How does the mega backdoor Roth save on taxes?

The mega backdoor Roth lets high earners move large after-tax dollars into Roth accounts, where future growth and withdrawals are tax-free. It works only if your 401(k) plan allows both after-tax (non-Roth) contributions beyond the $23,500 employee limit and either in-plan Roth conversions or in-service withdrawals to a Roth IRA. The total 2025 cap on all 401(k) contributions (yours, employer match, and after-tax) is $70,000, so the after-tax room is whatever's left after your regular contributions and match. You contribute after-tax dollars, then quickly convert them to Roth so little or no growth is taxed. The tax win isn't upfront, it's decades of tax-free compounding and tax-free retirement withdrawals far beyond the normal Roth limits. Compare account types at wealthserene.com/tools/roth-vs-traditional.