Roth IRA conversion: The tax bet that depends on your future withdrawal rate

Two brothers, starting with the same $450,000 in traditional IRAs and experiencing similar market returns over a decade, ended up with different financial outcomes based on their tax strategies. One brother converted $40,000 annually into a Roth IRA, paying a 12% federal tax on each conversion from funds held outside his retirement accounts. The other brother made no changes to his traditional IRA, allowing it to grow to $730,000.

The core of the decision lies in when taxes are paid. Money in a traditional IRA is pre-tax, meaning withdrawals are taxed as ordinary income in retirement. In contrast, money in a Roth IRA has already been taxed, making qualified withdrawals tax-free. The scenario assumes a mid-single-digit annual return, a 12% tax rate on conversions, and a 22% withdrawal rate for the non-converter.

Under these assumptions, the brother who converted to a Roth IRA has an account balance that can be spent dollar-for-dollar, while the larger balance in the traditional IRA is diminished by future taxes. The opportunity cost of paying taxes now, rather than investing that money, is a key consideration. However, the decision can flip if future tax rates are lower than the current conversion rate. If the non-converting brother's retirement income keeps him within the two lowest tax brackets, he might pay less than 12% in taxes, making his larger traditional IRA balance more advantageous.

Beyond tax rates, Roth IRAs offer advantages such as the absence of lifetime required minimum distributions for the original owner, which can prevent forced income realization and potentially lower Social Security taxation and Medicare premium surcharges. Heirs also benefit, as inherited Roth IRAs are tax-free, unlike traditional IRAs which are generally taxed within ten years of inheritance.