2025 mean three forms. The penalty schedule is the same as for W-2s and 1099s.
State Unemployment Insurance (SUI) Filings
Many states require annual SUI filings in January or February. Check with your state's labor department. Failure to file on time can result in loss of experience rating credits, meaning higher rates the following year.
March 2026
March 1, 2026: Delaware Franchise Tax and Annual Report
If you incorporated in Delaware, and most venture-backed startups do, the franchise tax and annual report are both due March 1. Delaware's Division of Corporations does not extend for weekends the way the IRS does, so the Sunday date stands and the filing system is open.
The notice Delaware sends is calculated under the Authorized Shares method, which produces alarming numbers for a company with millions of authorized shares. Most startups should recalculate under the Assumed Par Value Capital method instead, where the minimum is $400. The minimum under Authorized Shares is $175, and the annual report fee is $50 on top of either. Missing the date costs $200 plus 1.5% interest per month and puts your good standing at risk, which can stall a financing or a new bank account.
March 16, 2026: S-Corp and Partnership Tax Return Deadline (for 2025)
If your startup is taxed as an S-Corporation (Form 1120-S) or a Partnership (Form 1065), your 2025 return is due today. The statutory date is March 15, which falls on a Sunday in 2026, so the deadline moves to Monday, March 16. Form 7004 buys an automatic six-month extension to September 15, but it extends the filing, not the payment.
Form 1120-S requires detailed reporting of income, deductions, depreciation, and pass-through allocation to shareholders, and the partnership issues a Schedule K-1 to each owner. Filing late costs $255 per shareholder or partner per month, for up to 12 months, on returns required to be filed in 2026. A four-shareholder S corp that files three months late owes $3,060 before anything else.
Payroll Tax Deposits for Q1
If your startup has employees, monthly or semi-weekly payroll tax deposits for January and February are due throughout March, depending on your deposit schedule. Check your IRS deposit schedule letter.
April 2026
April 15, 2026: C-Corporation Tax Return Deadline (Form 1120, for 2025)
This is the marquee tax day. C-Corporations file Form 1120 for their 2025 year by April 15, 2026. Most venture-backed startups are Delaware C-Corps, so this is likely your deadline.
Form 1120 requires comprehensive reporting of corporate income, business expenses, depreciation, capital gains, and losses. If your startup is unprofitable, which many early-stage startups are, you still file to document the losses that can offset future gains.
April 15, 2026: First Estimated Tax Installment for 2026
A calendar-year corporation's estimated tax installments are due on the 15th day of the 4th, 6th, 9th, and 12th months of its tax year, which means April 15, June 15, September 15, and December 15. March 15 is not a corporate estimated tax date. It belongs to pass-through returns.
Corporations must make estimated payments if they expect to owe $500 or more for the year. Individuals follow a different fourth installment, due January 15 of the following year rather than December 15.
April 15, 2026: Individual Tax Day
Founders and employees file their personal 1040 returns today. Even if your startup hasn't made distributions, you may owe estimated taxes based on other income sources.
April 15, 2026: Form 5472 for Foreign-Owned LLCs
A single-member LLC with a foreign owner files Form 5472 attached to a pro forma Form 1120, even with no US income and no tax due. The penalty for not filing starts at $25,000, which makes this the highest-cost omission on the calendar for international founders.
April 30, 2026: Q1 2026 Payroll Tax (Form 941)
Form 941 for Q1 2026, covering January through March, is due today. This covers federal income tax withholdings, Social Security, and Medicare taxes for all employees.
June and July 2026
June 15, 2026: Second Estimated Tax Installment for 2026
The second installment for both corporations and individuals. June 15, 2026 is a Monday, so the date does not shift. Failing to make estimated payments can trigger penalties and interest even where the year ends up at a loss.
July 31, 2026: Q2 2026 Payroll Tax (Form 941)
Form 941 for Q2 2026, covering April through June, is due today.
September 2026
September 15, 2026: Partnership and S-Corp Extension Deadline
If you filed Form 7004 by March 16, your extended 2025 return for a partnership (Form 1065) or S-Corp (Form 1120-S) is due today. The extension is automatic once the form is filed, but you still have to submit the actual return by this date.
September 15, 2026: Third Estimated Tax Installment for 2026
The third of four installments for corporations, and the third of four for individuals.
October and November 2026
October 15, 2026: C-Corporation and Individual Extension Deadline
If you filed Form 7004 by April 15, your extended 2025 C-Corp return (Form 1120) is due today. Extended individual returns are due the same day.
November 2, 2026: Q3 2026 Payroll Tax (Form 941)
Form 941 for Q3 2026, covering July through September, is due today. The statutory date is October 31, which is a Saturday in 2026.
December 2026
December 15, 2026: Fourth Estimated Tax Installment for Corporations
The final corporate installment for the 2026 tax year. Individuals do not have a December installment; theirs falls on January 15, 2027. A calendar that blends the corporate and personal dates into one list has nothing to put on this line.
December 31, 2026: Tax Year Cutoff
All business transactions, depreciation, and income must be captured by December 31 to be included in your 2026 tax year. Any transactions after this date belong to 2027. This is also the date Delaware measures issued shares and gross assets against for next year's franchise tax calculation.
January and February 2027
January 15, 2027: Fourth Estimated Tax Installment for Individuals
Founders taking distributions from a pass-through entity make their final 2026 installment today.
February 1, 2027: W-2s, 1099s, Form 940, and Q4 2026 Form 941
January 31, 2027 is a Sunday, so the cycle starts again on Monday, February 1. Remember that the 1099 reporting threshold for 2026 payments is $2,000 rather than $600.
How the Weekend Rule Moves Four of These Dates
When a federal filing deadline falls on a Saturday, Sunday, or legal holiday, it moves to the next business day. 2026 is an unusually bad year for this, and four of the dates a founder is most likely to have memorised are wrong this year:
January 31 is a Saturday, so W-2s, 1099s, Form 940, and the Q4 2025 Form 941 are due February 2.
March 15 is a Sunday, so Form 1120-S and Form 1065 are due March 16.
October 31 is a Saturday, so the Q3 Form 941 is due November 2.
January 31, 2027 is a Sunday, so the Q4 2026 filings are due February 1, 2027.
The rule does not apply everywhere. Delaware holds its March 1 franchise tax date regardless of the day of the week, so a founder who assumes the federal convention applies and files on March 2 has already incurred the $200 penalty.
The Dates That Move in Your Favour
The weekend rule pushes a deadline because the calendar forces it. Other dates move because you were on time all along, and they are worth knowing before you spend a weekend rushing a filing you had an extra week for.
Form 941 and the 10th-day rule. If you made your payroll deposits in full and on time for the quarter, you can file the return by the 10th day of the second month after the quarter ends rather than the last day of the first. For 2026 that means May 11 instead of April 30, because May 10 is a Sunday, then August 10 instead of July 31, November 10 instead of November 2, and February 10, 2027 instead of February 1. The deposits are the hard deadline. The return itself is not.
Form 940 and February 10. The same principle applies to the annual federal unemployment return. Deposit all your FUTA tax when it was due and you have until February 10 to file rather than February 2.
The January 15 individual installment can be skipped. A founder who files their 2026 personal return by February 1, 2027 and pays the entire balance with it does not have to make the January 15, 2027 estimated payment.
One number moves the other way. A return filed more than 60 days late, where the due date falls after December 31, 2025, carries a minimum penalty of $525 or 100% of the tax owed, whichever is less. On a small balance that floor is the whole penalty, and it does not scale down because the amount was small.
How to Structure Your Tax Calendar by Business Entity
C-Corporation (Most VC-Backed Startups)
If your startup incorporated in Delaware or another state and is taxed as a C-Corp, your primary dates are:
March 1: Delaware franchise tax and annual report, if you are a Delaware corporation
April 15: Form 1120 for 2025, and the first 2026 estimated tax installment
June 15: Second 2026 estimated tax installment
September 15: Third 2026 estimated tax installment
October 15: Extended 2025 return, if you filed Form 7004 by April 15
December 15: Fourth 2026 estimated tax installment
Plus quarterly payroll tax returns on Form 941 if you have employees
C-Corp structures are favoured by VCs because they allow multiple classes of stock, preferred equity, and clean cap tables. They face two layers of tax, corporate and then shareholder on distributions, which is one reason early-stage companies reinvest rather than distribute.
S-Corporation Election
A corporation can elect pass-through taxation by filing Form 2553, which sends income to shareholders' personal returns. The election comes with eligibility limits that rule it out for most venture-backed companies: no more than 100 shareholders, only one class of stock, and no partnerships or corporations as shareholders, which excludes venture funds. It suits profitable founder-owned businesses rather than companies planning to raise institutional money. If you have made the election:
March 16: Form 1120-S due for 2025
September 15: Extended Form 1120-S due, if Form 7004 was filed by March 16
Plus estimated quarterly payments on Form 1040-ES for shareholders, due April 15, June 15, September 15, and January 15, 2027
Plus quarterly payroll tax returns on Form 941 if you have W-2 employees
LLC or Partnership (Pass-Through Taxation)
If your startup is taxed as a partnership:
March 16: Form 1065 due for 2025
September 15: Extended Form 1065 due, if Form 7004 was filed by March 16
The partnership issues a Schedule K-1 to each partner, who reports it on a personal return due April 15
Plus estimated quarterly payments on Form 1040-ES for partners
Plus quarterly payroll tax returns on Form 941 if you have W-2 employees
Special Deadlines for Startups with R&D Activities
If your startup conducts research and development, you may qualify for the federal R&D credit under Section 41. The regular credit is 20% of qualified research expenses above a base amount. Most startups instead use the Alternative Simplified Credit, which is 14% of qualified expenses above 50% of the average for the prior three years, or 6% of current-year expenses where the company had no qualified research expenses in any one of the three preceding years, which is the rate most first-time claimants end up on.
The critical deadline is your return filing date or extended deadline, October 15 for a calendar-year C-Corp. You claim the credit on Form 6765 filed with the return, and documentation of R&D activities including engineer time, equipment, and failed experiments must be preserved for audit defence.
A pre-revenue startup can elect to apply up to $500,000 of the credit per year against payroll tax, which is what makes the credit worth cash rather than a carryforward. Only the first $250,000 offsets the employer share of social security tax. Anything above that offsets the employer share of Medicare tax. The election has to be made on an originally filed return, not an amended one, and the company needs gross receipts under $5 million for the credit year and no gross receipts at all in any year before the five-year period ending with it. That second test is stricter than it reads: for a 2025 credit year the window runs 2021 through 2025, so one invoice collected in 2020 rules the company out however small it is now. Once elected, the offset is claimed on Form 8974 alongside the Form 941 for the first quarter that begins after the return is filed, and it draws down against payroll as it runs rather than arriving as a refund.
For a detailed guide on claiming the R&D credit and qualifying activities, see our comprehensive guide: R&D Tax Credits for Startups: The Complete Guide to Claiming What You've Earned
State and Local Tax Deadlines
Federal deadlines are only part of the picture. Most states impose their own filing and payment requirements:
Sales Tax Filings (Monthly, Quarterly, or Annual depending on state and volume)
Due dates vary by state; many are the 20th of the following month
Illinois, Texas, Florida, and New York have unique rules
Economic nexus thresholds determine filing requirements across states
State Income Tax Returns
Most states follow the federal due dates, April 15 for corporations and March 16 for pass-throughs in 2026
A handful set their own dates or require a separate state extension even where you have a federal one
Multi-state startups must file in every state where they have nexus
Some states impose a minimum tax regardless of income, and California's $800 minimum franchise tax is the one startups meet most often
State Payroll Tax Filings (Monthly or Quarterly)
Combined state and local withholding deposits due throughout the month
SUTA filings due annually
Varies significantly by state
Local Tax Filings (Varies by City/County)
City business licenses and gross receipts taxes
County registration fees
Industry-specific permits
For multi-state startups, track all state deadlines in a separate calendar. Consider working with a CPA familiar with multi-state requirements.
Bookkeeping Best Practices to Never Miss a Deadline
The best way to meet every startup tax deadline is to maintain clean, accurate books throughout the year. Here's how:
Use Cloud-Based Accounting
Tools like QuickBooks Online, Xero, or FreshBooks generate financial reports automatically and sync with your bank accounts. These systems are designed to help you track your tax obligations in real-time.
Monthly Bookkeeping Review
Spend 30 minutes each month reviewing your P&L, balance sheet, and tax liability estimates. This prevents surprises at tax time.
Quarterly Tax Planning
Every three months, estimate your tax liability and set aside funds for estimated payments. This prevents cash flow crises when taxes are due.
Document Your Expenses
For startup founders, maintaining thorough documentation of business expenses is critical. The IRS allows deductions for office supplies, software subscriptions, home office expenses, meals with business purposes, and professional services. On how long to keep them, the IRS period of limitations is three years for most records. Seven years applies only to a claim for a loss from worthless securities or a bad debt deduction, six years if you leave out income worth more than 25% of the gross income shown on your return, and four years for employment tax records, counted from the date the tax was due or paid.
For more details on organizing your bookkeeping throughout the year, check out our guide: The Complete Startup Bookkeeping Guide: From Shoebox to Clean Books
Preparing for Investor Due Diligence
If your startup is raising capital, potential investors will review your tax compliance and filing history. A clean tax record demonstrates operational maturity and founder competence.
Investors typically request:
Financial statements for the life of the company, reviewed or audited where the stage warrants it
Corporate tax returns
Evidence of estimated tax payments
Proof of timely payroll tax filings
State tax compliance certificates and Delaware good standing
Missing deadlines in previous years can be remedied by filing amended returns and paying back taxes with interest. That creates friction during due diligence and raises questions about your financial controls.
For a detailed look at what investors review during due diligence, see: Investor Financial Due Diligence: Preparing Your Startup's Books for Scrutiny
Key Takeaways: Your Startup Tax Deadline Checklist
Identify your business structure: C-Corp, S-Corp, or partnership. Your structure determines your filing deadlines and tax treatment.
Map out your calendar: Add every applicable date to your calendar with 10-day reminders, using the 2026 dates above rather than the statutory ones.
Put December 15 on the list: The fourth corporate estimated installment has no individual equivalent, so it is absent from every personal tax calendar a founder might be working from.
Set up bookkeeping systems: Use cloud accounting to track income, expenses, and tax liabilities in real time.
Plan estimated payments: If you project profits, estimate the liability and set aside funds monthly.
Document everything: Keep detailed records of business expenses, R&D activities, and capital purchases.
File extensions if needed: Form 7004 buys six months. It is better to extend than to file numbers you are not confident in.
Never Miss a Deadline Again
Staying on top of startup tax deadlines is non-negotiable for compliance, audit defence, and investor credibility. Tracking federal, state, and payroll deadlines across an entire year is logistically complex, and as 2026 shows, the dates themselves move.
That's where Median comes in. We work with startup founders to organize their books, forecast tax liabilities, and make sure every deadline is met on time. Our team understands the compliance challenges of early-stage companies, from cap table complexity to multi-state nexus to R&D credit documentation.
Median tracks your tax obligations and keeps your books ready for filing. Get a free assessment today.
And if the deadlines above are the part that keeps slipping, the fix usually sits upstream of the calendar: Why Daily Bookkeeping Changes Everything: A Founder's Guide to Current Books
If your startup is in a tech, software, engineering, or scientific field, check whether you qualify for the R&D Tax Credit. Our R&D Tax Credits guide walks through eligibility and documentation requirements.