First, if there is capital gain it would be added to total income Line 13 Sch. 1.
Yup, it's certainly a factor in taxes. It definitely adds to the AGI. But for MFJ with taxable income below $78,570 in 2019, it shouldn't cause any extra taxes - the Qualified Dividends and Capital Gains Tax Worksheet should result in it not increasing taxes.
Though there are actually a couple of secondary effects that makes the above not completely true (thanks to the "stealth taxes" ):
(#1#) Cap gains also adds to the MAGI used for determining how much of Social Security is taxed -- a complicated worksheet. So you could end up with more of your Social Security benefits being taxed, which would increase your taxes. Ultimately the amount of Social Security that is taxed ends up on 1040 line 5b (2018 return)
(#2#) If one's AGI is over $170,000 (in 2017, for MFJ couple, which would impact 2019 premiums), then one pays higher premiums on Medicare Part B and Part D. (The threshold is $85,000 for singles). As sinkingfeeling found out. For 2018 AGI (affecting 2020 premiums), the threasholds are $87,000 single / $174,000 MFJ. Similarly for Part D premiums - i think the same threshholds.
There are some other stealth taxes that go up with AGI, but at higher levels.
But... this discussion sent me to look at the worksheet for qualified dividends where one would also calculate the various taxes on capital gain and provides the tax amount. Since we did not have capital gain I ignored this worksheet but, of course, we do have qualified dividends from the taxable account so I went back and, lo and behold, lowered the taxes.
Wow! But how did you lower your taxes by looking at this worksheet?