Thanks Gumby. All "Long term" gains. I will add that to my original post.
I know $50k of Roth Conversions would result in the future gains being not taxed.
Whereas if I harvest the gains in taxable and re-invest in stock index funds, those future gains would be taxable.
Does the gain itself get added to AGI?
I guess I was assuming you needed the money to spend this year, but you are quite correct. If you Roth convert, future gains will be untaxed.
AGI will include the amount of any capital gains if you sell taxable assets. So, for instance, if you are married filing jointly, had Zero other income, sold assets sufficient to generate $125,900 in long term capital gains*, and took the standard deduction of $25,900 -- you would have an AGI of $125,900, you would have taxable income of $100,000, which would be all long term capital gains. You would pay 0% tax on the first $83,350, You would pay 15% tax on $16,650 for a tax bill of $2498.
If you Roth convert $125,900 out of your tIRA and had Zero other income, you will also have an AGI of $125,900 and taxable income of $100,000. But it will be all ordinary income, so you'll pay 10% of the amount up to $20,550; 12% of the amount from $20,551 to $83,550, and 22% of the amount from $83,551 to $100,000. For a total tax bill of $13,234.
* so, for example, you bought 10,000 shares of XYZ at $1.00 per share and sold it at $13.59 per share. You actually get $135,900 in proceeds from the sale, but $10,000 was your basis, so the capital gain is only $125,900
Note that once your taxable income reaches $459,750, the capital gains tax goes to 20%.
Note also that I assume you have no after tax contributions in your tIRA.
And, most importantly, note that it is AGI that drives taxation of social security, ACA subsidies and IRMAA premiums.