The Net Investment Income Tax (NIIT) is an additional 3.8% federal tax on certain investment income for individuals, estates, and trusts whose income clears IRS thresholds. It sits on top of ordinary income tax and capital-gains tax - not instead of them. Everyday investors meet it most often through taxable brokerage activity at Fidelity, Schwab, Vanguard, or similar (Taxable brokerage basics).
NIIT is Form 8960 territory. Thresholds and definitions change; confirm current IRS figures when you file (Filing taxes for beginners).
Who is in scope (simplified)
NIIT generally applies when both are true:
- You have net investment income, and
- Your modified adjusted gross income (MAGI) exceeds the filing-status threshold in effect for that year (historically tied to figures such as $200,000 single / $250,000 married filing jointly - verify the year’s IRS numbers).
The tax is 3.8% of the lesser of (a) net investment income or (b) the amount by which MAGI exceeds the threshold.
Ordinary wage brackets vs effective rate: Tax bracket vs effective rate. Preferential long-term capital-gains rates still apply first; NIIT can stack on top (Capital gains basics).
What usually counts as net investment income
| Often included (simplified) | Often not NIIT investment income |
|---|---|
| Taxable interest | Wages / salary |
| Dividends (qualified and ordinary) | Social Security benefits (separate rules) |
| Capital gains (including fund distributions) | Municipal bond interest that is tax-exempt (generally excluded from NIIT) |
| Rental and royalty income (with exceptions) | Distributions from retirement accounts that are not investment income under the regs |
| Passive business income in many cases | Active trade/business income that is not passive and not trading in financial instruments |
Qualified vs ordinary dividends still matter for regular tax; both can feed NIIT when thresholds are met (Qualified dividends basics). Asset location (bonds in tax-advantaged wrappers, broad stock index in taxable) is a common planning theme - not a guarantee you avoid NIIT (Taxable vs tax-advantaged).
Worked example: MAGI just over the line
Sam files single. For the year:
- W-2 wages: $185,000
- Taxable brokerage: $12,000 qualified dividends + $18,000 long-term capital gains
- MAGI (illustrative): $215,000
- Assume the single threshold is $200,000 (confirm current law)
Net investment income ≈ $30,000 (dividends + gains, ignoring investment expenses for simplicity).
Excess MAGI over threshold = $15,000.
NIIT base = lesser of $30,000 and $15,000 = $15,000.
NIIT ≈ $15,000 × 3.8% = $570.
Sam still owes regular tax on the dividends and long-term gains; the $570 is additional. If Sam had only $8,000 of investment income, NIIT would be $8,000 × 3.8% instead - because the lesser-of rule binds to NII when it is smaller than the MAGI excess.
Planning cues (education, not advice)
- NIIT is a MAGI and NII problem - raising wages alone can pull you over the threshold even if investment income is modest.
- Tax-loss harvesting and basis management change capital gains that feed NII (Capital loss carryforwards when losses exist). Niche deferral ideas such as QOFs have their own inclusion rules—do not assume they erase NIIT analysis.
- Roth conversions increase MAGI in the conversion year and can unexpectedly create or enlarge NIIT exposure.
- Estimated taxes: if you owe NIIT, quarterly vouchers may need to cover it alongside income tax (Quarterly estimated taxes).
Checklist
- Estimate MAGI against the current-year NIIT threshold for your filing status.
- List dividends, interest, rents, and realized capital gains that may count as NII.
- Compute 3.8% of the lesser of NII vs MAGI-over-threshold.
- Do not confuse NIIT with the additional Medicare tax on wages or with AMT (separate rules).
- Save Form 8960 worksheets with your return when thresholds are close.
- Confirm figures on IRS.gov or with a CPA/EA before year-end moves.
Educational only. Not tax, legal, or investment advice. Thresholds, definitions, and exceptions change; confirm with current IRS publications or a qualified tax professional.