Net Investment Income Tax (NIIT) 2026: A Business Owner's Guide

The 3.8% net investment income tax is one of the most misunderstood taxes affecting business owners at exit. Enacted in 2013 as part of the ACA, it has never been adjusted for inflation — meaning more owners cross the threshold every year without realizing it. This guide explains what triggers it, why the S-corp material participation exception protects most active owners, when business sale proceeds fall in, and five strategies to reduce your exposure.

What is the Net Investment Income Tax?

The Net Investment Income Tax (NIIT) is a 3.8% surtax under IRC § 1411, effective since January 1, 2013.¹ It applies to the lesser of:

• Your net investment income (NII), or
• The excess of your modified adjusted gross income (MAGI) over the applicable threshold

The "lesser of" structure means NIIT only hits income in excess of the threshold — but once you're over it, every dollar of NII in that excess is taxed at 3.8%.

2026 NIIT thresholds

The thresholds have never been indexed for inflation since NIIT was enacted. They remain the same in 2026 as they were in 2013:¹

Because the thresholds have never moved while business income has risen, a successful owner who would have been safely below the line a decade ago may now be well above it. A married S-corp owner with $280,000 in W-2 wages and $120,000 in K-1 distributions has $400,000 MAGI — $150,000 over the MFJ threshold. Every dollar of NII they have is taxed at 3.8%.

What counts as Net Investment Income?

Under IRC § 1411(c), NII includes:⁴

• Dividends (qualified and ordinary)
• Interest (taxable bond interest, bank accounts)
• Capital gains (short-term and long-term, net of capital losses)
• Rental and royalty income from passive activities
• Passive activity income — income from a trade or business in which you do not materially participate under IRC § 469
• Passive activity gains — capital gain from disposing of a passive activity interest

NII does not include:

• W-2 wages and self-employment income
• S-corp or partnership business income where you materially participate (active income — see section below)
• Distributions from qualified plans (401(k), IRA, SEP, pension — taxed as ordinary income, not NII)
• Social Security benefits
• Gain excluded from gross income under QSBS (IRC § 1202) — excluded gain is not NII

The S-corp exception: material participation protects active owners

Here's what most business owners and even some advisors get wrong: if you actively run your S-corp and materially participate in it, your K-1 business income is almost certainly not subject to NIIT.

NIIT applies to "passive activity" income as defined by IRC § 469. If you materially participate in your S-corp, the business income flowing to your personal return is active — not passive — and therefore not NII. The 3.8% does not apply to it.²

Under Temp. Reg. § 1.469-5T, you materially participate if you satisfy any one of seven tests. The most common for active business owners:

1. 500-hour test: You worked more than 500 hours in the activity during the year. Most full-time owners satisfy this without counting.
2. Substantially all participation: Your participation constituted substantially all participation in the activity for the year.
3. Prior history: You materially participated in any 5 of the preceding 10 taxable years (useful for semi-retired owners).

The risk arises when an owner has stepped back significantly. A passive investor who no longer works in the business does not materially participate — their K-1 income becomes passive and therefore NII. If you're transitioning out ahead of a sale, be aware of when you cross from active to passive status.

When business sale proceeds trigger NIIT

This is where the stakes are highest — and where entity structure chosen years earlier has consequences.

C-corp stock sale: NIIT applies in full

If you sell C-corp stock, the gain is a capital gain. Capital gains are NII by definition. There is no look-through to the underlying business assets for a C-corp stock sale — the entire gain is NII.³

The combined 2026 federal rate on long-term capital gains for a married owner well above the thresholds:

On a $5M C-corp stock sale gain at the 23.8% combined rate: $1,190,000 in federal capital gains + NIIT. The 3.8% NIIT component alone is $190,000.

S-corp and partnership sale: look-through election can eliminate NIIT

For S-corp and partnership interests, Treas. Reg. § 1.1411-7 provides an elective look-through rule.³ When you make the election, the gain on sale is bifurcated based on the underlying assets:

• Gain attributable to assets where you materially participated → not NII
• Gain attributable to passive assets held inside the entity (idle investment accounts, passive real estate) → IS NII

For an active owner-operator selling a business where virtually all assets are active — equipment, receivables, goodwill, IP — the look-through election typically reduces NIIT on the sale to near zero. The savings on a $5M gain: $190,000 compared to an equivalent C-corp stock sale.

Asset sales

In an asset sale, the character of each asset category drives both ordinary income tax and NIIT exposure. § 1245 recapture is ordinary income — not NII. Goodwill allocated directly to the owner as personal goodwill is capital gain, and if the owner materially participated, it may be exempt from NIIT under the look-through logic. See the asset sale vs. stock sale guide for the full tax treatment by asset category.

Five strategies to reduce NIIT exposure

1. Maximize retirement plan contributions to reduce MAGI

Contributions to a Solo 401(k), Cash Balance Plan, or SEP-IRA reduce your AGI — which reduces your MAGI — which reduces the spread above the NIIT threshold. A 52-year-old S-corp owner contributing $72,000 to a Solo 401(k) plus $200,000 to a Cash Balance Plan reduces MAGI by $272,000. That could push total MAGI below the $250,000 MFJ threshold entirely, eliminating NIIT on investment income for the year.

Use the retirement plan calculator to estimate your maximum tax-deferred contribution based on your income, entity, and age.

2. Installment sale to spread gain across tax years

Under § 453, gain is recognized when principal payments are received — not in the year of sale. If the gain recognized in each installment year stays below the NIIT threshold (or keeps total MAGI low), NIIT is reduced or eliminated in those years. The installment sale calculator models year-by-year gain recognition with NIIT and LTCG bracket stacking across multiple payment schedules.

Caveats: § 453A imposes an interest charge on installment obligations exceeding $5M (charge applies to tax deferred, not the face amount). § 453(i) requires immediate recognition of depreciation recapture regardless of installment payments — you pay tax on recapture in year one even on an installment sale.

3. QSBS exclusion eliminates NIIT on excluded gain

If your C-corp qualifies under IRC § 1202, excluded gain is not included in gross income and therefore is not NII at all. QSBS is the only C-corp exit strategy that eliminates both LTCG and NIIT on the excluded portion simultaneously. The exclusion is up to $10M for pre-OBBBA QSBS (stock issued before July 4, 2025) and up to $15M for post-OBBBA QSBS — subject to the 10× adjusted basis cap.

Use the QSBS calculator to estimate your exclusion. See the full QSBS guide for eligibility requirements, the tiered holding periods, and post-OBBBA changes.

4. Charitable vehicles: CRT and DAF

A Charitable Remainder Trust (Flip CRUT) accepts contributed appreciated business interests before sale. The trust sells the business interest — as a tax-exempt entity, it pays no capital gains tax on the sale — and distributes an income stream to you over time. The full sale proceeds are never recognized on your personal return in the year of sale, so no NIIT exposure in year one.

A Donor Advised Fund accepts contributed appreciated stock pre-sale. The stock is removed from your estate and return entirely, generating a current-year charitable deduction (subject to OBBBA-updated limits: 0.5% AGI floor for itemized charitable deductions; 35% deduction benefit cap for high-income filers).

5. S-corp structure and the look-through election at exit

As described above: for active S-corp owners, ordinary business income is not NII (material participation exception), and the Treas. Reg. § 1.1411-7 look-through election can eliminate NIIT on the business sale gain. If you're currently structured as a sole proprietorship or C-corp and exit is on the horizon, evaluating S-corp structure before the sale matters for this reason — in addition to the FICA savings the S-corp calculator models.

NIIT on rental and investment income inside your business

If your S-corp or partnership holds passive investments — idle cash in a brokerage account, investment real estate rented passively, minority stakes in other businesses where you don't participate — income from those assets is passive and therefore NII. The business wrapper does not protect passive investment returns from NIIT.

Rental income from real estate is NII unless you qualify as a real estate professional (750 hours in real estate activities, more than 50% of personal services — IRC § 469(c)(7)). Real estate professionals who materially participate in each rental activity can treat rental income as non-passive and exclude it from NII. A grouping election under Reg. § 1.469-9 lets you satisfy the 750-hour test across all rental activities combined rather than per property.