Use Form 8960 Net Investment Income Tax—Individuals, Estates, and Trusts to figure the amount of your Net Investment Income Tax (NIIT).
Per IRS Instructions for Form 8960, on page 1:
Attach Form 8960 to your return if your modified adjusted gross income (MAGI) is greater than the applicable threshold amount.
U.S. citizens and residents. Individuals who have for the tax year (a) MAGI that’s over an applicable threshold amount, and (b) net investment income, must pay 3.8% of the smaller of (a) or (b) as their NIIT.
The applicable threshold amount is based on your filing status.
To make entries for Form 8960 in TaxAct:
Note that any link in the information above is updated each year automatically and will take you to the most recent version of the document at the time it is accessed.