Net Investment Income Tax (NIIT)

The net investment income tax is a 3.8% federal tax imposed by section 1411 of the Internal Revenue Code on the investment income of individuals, estates and trusts above a threshold. For an individual, section 1411(a) imposes it on "the lesser of" net investment income for the year or "the excess (if any) of the modified adjusted gross income for such taxable year, over the threshold amount." That lesser-of structure is the part most descriptions skip and the part that determines the number.

The tax is sometimes called the Medicare surtax, and the label needs one sentence of unpicking because it is half right. Section 1411 does sit in a chapter of the Code titled "Unearned Income Medicare Contribution," so the name is not invented. But that is a different chapter from the one imposing the Medicare payroll tax: this charge is not withheld from anything, is not imposed on wages, and does not enter the Medicare-covered earnings record that eligibility for Medicare is built from. It is also routinely confused with the additional 0.9% Medicare tax, which is a separate charge on different income. The name used by the IRS, and by Form 8960 where the tax is computed, is the net investment income tax.

The lesser-of rule is the whole design. A household with an enormous portfolio and modest total income pays on the modified adjusted gross income excess, which may be nothing. A household with a very large salary and one small dividend pays on the dividend. Neither pays 3.8% of everything. The practical result is that the tax phases itself in: a couple whose modified adjusted gross income has just crossed the joint threshold owes the tax only on the small amount by which they crossed, however large their investment income happens to be.

What counts as net investment income. Section 1411(c) builds it from three categories: gross income from interest, dividends, annuities, royalties and rents; gross income from a passive trade or business or from trading in financial instruments or commodities; and net gain from the disposition of property. Income from a trade or business the taxpayer is not passive in is excluded from the first two categories. Allocable deductions are then subtracted, which is why the figure is a net one and why investment interest expense and the portion of state income tax attributable to investment income can reduce it.

The notable exclusions are as informative as the inclusions. Wages and self-employment income are not investment income, and section 1411 expressly keeps out income already subject to self-employment tax. Distributions from qualified retirement plans and individual retirement accounts under sections 401(a), 403(a), 403(b), 408, 408A and 457(b) are excluded. Interest that is excluded from gross income, most municipal bond interest among it, never enters the computation, because section 1411(c) starts from gross income from interest and tax-exempt interest is not in gross income. The distinction matters in both directions: a large traditional retirement distribution owes no net investment income tax on itself, but it raises modified adjusted gross income and can therefore drag other investment income into the tax.

The thresholds do not move, and that is a fact about the statute rather than a claim about anyone's intentions. Section 1411(b) sets $250,000 for a joint return or a surviving spouse, half that amount for a married person filing separately, and $200,000 in any other case. There is no inflation-adjustment provision anywhere in section 1411, and the tax has been in force for taxable years beginning after December 31, 2012. So the figures are the ones enacted more than a decade ago, in nominal dollars, and the share of households they reach grows as nominal incomes grow.

Two interactions worth knowing. Modified adjusted gross income for this purpose is defined in section 1411(d) and is not the same modified figure used by other provisions, which is a general hazard of that label. And because the tax stacks on the preferential capital gains rates rather than replacing them, the top federal rate on a long-term gain for a household above the threshold is 20% plus 3.8%.

The computation on Form 8960 runs in four steps.

1. Add up investment income in the section 1411(c) categories: interest, dividends, annuities, royalties, rents, passive business income, income from trading financial instruments, and net gain on dispositions of property.

2. Subtract allocable deductions to arrive at net investment income.

3. Compute the modified adjusted gross income excess over the threshold for your filing status.

4. Multiply the smaller of steps 2 and 3 by 3.8%.

A hypothetical example, run twice to show the lesser-of rule working.

Owen and Rita file jointly. Their modified adjusted gross income is $290,000, and $30,000 of it is net investment income from dividends and a fund's capital gain distribution.

• Excess over the $250,000 joint threshold: $290,000 − $250,000 = $40,000.
• Net investment income: $30,000.
• The lesser is $30,000, so the tax is 3.8% × $30,000 = $1,140.

Now change one fact. Suppose their modified adjusted gross income is $260,000 and their net investment income is $12,000.

• Excess over the threshold: $260,000 − $250,000 = $10,000.
• Net investment income: $12,000.
• The lesser is now $10,000, so the tax is 3.8% × $10,000 = $380, not 3.8% of the $12,000.

The second case is the common one, and it is why a household that has just crossed the threshold often finds the tax smaller than expected. It is also why anything that reduces modified adjusted gross income, a deductible retirement contribution or a health savings account contribution among them, can reduce this tax even when it does not touch the investment income itself.

What is straightforward about it

• The rate is flat and the mechanics are arithmetic, so the number is predictable once the two inputs are known.
• Because it is the lesser of two figures, crossing the threshold by a small amount produces a small tax rather than a cliff.
• Retirement account distributions are excluded from investment income outright, so the tax does not reach ordinary retirement withdrawals on their own account.
• Deductions allocable to investment income reduce the base, so the tax is charged on a net figure rather than on gross investment receipts.

What makes it awkward

• Nobody withholds it. It arrives at filing or has to be covered through estimated tax payments, and a one-off sale is the classic way to be caught out.
• The thresholds have no inflation adjustment, so a household reaches the tax over time on unchanged real income.
• The married filing separately threshold is half the joint amount, so separating does not avoid it and often accelerates it.
• Modified adjusted gross income here is defined by section 1411(d) and is not interchangeable with the modified figures used elsewhere, which makes planning off a single "MAGI" number unreliable.
• Because retirement distributions raise modified adjusted gross income without being investment income themselves, they can expose other investment income to a tax they do not pay.