NIIT on the Sale of a Business: When the 3.8% Tax Applies

The net investment income tax (NIIT) is a 3.8% federal tax on certain investment income, including gains from the sale of a business, imposed on high-income individuals and trusts. Whether a business sale is subject to NIIT depends on the seller’s modified adjusted gross income (MAGI), the nature of the business (active vs. passive), and specific statutory exceptions—particularly the “passive activity” and “S corporation” carve-outs.

The NIIT was introduced in 2013 as part of the Affordable Care Act. It operates independently of ordinary income tax, triggered by a separate income threshold. Many business owners discover, too late, that selling their business pushes them above the MAGI threshold and subjects the gain to an unexpected 3.8% federal tax on top of ordinary capital gains tax.

The 3.8% tax: how it works

The NIIT applies to “net investment income,” which includes:

• Interest, dividends, and annuities.
• Capital gains (including those from business sales).
• Passive activity income and gains.
• Foreign investment income.

The tax applies to the lesser of:

1. Net investment income for the year, or
2. Excess modified adjusted gross income (MAGI) above the threshold.

For a married couple filing jointly in 2024, the MAGI threshold is $250,000. If MAGI is $300,000 and net investment income is $150,000, NIIT applies to the lesser of $150K (net investment income) or $50K (excess MAGI), meaning 3.8% × $50K = $1,900.

If MAGI is $350,000 and net investment income is $150,000, NIIT applies to the lesser of $150K or $100K, so 3.8% × $100K = $3,800.

MAGI threshold and business sale income

When a business is sold, the gain enters the numerator of MAGI. This often pushes a seller over the threshold in the year of sale, triggering NIIT on the gain itself.

Example: An owner of an S corporation sells the business for a $2M gain. In the year of sale, MAGI (before the gain) is $180,000. Including the $2M gain, MAGI becomes $2,180,000. The excess over the $250K threshold is $1,930,000, so NIIT applies to the lesser of:

• Net investment income ($2M in gain), or
• Excess MAGI ($1,930,000).

NIIT is 3.8% × $1,930,000 = $73,340 on top of regular capital gains tax.

The passive-activity exception: material participation

The critical escape hatch is the passive activity exception. If the seller materially participated in the business during the tax year of sale (or certain prior years), the gain is not investment income and thus not subject to NIIT.

“Material participation” is defined in the tax code as involvement in business operations that is:

• Regular, continuous, and substantial, or
• Meeting specific safe harbors (e.g., >500 hours of work per year, or 100+ hours and no one else does more).

The test is fact-intensive:

• An owner-operator of a restaurant who works there daily clearly materially participates.
• A passive investor who owns 30% of an LLC and has no say in operations does not.
• A managing partner in a partnership with significant decision-making authority likely qualifies.

Passive investors (those with no material participation) face NIIT on the full gain. Active owners escape it entirely.

Passive activity loss limitations and NIIT

Separately, passive activities generate passive losses (from real estate, partnerships, rental properties, etc.). These losses also count toward net investment income for NIIT purposes if they exceed passive gains. A real-estate developer with $500K in passive losses might owe NIIT on the differential.

This creates a counterintuitive scenario: a business owner with significant passive losses could owe NIIT despite net investment losses, if their MAGI from active business still exceeds the threshold.

Installment sales and spread of NIIT

If a business is sold via an installment contract—the buyer pays over time rather than upfront—the gain is recognized in the years payments are received, not in the year of sale.

NIIT likewise spreads. If a $2M gain is recognized $400K per year over five years, NIIT is computed on each year’s $400K (assuming MAGI stays elevated), rather than on the full $2M upfront.

This can reduce NIIT burden: in years where MAGI (including the installment payment) dips below the threshold, no NIIT is owed.

Example: A seller receives $400K/year for five years. Year 1 MAGI (before payment) is $150K. Adding the $400K payment, MAGI becomes $550K, $300K over the $250K threshold. NIIT = 3.8% × lesser of $400K (installment gain) or $300K (excess MAGI) = 3.8% × $300K = $11,400.

In Year 5, if the seller has retired and MAGI is $100K, the $400K payment pushes MAGI to $500K, $250K over threshold. NIIT = 3.8% × $250K = $9,500.

Total NIIT over five years is less than if the full $2M gain triggered the tax in year one.

S corporations and partnerships

Gains from the sale of an S corporation or partnership interest are subject to NIIT unless the entity (or the seller’s interest in it) qualifies as an active business.

An S corp that manufactures widgets and generates $1M in operating income per year is an active business; a gain on its sale may escape NIIT if the seller materially participated.

An S corp that is a holding company, investing in marketable securities, is more likely treated as a passive investment; gains on sale are subject to NIIT.

The distinction turns on the nature of the entity’s activity, not just the amount of income.

Trusts and estates

NIIT applies to trusts and estates with high net investment income. The threshold is much lower: $12,950 (2024). A trust that realizes a $500K capital gain on the sale of a business interest owed by the estate likely owes 3.8% × ~$487K = ~$18,500 in NIIT (assuming income above the low threshold).

This is a major driver of NIIT for family-business succession planning.

Real example

A physician and business partner sell their medical practice to a hospital system for a $5M gain. The physician’s W-2 salary for the year is $250K; the gain makes their MAGI $5,250,000.

Thresholds:

• Ordinary long-term capital gains tax on $5M at 20% = $1,000,000.
• NIIT at 3.8% on lesser of $5M (gain) or $5M (excess MAGI over $250K) = $190,000.
• Total federal tax on gain = $1,190,000.

If the physician could prove material participation in the practice (likely, given it’s a medical partnership), the NIIT drops to $0. Only the $1M capital gains tax applies.

If instead the practice was owned in a holding company and the physician was a passive investor, NIIT sticks.

Planning around NIIT

Common strategies:

1. Timing: Spread the sale over multiple years (installment sale) to keep MAGI below the threshold in some years.
2. Charitable gifts: Donating appreciated business interests to charity avoids NIIT on those gains (and yields a deduction).
3. Grantor retained annuity trusts (GRATs): Transferring the business to a GRAT can reduce or eliminate NIIT on appreciation post-transfer.
4. Material participation: Ensure documented involvement in the business to support the passive-activity exception.
5. Business structure: Certain entity types or arrangements may qualify for exceptions; consult a tax advisor.

See also