there is a net investment income tax of 3.8 % form 8960
the NIIT equals 3.8 % of the lesser of
1) net investment income or
2) the excess (if any) of Modified Adjusted Gross Income over $250,000 for married taxpayers.
also the way the capital gain tax works. say you had taxable income all LTCG of $1 million
$80K taxed at 0
$416.6K taxed at 15%
$503.4k taxed at 20%
about $775K taxed at 3.8 %
so total federal tax would be about $193K
look at the capital gain worksheet to see the actual comps under your assumptions
now add $100K to taxable income that's ordinary income. your federal tax bill goes up by about $32.4K
that $80k taxed at 0% almost disappears
an additional $100K is taxed at 20%
the $100K of ordinary income is taxed less the standard deduction of $24,800 (net $75,200) taxes about $8.6K
then there's the additional NIIT of $3.8K
