Your Summary Plan Description (SPD) is the document that actually decides whether the Mega Backdoor Roth is available to you. HR can give it to you, and most of what you need is in three places. Here is what to look for.
Fact 1 — Are after-tax (non-Roth) contributions permitted?
Search the SPD for the phrase after-tax in the contributions section. You are looking for language that the plan accepts employee contributions beyond pre-tax and Roth elective deferrals.
“Participants may make after-tax (non-Roth) contributions to the Plan, in addition to elective deferrals, up to the limits prescribed under Code §415(c).”
If instead the SPD says contributions are limited to pre-tax and Roth deferrals under §402(g), there is no after-tax bucket — and the Mega Backdoor Roth is not available, regardless of anything else.
Fact 2 — Is in-plan Roth conversion permitted?
This is the optimal conversion path. Look for in-plan Roth rollover or in-plan Roth conversion.
“A Participant may elect to convert all or a portion of their non-Roth account balances to a Roth account within the Plan (an ‘in-plan Roth rollover’).”
The detail that matters most is automation. The best plans convert after-tax contributions to Roth automatically every pay period, so earnings have almost no time to accrue. If conversion is manual and infrequent, you may owe tax on the earnings that built up between contribution and conversion.
Fact 3 — Is in-service distribution permitted?
Only relevant if Fact 2 is “no.” This is the substitute path: withdraw after-tax money while still employed and roll it to a Roth IRA.
“A Participant may request an in-service withdrawal of after-tax contributions and associated earnings at any time.”
Watch for an age gate. If the SPD says in-service withdrawals from after-tax accounts are not permitted before age 59½, then a younger participant with no in-plan conversion has no actionable path — the decoder flags this as MBR-Blocked.
Putting it together
The verdict is deterministic: an after-tax bucket plus in-plan conversion is MBR-Eligible; an after-tax bucket with only in-service distribution is MBR-Conditional; no after-tax bucket, or a bucket with no conversion path, is MBR-Blocked. Run your three answers through the decoder above for the statute-cited result, then verify the specifics with your benefits administrator.