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Net Investment Income Tax (NIIT) is a 3.8% tax applicable to certain net investment income of individuals, estates, and trusts. Types of income that can be subjected to NIIT include but are not limited to, rental property income, dividends, capital gains, interest, and royalties. This tax is calculated separately from regular income tax and is applicable even if you do not owe any regular tax. However, it usually only applies when income is above a certain threshold. The applicable threshold levels depend on the investor's filing status and modified adjusted gross income.