Your income tax liability is determined by your earnings ... you will likely get a higher number than what you're actually responsible for paying. The tax code includes a number of deductions ...

If you don’t know what Net Investment Income Tax (NIIT) is, you’re not alone. This relatively new tax began a little over a decade ago but the number of taxpayers subject to the tax has increased.

The lower number of the two is the one that will be subject to the 3.8 percent additional tax. The income thresholds are not indexed for inflation. Here’s an example of how to calculate NIIT.