When I started thinking seriously about Roth conversions, I went looking for a tool that could help me model the decision properly. I found plenty of calculators — but almost all of them had the same frustrating limitations.
They’d ask for a “current tax rate” and a “future tax rate” and spit out a break-even number. That framing misses most of what actually makes Roth conversions complicated. The real question isn’t just what rate you pay today versus tomorrow — it’s how a conversion interacts with Social Security taxation, Medicare surcharges, required minimum distributions, and the hidden tax liability sitting inside your traditional IRA. Those interactions are nonlinear and they compound over decades.
So I worked with Claude Code and we built the tool I wished existed.
The problem
Most calculators ignore your actual income picture. They ask for a single tax rate, with no way to enter wages, Social Security benefits, or pension income — let alone model how those income sources interact.
What this tool does
You enter your full income profile: ordinary income before retirement, your Social Security benefit and claiming age, and any pension or post-retirement income. The engine stacks these together the way the IRS does — including the portion of Social Security that becomes taxable when your combined income crosses key thresholds. A Roth conversion can push you over those thresholds in ways a flat-rate comparison completely misses.
The problem
Couples are an afterthought. Most tools model a single person, or treat a spouse as just a checkbox that doubles the standard deduction.
What this tool does
You can enter separate income, retirement age, Social Security benefit, and claiming age for each spouse. The projection tracks both timelines independently and handles the transition when each spouse retires.
The problem
You can only model one scenario at a time. Comparing 'what if I convert $50K/year' against 'what if I fill the 22% bracket' means running two separate calculators and trying to hold the numbers in your head.
What this tool does
You can run up to three custom scenarios side by side — different amounts, different strategies, different start and stop ages, even different states of residence. All charts and tables update instantly as you adjust inputs.
The problem
IRMAA is ignored. Medicare's income-related surcharges can add thousands of dollars per year, and they're triggered by income from two years earlier — meaning a large conversion today creates a bill you won't see until year three.
What this tool does
IRMAA surcharges are modeled year by year using a two-year lookback on MAGI, applied against current IRMAA tiers inflated forward. Each scenario's table shows when and by how much you'd enter a surcharge tier.
The problem
Traditional IRA balances are shown at face value, making them look bigger than they are. A $1M traditional IRA isn't worth $1M — a meaningful chunk of it belongs to the IRS.
What this tool does
The 'After-Tax Equivalent Wealth' metric deducts the estimated embedded federal tax liability from your traditional balance before comparing scenarios. This is the apples-to-apples number that actually tells you whether conversions improved your position.
The problem
There's no way to know if you're even searching the right space. Choosing a conversion amount and bracket ceiling involves a lot of guesswork.
What this tool does
In addition to your three custom scenarios, the tool automatically searches across dozens of combinations — different brackets to fill, flat annual amounts, start ages, and end ages — and surfaces the strategy that maximizes your after-tax wealth. It also explains why: showing the projected marginal rate on conversions now versus the rate you'd pay on RMDs if you converted nothing.
All projections use 2024 federal tax brackets, IRMAA tiers, and RMD tables as a baseline, inflated forward using your specified inflation rate. State income tax rates auto-fill from current published rates and can be overridden for your situation. RMD start ages follow SECURE 2.0 rules based on birth year.
This tool is designed for illustrative purposes and does not substitute for professional tax or financial advice. Consult a qualified advisor before making conversion decisions.
Read the full methodology →This is a work in progress and I’d love to hear what’s useful, what’s missing, or what’s confusing. If you have thoughts, reach out at bill@horyzen.com.
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