Partnership Taxation Fundamentals: Basis, Losses, and K-1s

Partnership taxation is a pass-through system: the partnership itself owes no federal income tax, and each partner reports their share of the business’s income, losses, deductions, and credits on their own return and pays tax at their individual rate.¹ That structure sidesteps the double taxation that hits C-corporations, where profits are taxed at the corporate level and again when distributed as dividends. The trade-off is complexity. Partners have to track basis, work through three separate loss limitation rules, handle self-employment tax on their own, and coordinate individual filings with the partnership’s return.

One consequence catches new partners off guard immediately: you owe tax on your allocated share of partnership income even in years when the partnership distributes no cash to you at all.

Which Businesses Are Taxed as Partnerships

The classification covers traditional general and limited partnerships, but it also picks up most multi-member LLCs by default. Under the federal check-the-box regulations, any domestic entity with two or more members that doesn’t elect to be taxed as a corporation is automatically classified as a partnership for tax purposes.² That default is why the vast majority of multi-member LLCs end up following partnership tax rules without ever filing a formal election.

What the Partnership Files

Every partnership files Form 1065, an informational return that reports gross receipts, cost of goods sold, operating expenses, and net results for the year.³ The return has to separately state different categories of income, gains, losses, and deductions so each item flows to the right line on each partner’s return.⁴ Rental real estate income, portfolio interest and dividends, and capital gains are all reported separately rather than lumped together, because they may be taxed differently or hit different limitations on a partner’s return.⁵

Alongside Form 1065, the partnership issues a Schedule K-1 to every partner. Each K-1 breaks out that specific partner’s share of each income and deduction item, along with the partner’s name, taxpayer identification number, and ownership percentage.³ Mismatches between what a K-1 reports and what a partner reports on their individual return are a routine audit trigger.

Deadlines and Late Filing Penalties

Calendar-year partnerships have to file Form 1065 by March 15. Filing Form 7004 before that deadline grants an automatic six-month extension to September 15.⁶ The extension covers the filing, not payment of any tax due.

Late filing costs $255 per partner per month or partial month, up to 12 months.⁷ For a 10-partner partnership that files three months late, that’s $7,650, and the IRS doesn’t send a reminder before assessing it.

How Income and Losses Are Divided Among Partners

The partnership agreement controls how income, losses, deductions, and credits are split among partners. Allocations don’t have to follow ownership percentages. Partners can agree to any split they want, as long as it has “substantial economic effect.”⁸ In plain terms, the partner who gets the tax benefit also has to bear the corresponding economic risk. A partner allocated 90 percent of losses must actually stand to lose money in that proportion if the business fails. The IRS can reallocate items if it decides the arrangement is just tax shifting with no real economic substance.

If the agreement is silent on a particular item, or the allocation fails the substantial economic effect test, the IRS determines each partner’s share based on their overall interest in the partnership: their share of capital, profits, losses, and cash distributions viewed as a whole.⁸

Guaranteed Payments

When a partnership pays a partner a fixed amount for services or the use of their capital, regardless of whether the business earns a profit, that payment is a “guaranteed payment.” Guaranteed payments are taxed like wages to the recipient. They count as ordinary income on the partner’s return and are generally deductible by the partnership as a business expense.⁹ They also count as self-employment income, so the receiving partner owes self-employment tax on them even if their share of ordinary partnership income would otherwise be exempt.

Partner Basis

Every partner has an “outside basis,” a running account of how much they’ve invested in the partnership on an after-tax basis. That number controls three things: how much loss you can deduct, whether a distribution is tax-free, and your gain or loss when you eventually sell your interest. Getting basis wrong usually means paying tax twice on the same income or claiming deductions you weren’t entitled to.

Basis goes up with initial and additional capital contributions, allocated shares of partnership taxable income, and shares of tax-exempt income like municipal bond interest earned by the partnership.¹⁰ The tax-exempt income adjustment exists so a partner selling their interest doesn’t recognize taxable gain equal to income that was supposed to be tax-free.¹¹

Basis goes down with cash distributions, the partner’s share of partnership losses, and their share of nondeductible expenses such as penalties or the nondeductible portion of meals.¹⁰ The nondeductible expense reduction prevents a partner from converting those costs into a capital loss on sale.¹¹ Basis cannot drop below zero. Once it hits zero, additional losses are suspended and further cash distributions trigger capital gains tax.

Partnership Debt and Basis

Unlike shareholders in a corporation, partners get basis credit for their share of partnership debt. An increase in a partner’s share of liabilities is treated like a cash contribution and raises basis; a decrease is treated like a cash distribution and reduces it.¹² This is one of the biggest practical advantages of partnership taxation. It lets partners deduct losses financed by borrowed money, subject to the loss limitation rules below.

How that debt gets divided depends on whether it’s recourse or nonrecourse. A recourse liability is allocated to the partner who would bear the economic loss on default. Nonrecourse liabilities, where no partner is personally on the hook, are allocated using a three-tier system based on minimum gain, built-in gain on contributed property, and profits.¹³

Three Layers of Loss Limitations

Partners allocated a share of partnership losses can’t always deduct those losses right away. Three separate rules apply in order, and a loss has to clear all three before it lands on your tax return.

Basis Limitation

Your deductible share of partnership losses can’t exceed your adjusted basis at year end.⁸ Any excess is suspended and carried forward indefinitely, becoming deductible in a future year when your basis rises through contributions, allocated income, or an increased share of debt.

At-Risk Limitation

Losses that clear basis have to pass the at-risk rules next. A partner is “at risk” for amounts they contributed to the partnership, plus any partnership debt they’re personally liable for or have pledged separate property to secure.¹⁴ Nonrecourse financing, where the lender can look only to the partnership’s assets, generally doesn’t count. The rule prevents partners from writing off losses backed entirely by borrowed money they’ll never have to repay. Disallowed losses carry forward.

Passive Activity Limitation

Losses surviving the first two hurdles still face the passive activity rules. If a partner doesn’t materially participate in the business, roughly 500 hours a year being the main threshold, their share of losses is passive and can only offset passive income. Passive losses can’t reduce wages, interest, or other nonpassive income. Limited partners face a steeper version of the same rule: a limited partnership interest is generally passive regardless of hours worked.¹⁵ Suspended passive losses are released and become fully deductible when the partner disposes of their entire interest in the activity in a taxable transaction.

The three layers interact in ways that surprise people. Basis might be high enough after debt allocations to clear the first hurdle, but the at-risk rules disallow the same loss because the debt is nonrecourse. Or the loss clears basis and at-risk but freezes at the passive activity stage because the partner is a limited partner or doesn’t put in enough hours. Tracking all three limitations separately, year by year, is where partnership tax compliance gets genuinely difficult.

Self-Employment Tax on Partnership Income

General partners owe self-employment tax on their share of partnership ordinary business income whether or not the partnership distributes any cash. The combined rate is 15.3 percent: 12.4 percent for Social Security on the first $184,500 of combined earnings in 2026, and 2.9 percent for Medicare on all earnings with no cap.¹⁶ Partners earning above $200,000 (single) or $250,000 (married filing jointly) also owe an additional 0.9 percent Medicare surtax on earnings above those thresholds.

Limited partners get a statutory break: their share of ordinary partnership income is generally exempt from self-employment tax. The exemption doesn’t cover guaranteed payments for services, which are always subject to self-employment tax regardless of partner status. The IRS never finalized regulations defining who counts as a “limited partner” for this purpose, but under proposed rules from 1997 that the IRS will respect, a partner is generally not treated as limited if they have personal liability for partnership debts, have authority to contract on behalf of the partnership, or participate more than 500 hours a year.¹⁷ Partners in service fields like law, accounting, health care, and consulting can’t claim the limited partner exemption if they provide services in that trade or business.

Partners whose income from the partnership is passive rather than earned from active work may face the 3.8 percent net investment income tax instead of self-employment tax. The thresholds are the same $200,000 and $250,000.¹⁸ A partner generally can’t owe both on the same dollar of partnership income, but it’s entirely possible to owe one on active income and the other on passive income from a different partnership.

Qualified Business Income Deduction

Partners may be eligible to deduct up to 20 percent of their qualified business income from the partnership under Section 199A.¹⁹ The deduction was originally set to expire after 2025 but was made permanent by legislation signed in July 2025. It’s taken on the partner’s individual return, not at the partnership level, and reduces taxable income without reducing adjusted gross income or self-employment income.

Partners whose taxable income falls below certain thresholds get the full 20 percent without limitation. Above those thresholds, the deduction may be reduced or eliminated for partners in “specified service trades or businesses” like health, law, accounting, consulting, athletics, and financial services. Partners in non-service businesses above the thresholds face a different cap: the deduction is limited to the greater of 50 percent of W-2 wages paid by the business, or 25 percent of W-2 wages plus 2.5 percent of the unadjusted basis of qualified property held by the business.¹⁹ Since partnerships often pay no W-2 wages (partners receive distributive shares, not wages), the W-2 limitation can significantly reduce or eliminate the deduction for higher-income partners in partnerships with few employees.

What Partners Do at Filing Time

Once you receive your Schedule K-1, you transfer each line item to Schedule E of your Form 1040.²⁰ The K-1 breaks income into categories (ordinary business income, rental income, interest, dividends, capital gains) and each category lands on a different part of your return. Every line on the K-1 has to match your individual filing; inconsistencies are among the easier things for the IRS’s automated systems to flag.

Because partnerships don’t withhold taxes from distributions the way employers withhold from paychecks, most partners need to make quarterly estimated tax payments. For 2026, those are due April 15, June 15, September 15, and January 15 of 2027.²¹ The safe harbor to avoid underpayment penalties is paying at least 100 percent of your prior-year tax liability (110 percent if your adjusted gross income exceeded $150,000) or 90 percent of your current-year liability. New partners often underestimate this obligation because they’re not used to paying taxes outside of payroll withholding.

One practical headache: partnerships have until March 15 (or September 15 with an extension) to issue K-1s, but partners need K-1 data to file their own returns by April 15. If the partnership extends, the partner either has to file their own extension or estimate the K-1 figures and amend later. Filing a personal extension using Form 4868 is the cleaner option in most cases.

1
Office of the Law Revision Counsel. 26 USC 701 – Partners, Not Partnership, Subject to Tax
2
eCFR. 26 CFR 301.7701-3 – Classification of Certain Business Entities
3
Internal Revenue Service. About Form 1065, U.S. Return of Partnership Income
4
Office of the Law Revision Counsel. 26 USC 6031 – Return of Partnership Income
5
Internal Revenue Service. Instructions for Form 1065
6
Internal Revenue Service. Instructions for Form 7004
7
Internal Revenue Service. Failure to File Penalty
8
Office of the Law Revision Counsel. 26 USC 704 – Partners Distributive Share
9
Office of the Law Revision Counsel. 26 USC 707 – Transactions Between Partner and Partnership
10
Office of the Law Revision Counsel. 26 USC 705 – Determination of Basis of Partners Interest
11
Internal Revenue Service. Revenue Ruling 96-11
12
Internal Revenue Service. Determining Liability Allocations
13
eCFR. 26 CFR 1.752-3 – Partners Share of Nonrecourse Liabilities
14
Office of the Law Revision Counsel. 26 USC 465 – Deductions Limited to Amount at Risk
15
Office of the Law Revision Counsel. 26 USC 469 – Passive Activity Losses and Credits Limited
16
Social Security Administration. Contribution and Benefit Base
17
Internal Revenue Service. Self-Employment Tax and Partners
18
Internal Revenue Service. Net Investment Income Tax
19
Internal Revenue Service. Qualified Business Income Deduction
20
Internal Revenue Service. About Schedule E (Form 1040), Supplemental Income and Loss
21
Taxpayer Advocate Service. Your Tax To-Do List: Important Tax Dates