The Roth Optimizer suggests year-by-year Roth conversion amounts sized to maximize after-tax wealth across the simulation. It runs as a follow-up step on top of a completed Proof view: pick a goal mode, hit calculate, and the optimizer returns a list of recommended conversion sizes per year.

Goal modes

Fill to bracket

The classic mode. The optimizer finds the next federal income tax bracket above your current taxable income and converts just enough to reach the top of it. Anything more would spill into the next bracket and pay a higher marginal rate; anything less leaves cheap bracket headroom on the table.

Fill to IRMAA

For households on Medicare, the cliff that matters most is often the next IRMAA tier. The Medicare premium surcharge that kicks in when modified AGI crosses certain thresholds. fill_to_irmaa targets the IRMAA threshold instead of a federal bracket, so each year's conversion respects the surcharge cliff.

The NIIT threshold cap applies here too

With the NIIT cap turned on in your Tax Strategy settings, an auto-sized conversion is also held so the year's MAGI stays at or below the Net Investment Income Tax threshold. It runs after the bracket and IRMAA ceilings, so whichever leaves the least room is the one that sizes the conversion, and the year detail's "Why this conversion?" panel names which one bound. A conversion you entered as a fixed amount is never trimmed by it - that number is what you asked for.

If your income is already past the threshold before converting, the cap leaves no room and the conversion does not fire that year.