I want to share my experience with the mega backdoor Roth because the instructions online are often incomplete.

The mega backdoor Roth allows you to contribute up to $43,500 in after-tax contributions to a 401k (beyond the $23,000 standard limit) and then convert those to Roth. Not all 401k plans support this — it requires both the ability to make after-tax contributions and in-service withdrawals or in-plan Roth conversions.

Step 1: Confirm your plan allows after-tax contributions (check your Summary Plan Description or ask HR).
Step 2: Confirm in-plan Roth conversion is available, OR that in-service withdrawals to a Roth IRA are allowed.
Step 3: Elect to make after-tax contributions up to the combined limit ($66,000 total in 2024 including employer contributions).
Step 4: Immediately convert the after-tax contributions to Roth. Do this quickly to minimize taxable earnings on the after-tax money before conversion.

The tax impact: you pay no tax on the conversion if you convert immediately (the after-tax contributions have no growth yet). Any earnings before conversion are taxable.

This is the most tax-advantaged savings strategy available to high earners whose 401k plans support it.