What happens if I file my federal return after April 15?

If you owe tax and file after April 15 without an approved extension, the IRS assesses a failure-to-file penalty (5% of unpaid tax per month, up to 25%) plus a failure-to-pay penalty (0.5% per month) plus interest. If you're due a refund, there's no late-filing penalty — but you have only three years from the original deadline to claim it. CPAs should advise extension filers that an extension extends time to file, never time to pay.

Do state filing deadlines automatically follow federal?

Most states conform to the federal April 15 individual deadline, but conformity varies for corporate, pass-through, and franchise filings. Several states have unique deadlines: Louisiana (May 15), Virginia (May 1), Iowa (May 1 for individual), Delaware (April 30 for individual). Always verify against the state revenue department site — TaxScout links to the authoritative source on every state page.

Are estimated quarterly payments mandatory for CPA-firm clients?

Federally, individuals must pay estimated tax if they expect to owe $1,000 or more after withholding. C-corporations have a $500 threshold. Underpayment triggers Form 2210 / 2220 penalties calculated quarterly. Most CPA firms advise clients to safe-harbor at 100% of prior-year liability (110% for AGI over $150K) to avoid the penalty entirely.

Tax Year 2026 · Filed 2027State-revenue-department verified

Colorado tax deadlines 2026

Last updated May 14, 2026

Colorado: Flat 4.40% individual income tax (reduced from 4.55% in 2023 under TABOR refund mechanism). May reduce further when state revenue exceeds TABOR cap.
Colorado offers a Pass-Through Entity Tax (PTET) election — federal SALT cap workaround.

Colorado individual income tax

Filing deadline

April 15, 2027

Extension deadline

October 15, 2027

Colorado conforms to the federal April 15 individual income tax deadline. Flat 4.40% individual income tax (reduced from 4.55% in 2023 under TABOR refund mechanism). May reduce further when state revenue exceeds TABOR cap.

Quarterly payments

Q1 2026

Jan 1 – Mar 31, 2026

April 15, 2026

Q2 2026

Apr 1 – May 31, 2026

June 15, 2026

Q3 2026

Jun 1 – Aug 31, 2026

September 15, 2026

Q4 2026

Sep 1 – Dec 31, 2026

January 15, 2027

Authoritative source

Colorado Pass-Through Entity Tax (PTET)

Colorado SALT Parity Act allows pass-through entity election (Form DR 0106). Annual election; entity-level tax flows through as state tax credit to owners.

Authoritative source

Federal individual income tax (Form 1040)

Filing deadline

April 15, 2027

Extension deadline

October 15, 2027

Quarterly payments

Q1 2026

Jan 1 – Mar 31, 2026

April 15, 2026

Q2 2026

Apr 1 – May 31, 2026

June 15, 2026

Q3 2026

Jun 1 – Aug 31, 2026

September 15, 2026

Q4 2026

Sep 1 – Dec 31, 2026

January 15, 2027

Penalty summary

Failure-to-file: 5%/month up to 25%. Failure-to-pay: 0.5%/month. Interest accrues on unpaid balances.

Authoritative source

Federal S-corp & partnership (Form 1120-S / 1065)

Filing deadline

March 16, 2027

Extension deadline

September 15, 2027

Authoritative source

Federal C-corporation (Form 1120, calendar year)

Filing deadline

April 15, 2027

Extension deadline

October 15, 2027

Authoritative source

Federal payroll (Form 941)

Quarterly payments

Q1 941

Jan – Mar 2026 wages

April 30, 2026

Q2 941

Apr – Jun 2026 wages

July 31, 2026

Q3 941

Jul – Sep 2026 wages

November 2, 2026

Q4 941

Oct – Dec 2026 wages

February 1, 2027

Authoritative source

Authoritative resources

File these Colorado returns with confidence

TaxScout's tax form library walks through every IRS form CPA firms file for Colorado clients — who must file, key fields, common mistakes, and step-by-step instructions.

Browse tax forms Other state deadlines

Frequently Asked Questions

Colorado-specific and federal questions for tax year 2026.

Colorado's Taxpayer Bill of Rights (TABOR) requires the state to refund excess revenue to taxpayers. In years when state revenue exceeds the TABOR cap, Colorado may temporarily reduce the flat income tax rate (e.g., from 4.55% to 4.40% in 2023) or issue direct refunds. CPAs should monitor TABOR announcements each November.

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