Saying that the Mega-Backdoor Roth strategy can supercharge your retirement saving is not a hyperbole. In most situations, maximizing this feature MORE THAN DOUBLES the amount you can save to Roth accounts each year. Once those savings are invested in a Roth account, they can grow tax free for years or decades and be withdrawn tax free* in retirement.
The “mega-backdoor” can have mega tax impacts
A quick example illustrates just how powerful those tax savings can be:
Each year for 20 years, “normal 401k Charlie” maxes out his Roth 401k with $20,500 and invests an additional $40,500 in a taxable investment account. Both accounts grow at 6% per year. At the end of 20 years, Charlie has $1,497,169 in his taxable account and $757,826 in his 401k; $2,254,995 in total. While Charlie’s Roth 401k savings can be withdrawn tax free in retirement, he will owe capital gains tax on the $687,169 of his investment gains in the taxable account. At long term capital gains rate of 15% that would be $103,075 in taxes! This doesn’t even include taxes he would likely have paid on dividends in the taxable account over 20 years.
“Mega-backdoor Bettie” on the other hand maxes out her normal $20,500 Roth 401k contribution and is able to invest an additional $40,500 into her Roth 401k through the Mega-backdoor strategy. After 20 years, with the same investment return as Charlie, Bettie has the same total savings of $2,254,995. However, Bettie can withdraw that full amount tax free in retirement.
In this example the Mega-backdoor saved Bettie $103,075 on taxes! She also didn’t pay tax on any dividends along the way.
How to implement the Mega-Backdoor strategy
Hopefully this example helps illustrate that the Mega-Backdoor Roth can be a powerful tax-saving strategy, but how does it work? By making after-tax 401k contributions and in-plan Roth conversions. Let’s break those two steps down: