401(k) & Retirement
Should You Convert Your 401(k) to a Roth IRA? The Tax Math Explained
Same conversion, same rate, same growth — but one choice about how you pay the tax bill can swing the outcome by nearly $90,000. Here's the math most explainers skip.
The fear that stops people who shouldn't be stopped
Ask around and you'll hear some version of: "isn't there a penalty for touching your 401(k) before 59½?" It's a reasonable thing to worry about, and it's wrong in this specific context. A conversion isn't a withdrawal — it's a transfer from one type of retirement account to another. No penalty applies to the conversion itself, at any age. The confusion usually comes from a real rule that applies later, not to the conversion itself, which we'll get to.
What actually happens when you convert
The mechanics are simple, even if the decision isn't: the full amount you convert gets added to your taxable income for that year, and taxed at your ordinary income rate — not capital gains rates, since the money was never taxed to begin with. Once it's in the Roth, it grows completely tax-free, and qualified withdrawals in retirement owe nothing at all, no matter how large the account has grown.
The variable that actually decides the outcome
Run the exact same conversion two different ways — same amount, same tax bracket, same growth rate, same number of years — and change only where the tax payment comes from, and the results aren't close.
Converting wins, in this scenario, because the full converted amount keeps growing tax-free.
Converting actually loses to just staying traditional — the same conversion, just funded differently.
That's a $100,000 conversion, 20-year horizon, 7% growth, taxed at 22% both now and hypothetically in retirement. The single decision of whether the tax bill comes from your bank account or from the retirement funds themselves is the difference between a clear win and a real loss — bigger than almost any other variable in the calculation, including the tax rate assumption itself.
The reason is straightforward once you see it: withholding the tax from the converted amount means less money actually lands in the Roth to grow tax-free. You're not just paying tax — you're permanently shrinking the asset that was supposed to benefit from decades of tax-free compounding.
The rule people actually mean when they worry about penalties
When converting genuinely makes sense
- Your income is unusually low this year — a gap between jobs, an early-retirement year before Social Security starts, or any year where your tax bracket is temporarily lower than you expect it to be later.
- You can pay the tax from savings, not the account. This is the one condition worth treating as close to a requirement.
- You have a long runway before you'll need the money — more years of tax-free growth means more benefit from having converted.
- You want to avoid future required minimum distributions. Traditional accounts force withdrawals starting at 73 (75 for those born in 1960 or later) whether you need the money or not. Roth accounts don't, during your lifetime.
The other thing people get backwards
Direct Roth IRA contributions phase out once your income crosses a threshold — a well-known limitation. Conversions have no income limit at all. You can convert any amount, regardless of how much you earn, which is exactly why high earners who can't contribute directly still find their way into Roth accounts through conversion.
| Lever | Reduces the conversion's tax bill |
|---|---|
| Paying from outside savings | The single biggest factor in the whole decision |
| Converting in a low-income year | Meaningful — lowers the rate applied to the conversion itself |
| Long time horizon before withdrawal | Increases the total benefit, doesn't change the upfront tax |
None of this is a reason to avoid converting — it's a reason to convert deliberately. Run your specific numbers, including how you'll actually pay the resulting tax bill, with our 401(k) to Roth IRA Conversion Calculator, which compares both funding scenarios side by side using your own figures.
Conversion tax mechanics, the 5-year rule for conversions, and current RMD ages verified against multiple independent 2026 tax and financial planning guides. This is general information, not personalized financial advice — a financial advisor can help model your specific tax bracket, timeline, and funding source.
Frequently asked questions
Does converting a 401(k) to a Roth IRA trigger the 10% early withdrawal penalty?
No — the conversion itself is never penalized, regardless of age. The penalty can apply later if you withdraw the converted amount within five years of that specific conversion while under 59½.
What's the single most important factor in whether a conversion is worth it?
Whether you can pay the resulting tax bill from savings outside the retirement account, rather than withholding it from the converted funds themselves. This one choice can be the difference between a large net benefit and a net loss on an otherwise identical conversion.
Is there an income limit on Roth conversions?
No. Direct Roth IRA contributions phase out at higher incomes, but conversions don't have any income limit at all.
Why do people convert specifically to avoid RMDs?
Traditional 401(k)s and IRAs require minimum distributions starting at age 73 (75 for those born in 1960 or later), forcing taxable withdrawals whether needed or not. Roth IRAs have no required minimum distributions during the original owner's lifetime.
Is a Roth conversion taxed at capital gains rates or income tax rates?
Ordinary income tax rates. The full converted amount is added to your taxable income for the year, since the original contributions were never taxed.