Backdoor Roth and Mega Backdoor Roth: Step-by-Step

TL;DR

• The backdoor Roth is a two-step process: contribute to a non-deductible traditional IRA, then immediately convert to Roth. Legal, well-established, used by millions.
• The pro-rata rule is the main trap: if you have other pre-tax IRA balances, your conversion is taxed proportionally. Resolve this first by rolling pre-tax IRA funds into your 401k.
• The mega backdoor Roth allows up to ~$46,500 in additional Roth contributions — but only if your 401k plan allows after-tax contributions AND in-plan Roth conversion or in-service withdrawal. Most plans do not.

Part 1: The Backdoor Roth IRA

Why it exists

Direct Roth IRA contributions phase out at:

• $150,000–$165,000 MAGI (single filers)
• $236,000–$246,000 MAGI (married filing jointly)

Above these thresholds, direct Roth contributions are not allowed. The backdoor Roth is a two-step workaround that has no income limit.

The steps

Step 1: Open or use an existing traditional IRA. Make a non-deductible contribution — the maximum is $7,000 for 2025 ($8,000 if age 50+). You will not get a tax deduction (you are above the income limit), but that is fine.

Step 2: Convert the traditional IRA to a Roth IRA. If you do this immediately after contributing and the balance has not grown, there is zero tax owed — you are converting after-tax money to Roth.

Step 3: File Form 8606 with your tax return. This documents your non-deductible basis and proves the conversion was not taxable. Do not skip this step. Failure to file Form 8606 can result in the conversion being taxed twice.

Timeline: Contribute in January, convert in January. Do not let it sit for months — any growth between contribution and conversion is taxable.

The pro-rata rule — the most common mistake

If you have any pre-tax IRA money (traditional IRA, rollover IRA, SEP IRA, SIMPLE IRA), the IRS treats all your IRAs as one pool when calculating the tax on a Roth conversion. You cannot cherry-pick just your non-deductible basis.

Example:

• You have a rollover IRA with $93,000 (pre-tax)
• You contribute $7,000 non-deductible to a new traditional IRA
• Total IRA balance: $100,000 — of which $7,000 (7%) is after-tax basis

When you convert the $7,000, only 7% is tax-free. The remaining 93% ($6,510) is taxable at your ordinary rate.

At 35%, that is $2,279 in unexpected taxes — wiping out most of the benefit.

The fix: Roll your pre-tax IRA balances into your 401k before executing the backdoor Roth. Most 401k plans accept incoming rollovers. Once the rollover IRA is empty, your only IRA balance is the new non-deductible contribution — 100% after-tax, 100% tax-free to convert.

Annual backdoor Roth checklist

• Check MAGI — are you above the Roth direct contribution limit?
• Do you have pre-tax IRA balances (rollover, SEP, SIMPLE)? If yes, roll them into your 401k first.
• Contribute $7,000 to traditional IRA (non-deductible)
• Convert the balance to Roth immediately (same week if possible)
• Confirm no unexpected growth between contribution and conversion
• File Form 8606 with your tax return — both spouses if married
• Repeat annually

Part 2: The Mega Backdoor Roth

What it is

Your 401k has a total annual contribution limit of $70,000 in 2025 (employee + employer + after-tax combined). The standard employee contribution is $23,500. If your employer contributes, say, $10,000, you have $36,500 of potential “after-tax” 401k space remaining.

The mega backdoor Roth allows you to fill that gap with after-tax (non-Roth) 401k contributions, then convert them to Roth — either through an in-plan Roth conversion or by rolling them to a Roth IRA.

If fully executed: You could have $23,500 in traditional 401k + $10,000 employer match + $36,500 after-tax (converted to Roth) = $70,000 in your 401k, with most of it in Roth. Plus your backdoor Roth IRA on top.

Plan requirements — check both

Requirement 1: Your plan must allow after-tax (non-Roth) contributions. Not all plans do. This is in your Summary Plan Description (SPD) — ask HR or check your 401k portal.

Requirement 2: Your plan must allow either:

• In-plan Roth conversion: You contribute after-tax, then convert immediately to Roth within the 401k
• In-service withdrawal: You withdraw the after-tax contributions (and convert to a Roth IRA) while still employed

Without one of these two options, your after-tax contributions are stuck — they will eventually be taxable on withdrawal, and you lose the benefit.

The steps (if your plan allows)

Step 1: Increase your 401k contribution to include after-tax contributions, beyond the standard $23,500 limit. Set up the after-tax contribution percentage in your 401k portal.

Step 2: As soon as the after-tax contributions hit your account, convert them to Roth (in-plan conversion) or process an in-service withdrawal to a Roth IRA. The faster you convert, the less growth occurs before conversion — and growth before conversion is taxable.

Step 3: Any earnings on the after-tax contributions before conversion are taxable at ordinary income rates. This is why you want to convert as quickly as possible — ideally same day or weekly.

After-tax gains on conversion: the IRS Notice 2014-54 rule

IRS Notice 2014-54 allows you to separately roll after-tax contributions to a Roth IRA and pre-tax earnings to a traditional IRA in a single distribution. This lets you cleanly separate the two buckets. Important for in-service withdrawals.

Which one should you do?

Do the backdoor Roth IRA first — it is available to everyone regardless of plan. Then investigate whether your plan supports the mega backdoor before spending time on it.

What most content gets wrong

“The backdoor Roth is a loophole that may be closed.” Congress has discussed this but has not closed it. It has been in use since 2010. The Step Transaction doctrine risk exists in theory but has not been asserted by the IRS. The IRS’s own instructions for Form 8606 describe the conversion process. Execute it, file Form 8606, and document your steps.

“You can do the mega backdoor Roth at any 401k.” Most plans do not allow it. Before assuming you can do it, read your Summary Plan Description and confirm with HR. The plan must explicitly allow after-tax contributions and conversion/withdrawal.

“The backdoor Roth only makes sense if you expect higher taxes later.” You do not need to predict tax rates. The backdoor Roth gives you a pool of tax-free money permanently — useful regardless of future rate direction. It is especially valuable if you might be in a high bracket in retirement due to RMDs.

Decision checklist

Backdoor Roth IRA:

• MAGI above direct contribution threshold?
• Pre-tax IRA balances resolved (rolled to 401k)?
• $7,000 contributed to traditional IRA as non-deductible?
• Converted to Roth immediately?
• Form 8606 filed for both spouses?

Mega Backdoor Roth:

• Plan allows after-tax contributions? (Check SPD)
• Plan allows in-plan Roth conversion OR in-service withdrawal?
• After-tax contributions set up in portal?
• Conversion/withdrawal executed promptly?
• IRS Notice 2014-54 split treatment used if applicable?

When to call a CPA

• If you have pre-tax IRA balances and need to confirm the rollover-to-401k strategy
• If you completed a backdoor Roth but forgot to file Form 8606 in prior years (correction is possible)
• If your plan’s after-tax contribution rules are ambiguous
• If you are making Roth conversions of large amounts and need to model optimal timing

Sources

• IRC §408A — Roth IRAs
• IRC §402(g) — 401k contribution limits
• IRS Notice 2014-54 — Guidance on after-tax 401k rollover tax treatment
• IRS Form 8606 — Nondeductible IRAs (instructions explain the conversion process)
• IRS Publication 590-A — Contributions to Individual Retirement Arrangements
• IRS Rev. Proc. 2024-40 — 2025 retirement plan contribution limits