Net Investment Income Tax (NIIT) Calculator
Find out if your investment income is subject to the 3.8% NIIT surtax — and how Roth conversions can unexpectedly trigger it.
Your total MAGI from all sources (Form 1040 line 11).
Net Investment Income Breakdown
Taxable interest from savings, bonds, CDs
Qualified and ordinary dividends
Net capital gains (long-term and short-term)
Net rental income after expenses
Royalties, passive business income, annuities
Roth conversions add to MAGI but are NOT net investment income. They can push you above the NIIT threshold.
What Is the Net Investment Income Tax?
The NIIT is a 3.8% surtax on net investment income that applies when your Modified Adjusted Gross Income (MAGI) exceeds specific thresholds. It was enacted as part of the 2010 health care reform legislation and took effect January 1, 2013. The thresholds have never been adjusted for inflation, meaning more taxpayers are affected each year.
NIIT Thresholds (2026)
| Filing Status | MAGI Threshold |
|---|---|
| Single | $200,000 |
| Married Filing Jointly | $250,000 |
| Married Filing Separately | $125,000 |
| Head of Household | $200,000 |
The Roth Conversion Trap
A common surprise: Roth conversions are notinvestment income, but they add to your MAGI. This means a large Roth conversion can push your MAGI above the NIIT threshold, causing your existing investment income (dividends, capital gains, rental) to become subject to the 3.8% surtax — even though the conversion itself isn't taxed by NIIT.
Related Retirement Tax Tools
Capital Gains Bump Zone
See how gains are taxed at 0%/15%/20% before NIIT adds another 3.8%.
IRMAA Medicare Planner
MAGI also triggers Medicare surcharges. Plan conversions to manage both.
Roth Conversion Calculator
Find your optimal Roth conversion amount considering NIIT, IRMAA, and tax brackets.
SS Tax Torpedo
Investment income also affects Social Security taxation thresholds.