A Mega Backdoor Roth IRA works by making after-tax contributions to a 401(k) plan and then rolling those contributions over to a Roth IRA. After-tax contributions are made with money that has already been taxed, so you will not have to pay taxes on those contributions again when you roll them over to a Roth IRA.
The annual contribution limit for Roth IRAs in 2023 is $6,500 or $7,500 for those aged 50 or older. But with a Mega Backdoor Roth IRA, high-earners can use their after-tax contributions to contribute up to the annual contribution limit in their 401(k). For 2024, the annual contribution limit is $7,000 for those under 50, and an additional $1,000 catch-up contribution for those 50 and older.