What this tool does, who built it, where the numbers come from, and what it cannot do.
Last reviewed: August 2026
The Roth Conversion Planner works out how much of a traditional 401(k) or IRA you could convert to a Roth this year, and what that conversion would actually cost.
Most conversion calculators apply your tax bracket to the amount converted and stop there. That answer is usually wrong, and often badly wrong. A conversion can drag Social Security benefits into taxable income, claw back deductions that were phasing out, push qualified dividends from the 0% capital gains bracket into the 15% one, trigger a Medicare IRMAA surcharge two years later, or wipe out an ACA premium tax credit entirely. Each of those is invisible in a bracket table.
This tool sweeps every possible conversion amount and reports what the next dollar really costs once all of it is counted. That number is frequently double the bracket rate, and knowing where those cliffs sit is the whole point.
This site is maintained by David Rose (admin@rothconvert.tax). As I near retirement, I need a convenient way to run scenarios for a Roth IRA conversion. The process of transferring tax-deferred assets into a tax-free vehicle can be complicated. Being able to see accurate estimates for various inputs is critical to making an informed decision.
This calculator is currently assisting me in making an actual conversion decision. It deepens my understanding of the process and increases my confidence in my future decisions.
Everything runs inside your browser. There is no server, no account and no database. The calculation happens on your own machine, and the numbers you type never leave the page. The site uses Google Analytics to count visits, which is separate from the calculator and cannot see what you enter — the privacy policy sets out precisely what is and is not collected.
The tax engine is around 600 lines of plain JavaScript, with roughly another 900 behind the help system, covering:
Every tax parameter is taken from a published primary source and cited in the code:
| Figure | Source |
|---|---|
| Brackets, standard deduction, capital gain breakpoints | IRS Revenue Procedure 2025-32 |
| Senior bonus deduction | One Big Beautiful Bill Act (2025) |
| Social Security taxability | IRS Publication 915 |
| Medicare IRMAA tiers and premiums | CMS 2026 Medicare Parts B & D announcement |
| Qualified charitable distribution limit | IRS Notice 2025-67 |
| ACA applicable percentages | IRS Revenue Procedure 2025-25 |
| Federal poverty guidelines | HHS 2025 guidelines |
| RMD ages and life expectancy factors | SECURE 2.0 Act; IRS Uniform Lifetime Table |
The calculation engine is covered by more than 450 automated checks: every bracket boundary for every filing status, the Publication 915 worksheet, capital gain stacking across band edges, deduction phase-outs, IRMAA tier changes, and the pro-rata basis rule.
Some of those verify the arithmetic. A separate set verifies the figures — the brackets, deductions and thresholds are checked against the cumulative tax amounts published in the revenue procedure itself, which is an independent check on whether they were transcribed correctly. That test found a real error: the head-of-household 24% bracket boundary was $201,775 (the single-filer figure) instead of $201,750. It has been corrected.
The multi-year projection is checked for conservation of dollars — every dollar must end up as either a balance or a tax paid — and against a deliberately rate-neutral case where converting should be close to a wash. That second test exists to catch a model that manufactures free money, which is the characteristic failure of optimistic financial projections.
Tests confirm the model does what it intends. They cannot confirm that the intent matches your situation. Check the line-by-line breakdown against your own return before relying on it.
These are listed because a model that hides its limitations is more dangerous than one that states them.
If you find an error in the tax logic, I want to hear about it. Tax law is intricate and this was not written by a tax professional. Get in touch — a specific citation is more useful than a general concern.