Small Business Tax Deadlines: Complete Calendar (2025-2026)

By Yarik Yarosh, CPA ·September 5, 2026 ·Updated October 4, 2026 ·6 min read

Written by Yarik Yarosh, CPA (US & Canada) September 5, 2026 · FL CPA license AC61704 · CPA Ontario

Missing a tax deadline triggers automatic penalties. The late-filing penalty for S-Corps is $255 per shareholder per month (up to 12 months) for returns due in 2026, and partnerships pay the same $255 per partner per month. These penalties are assessed automatically, even if no tax is owed. Estimated tax penalties are calculated per quarter. Knowing the deadlines and setting calendar reminders is the simplest way to avoid thousands in unnecessary penalties.

✓Key takeaway

Key deadlines by entity type:

Sole proprietors and single-member LLCs:

Deadline	What’s Due
January 15	Q4 estimated tax payment (or file return by January 31 to skip Q4 payment)
January 31	File W-2s for employees; file 1099-NEC for contractors paid $600+
April 15	File Form 1040 (Schedule C) + pay any balance due
April 15	Q1 estimated tax payment for current year
June 15	Q2 estimated tax payment
September 15	Q3 estimated tax payment
October 15	Extended Form 1040 due (if extension filed)

S-Corps (Form 1120-S):

Deadline	What’s Due
January 15	Q4 estimated tax payment (owner’s personal)
January 31	File W-2s for all employees (including owner); file 1099-NEC for contractors
March 15	File Form 1120-S (S-Corp return) + Schedule K-1s to shareholders
March 15	Form 2553 deadline for S-Corp election effective January 1 of current year
April 15	Owner files personal Form 1040 + pays any balance
September 15	Extended Form 1120-S due (if extension filed on Form 7004)

Partnerships (Form 1065):

Deadline	What’s Due
January 31	File 1099-NEC for contractors
March 15	File Form 1065 + Schedule K-1s to partners
September 15	Extended Form 1065 due (if extension filed)

C-Corps (Form 1120):

Deadline	What’s Due
April 15	File Form 1120 + pay corporate tax
October 15	Extended Form 1120 due

Payroll deadlines (all employers):

Deadline	What’s Due
Monthly or semi-weekly	Payroll tax deposits (941 deposits, schedule depends on liability amount)
April 30, July 31, October 31, January 31	File Form 941 (quarterly payroll tax return)
January 31	File Form 940 (annual FUTA return)
January 31	Furnish W-2s to employees

Late-filing penalties:

Form 1120-S (S-Corp): $255/month per shareholder (returns due in 2026), up to 12 months = $3,060 per shareholder for a full year late
Form 1065 (Partnership): $255/month per partner (returns due in 2026), up to 12 months = $3,060 per partner
Form 1040 (individual): 5%/month of unpaid tax, up to 25%
1099-NEC filed late: $60-$330 per form depending on how late (up to $660 for intentional disregard)
W-2 filed late: $60-$330 per form

What does a missed deadline actually cost?

Hypothetical example

Round numbers. For current guidance, see IRS tax calendar for businesses. An S-Corp with one shareholder (the owner). The S-Corp return (Form 1120-S) was due March 15 but not filed until September 15 (6 months late). No extension was filed.

Late-filing penalty: $255/month x 1 shareholder x 6 months = $1,530

This penalty applies even if the S-Corp owed $0 in tax (S-Corps are pass-through entities, so the entity itself rarely owes tax). The penalty is for failing to file the return on time, not for failing to pay.

How to avoid it: File Form 7004 (automatic 6-month extension) by March 15. This extends the filing deadline to September 15. The extension is automatic (no reason required). The extension gives more time to FILE, not more time to PAY (any estimated tax owed by the shareholder is still due by the original deadline).

The extension strategy: Most CPAs recommend filing Form 7004 for EVERY S-Corp and partnership return, every year, even if the return will be ready by March 15. Reason: if something delays the filing (missing documents, K-1s from other partnerships, unexpected issues), the extension is already in place. Filing a return early doesn’t cost anything extra, but a missed deadline without an extension costs $255/month per shareholder (returns due in 2026).

Estimated tax penalty scenario: The same S-Corp owner also missed all 4 estimated tax payments. Prior year tax: $20,000. Current year tax: $28,000.

Estimated tax penalty: approximately $1,100 (on the $20,000 prior year safe harbor amount, unpaid across 4 quarters).

Total cost of missed deadlines:

S-Corp late-filing penalty: $1,530
Estimated tax underpayment penalty: $1,100
Total: $2,630

Both penalties were avoidable. The extension (Form 7004) takes 5 minutes to file. The estimated tax payments take 15 minutes per quarter.

Illustrative numbers only, not a real client and not advice for your facts.

What penalties apply for underpaying estimated taxes?

The IRS charges an underpayment penalty under IRC 6654 when you owe more than $1,000 at filing and your withholding and estimated payments are less than the required annual amount. The penalty is calculated separately for each quarter, so a large payment in Q4 doesn’t fully cure an underpayment from Q1 through Q3. You avoid the penalty entirely if your payments cover at least 90% of the current year’s tax or 100% of last year’s tax (110% if your AGI exceeded $150,000). The penalty rate is the federal short-term rate plus 3 percentage points, compounded daily. If your income varies significantly during the year, the annualized income installment method (Form 2210, Schedule AI) can reduce or eliminate the penalty by matching payments to when income was actually earned.

What happens if I miss an estimated tax payment deadline?

The IRS assesses the underpayment penalty under IRC 6654 separately for each quarter you missed or underpaid. The penalty accrues from the due date of the missed payment until it is paid or until the return filing date, whichever comes first. Making a late payment reduces the penalty for that quarter but does not eliminate it. If you missed one quarter but overpaid another, the overpayment applies to subsequent quarters chronologically but does not retroactively cure an earlier underpayment. Filing Form 2210 with your return lets you calculate whether the annualized income installment method reduces or eliminates the penalty.

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Cite this page

Yarik Yarosh, CPA. "Small Business Tax Deadlines: Complete Calendar (2025-2026)." Blue Cloud CPA, September 5, 2026, updated October 4, 2026. https://bluecloudcpa.com/guides/small-business-tax-deadlines-complete-calendar

This guide is general information, not tax advice for your specific situation. Which points apply, and how, depends on your facts.