How Full Should You Fill Your Tax Bracket? I Ran the Numbers

“Fill up your tax bracket” skips the hard part: which bracket. I ran my Roth conversion optimizer 34 times to see where it stops, and why a big IRA should fill more than feels comfortable.

“Fill up your tax bracket” is the most common Roth conversion advice there is, and it skips the only part that’s hard: which bracket? The 12%? The 22%? Halfway into the 24% because it’s only two points more? I got tired of the hand-waving, so I ran my own Roth conversion optimizer 34 times, changing one thing at a time, and watched where it stopped.

Short version: it almost always stops exactly at a bracket edge, never somewhere in the middle. Which edge depends on a single comparison, and that comparison isn’t the one most people make.

Chart: total Roth conversions by assumed future tax rate. With a $1.2M IRA it is flat at about $196k up to 20%, then jumps to $565k at 23% and above. With a $3M IRA it rises steadily and converts the whole IRA from 25% up.
Same retiree, same accounts, only the future tax rate changes. The small IRA doesn’t slide gradually, it jumps from one bracket edge to the next.
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Roth Conversion vs ACA Subsidy: The Cliff Is Back

Two dollars of income cost a couple I was modeling last week $9,574. Not two thousand — two. That is the Roth conversion vs ACA subsidy problem in one line, and as of January it is live again for anyone retiring before 65.

The enhanced premium tax credits that ran from 2021 through 2025 expired on December 31 and were not extended. What came back with them is the old 400%-of-the-federal-poverty-line cliff: below it you get a subsidy, above it you get nothing at all. Not a taper. Nothing.

Chart of the 2026 ACA subsidy cliff: household premium rises gently with income, then jumps from $8,426 to $18,000 at $84,600 of MAGI
What a married couple pays for a benchmark silver plan as income rises. At 400% of the federal poverty line the premium tax credit disappears all at once — which is why a Roth conversion in a pre-Medicare year needs to be sized, not guessed.
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Roth Conversion Calculator With IRMAA and State Taxes

Somebody ran twelve impossible scenarios through my Roth conversion optimizer in seven minutes and then left with nothing. That’s the most useful thing I learned from finally sitting down and reading the tool’s usage log, and it’s why it’s now a Roth conversion calculator with IRMAA priced in, real state brackets for all fifty states, and the ability to tell you in plain English what your plan is actually doing.

The log covers roughly 190 solves across about 31 visitor sessions, which makes this the most-used of the optimization tools I’ve put online. Almost everyone got a plan out of it. Two people got nothing at all, and the way they failed turned out to be the most interesting thing in the whole file — they weren’t defeated by a bug, they were defeated by an answer that was technically correct and completely useless.

Plain-English notes under a Roth conversion plan, including one reading “Your income is pinned at $109,000 for 13 years”
The optimizer explaining its own plan — fixed rules, not generated text.
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A Withdrawal Rate Calculator That Draws Down to Zero

After I put the Roth conversion tool online, the question I kept getting wasn’t about conversions at all. It was simpler and scarier: “OK, but how much can I actually spend?” Almost every withdrawal rate calculator answers that by multiplying your savings by 4% and calling it a day. So I added a mode to my own optimizer that throws the flat percentage out and solves for the real number — the most you can spend every year while drawing every account down to zero by the end of the plan.

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Convert a Traditional IRA to Roth Without Paying Taxes?

People email me a version of the same hopeful question: how do I convert my traditional IRA to a Roth without paying taxes? I get why they ask. A Roth conversion lands a tax bill in the same year, and nobody enjoys writing the IRS a check for moving their own money from one pocket to another. So let me give you the honest answer first, and then the part that actually helps — the move that gets you as close to “tax-free” as the law allows.

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