Mega Backdoor Roth vs Traditional 401(k) Calculator (2026)
Mega Backdoor Roth lets high earners contribute up to $70,000 to a 401(k) in 2026 — including $46,500 after-tax that converts to Roth. This 2026 tool compares the long-term outcome to traditional 401k.
| Annual contribution | — |
| Years to retirement | — |
| Mega Backdoor Roth balance (tax-free) | — |
| Traditional 401k balance (pre-tax) | — |
| Trad 401k after retirement tax | — |
| Net advantage Mega Backdoor Roth | — |
The Mega Backdoor Roth lets high earners contribute up to $46,500 of after-tax money to a 401(k) in 2026 (the gap between the $23,500 elective deferral plus employer match and the $70,000 total 401(k) cap), then convert immediately to Roth. The converted dollars grow tax-free forever — a meaningful advantage over Traditional 401(k) for anyone expecting similar or lower tax rates in retirement.
How Mega Backdoor Roth Works
2026 401(k) limits are $23,500 elective deferral + up to $70,000 total contributions (employee + employer + after-tax). The 'mega backdoor' is the after-tax (non-Roth) contribution bucket up to fill the gap to $70,000. Two execution paths. (1) In-plan Roth conversion: convert after-tax balance to Roth 401(k) inside the plan. (2) In-service rollover: roll after-tax balance to an outside Roth IRA. Both work; the choice depends on your plan's investment menu vs an IRA's flexibility.
When MBR Beats Traditional
Mega Backdoor Roth wins for anyone who (1) maxes traditional 401(k) already, (2) has a plan that allows after-tax contributions AND in-plan or in-service Roth conversion (about 50% of large-employer plans), and (3) expects similar or lower tax rates in retirement. The math favors MBR most strongly for high earners in their 30s-40s with 20+ years of growth ahead — the tax-free compounding produces 20-40% more wealth than Traditional after retirement taxes. Critical execution detail: convert immediately after each after-tax contribution. Earnings that accumulate in the after-tax bucket before conversion are taxable. Pro-rata rule applies when converting, so frequent small conversions beat one big year-end conversion.
Last updated May 2026. Sources: IRS Notice 2014-54.