The Mega-Backdoor Roth, Explained (Up to $41,500 More Roth Space)
Most people know the 401(k) deferral limit ($23,500 in 2025; $31,000 at 50+). Fewer know the plan itself can hold far more: the IRS Section 415(c) "total additions" limit is $70,000 ($77,500 at 50+). The gap between those two numbers — after your employer's contributions — is space almost nobody uses. The mega-backdoor Roth is how you use it.
The three-step move
- 1. Max the normal deferral. Fill your regular pre-tax or Roth 401(k) space first — it's the better-taxed dollar.
- 2. Contribute after-tax dollars (a separate contribution type, not Roth deferrals) up to the 415(c) ceiling.
- 3. Convert immediately to Roth via an in-plan Roth rollover or an in-service withdrawal to a Roth IRA. Converted right away, there's little growth to tax — the dollars become Roth almost free.
The math, worked
Age 35, deferring the full $23,500, employer contributes $5,000: $70,000 − $23,500 − $5,000 = $41,500 of after-tax room — potential extra Roth space, every year, on top of everything else. For comparison, the regular backdoor Roth IRA moves $7,000.
The two plan features you need
Your 401(k) must allow both: (1) after-tax (non-Roth) contributions, and (2) in-service conversion — either in-plan Roth rollovers or in-service distributions of the after-tax subaccount. Roughly half of large-employer plans have them; check your Summary Plan Description or ask HR two questions: "Does the plan accept after-tax contributions above the deferral limit?" and "Can I convert them to Roth while employed?"
Why convert immediately
After-tax contributions come out tax-free, but their earnings are pre-tax. Left unconverted for years, you build an earnings layer that's taxable at conversion. Convert every payroll cycle (many plans automate this as a "daily sweep") and the earnings never accumulate.
Who this is for — honestly
This is a surplus-cash strategy. It only makes sense after the full deferral, the match, the HSA, and the IRA are handled — steps 4–6 of the financial order of operations. If you're not maxing the basics yet, do that first; the mega-backdoor will still be here.
Check your exact capacity
Tuesday's mega-backdoor calculator (Pro) computes your personal 415(c) room from your age, deferrals, and employer contributions. Find your after-tax room →
Frequently asked questions
- What is a mega-backdoor Roth?
- It is a strategy that uses after-tax 401(k) contributions above the normal deferral limit, converted immediately to Roth via an in-plan rollover or in-service withdrawal. It can add tens of thousands of dollars of Roth space per year on top of regular limits.
- How much can I put in a mega-backdoor Roth?
- The 2025 Section 415(c) limit is $70,000 ($77,500 age 50+). Subtract your own deferrals and all employer contributions; the remainder is your after-tax capacity. Someone deferring $23,500 with a $5,000 employer contribution has $41,500 of room.
- Does my 401(k) allow the mega-backdoor Roth?
- Only if the plan offers both after-tax (non-Roth) contributions and a way to convert them while employed — an in-plan Roth rollover or in-service distribution. Check the Summary Plan Description or ask HR; roughly half of large-employer plans qualify.