Short answer first, because that's what you came for: for a calendar-year partnership, March 15, or September 15 if the partnership extends.
The longer answer matters, because "due" means different things to the partnership that issues the K-1 and to the investor who's waiting on it.
When are partnership K-1s due?
A partnership must furnish K-1s by the due date of its Form 1065, including extensions. For a calendar-year partnership, that's March 15. With an automatic six-month extension, it's September 15. A fiscal-year partnership's return is due on the 15th day of the third month after its year ends, and the K-1 deadline follows.
The K-1 deadline isn't its own date. It's tied to the return's. That one sentence explains most of the confusion: a partnership that extends its return has, by law, extended its K-1s too. There's no separate request and no separate notice to investors, which is why so many investors find out in April.
What are the K-1 dates for the 2026 tax year?
For calendar-year entities filing 2026 returns: partnership and S corporation K-1s are due Monday, March 15, 2027, or Wednesday, September 15, 2027, if extended. Individual returns are due Thursday, April 15, 2027, or Friday, October 15, 2027, if extended. Trust and estate K-1s follow Form 1041, due April 15.
| Return | What it issues | Due (2026 tax year) | Extended due date | Extension form |
|---|---|---|---|---|
| Form 1065 (partnership, including most syndications) | Schedule K-1 (Form 1065) | Mon, Mar 15, 2027 | Wed, Sep 15, 2027 | Form 7004 |
| Form 1120-S (S corporation) | Schedule K-1 (Form 1120-S) | Mon, Mar 15, 2027 | Wed, Sep 15, 2027 | Form 7004 |
| Form 1041 (trust or estate) | Schedule K-1 (Form 1041) | Thu, Apr 15, 2027 | Thu, Sep 30, 2027 | Form 7004 |
| Form 1040 (you, the investor) | Receives the K-1s | Thu, Apr 15, 2027 | Fri, Oct 15, 2027 | Form 4868 |
These are federal dates for calendar-year entities. By statute, an S corporation's K-1s are due by the day it actually files its return, so its dates are the latest possible. States set their own, and most K-1s from a multi-state deal come with state filing obligations on each state's schedule.
Is there a penalty if a K-1 is late?
For the partnership, yes, once it misses the deadline that applies. Filing the return late draws a penalty per partner, per month, for up to 12 months, and failing to furnish K-1s on time draws a separate per-K-1 penalty. A return that was properly extended and filed by September 15 is on time.
The amounts are indexed for inflation each year, which is why I'm not printing them here; the current figures are in the Form 1065 instructions. The structure matters more than the number anyway. Because the filing penalty is multiplied by the number of partners, it scales with the size of the raise. A 150-investor syndication that files a month late owes 150 times the monthly amount.
For the investor, there's no penalty for receiving a K-1 late. There's only your own deadline, which didn't move. What to do about that is its own question: should I file an extension or file and amend while I wait on a K-1?
What if the due date lands on a weekend or holiday?
It moves to the next business day. When a federal filing deadline falls on a Saturday, Sunday or legal holiday, the return is timely if it's filed the next day that isn't one of those. For the 2026 tax year, none of the dates above is affected; each one falls on a weekday.
Watch April in particular. Emancipation Day, a legal holiday in Washington, D.C., falls on April 16, and in years when it sits next to a weekend it has pushed the individual deadline past April 15. In 2027 April 16 is a Friday, so Thursday, April 15 stands.
What about short years and final returns?
A partnership that ends mid-year files a short-year final return, due the 15th day of the third month after the month its year ends, not the following March 15. The final K-1s ride on that return. It's an easy deadline to miss, because it arrives while everyone is still celebrating the closing.
A partnership that sells its last property and winds up in July has a return due in October, and the deal team that assumes it has until next March is already late. I wrote about exactly that in the return was already late.
Why do so many K-1s arrive in September?
Because most partnerships extend, and an extended partnership has until September 15. Extending is automatic and costs the partnership nothing, while finishing by March 15 takes books closed in January and work done in the autumn. Unless someone plans for March, September is where the calendar lands.
The full chain, and which link usually breaks, is in why is my syndication K-1 late?
What should I do if my K-1 hasn't arrived by April 15?
File an extension and pay an estimate. Form 4868 gives you until October 15 to file, but the tax is still due April 15, so estimate what the missing K-1 will show and pay accordingly. Then file once, when everything is in. Filing on a guess and amending later is usually the costlier route.
That's the default I use for every LP client. If you're new to all of this, what a K-1 is and how it differs from a 1099 is the place to start.
The deadline isn't a mystery. It's March 15 or September 15, and the partnership chooses which. The useful question for an investor isn't when K-1s are due. It's which of your deals picks September every year, so you can plan around it.
This is general education, not tax advice. Deadlines depend on the entity type, its tax year and the states involved. Confirm the dates that apply to you with your tax advisor.