Net investment income tax
By Jude Wallis
Net investment income tax, or NIIT, is a 3.8 percent surtax on a defined slice of investment income for filers whose modified AGI clears a threshold.
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Interest, dividends, rents and capital gains can sit in that slice. Wages generally do not. A live computation takes the lesser of net investment income and MAGI minus a filing-status threshold.
NIIT is not FICA and not the additional Medicare tax on high wages.
How net investment income tax works is the explainer. The NIIT calculator applies 3.8 percent to a stated slice.