A backdoor Roth IRA is a two-step move that lets high earners fund a Roth IRA even though their income is too high to contribute to one directly. It isn't a secret account type or a special product some bank sells you. It's a sequence: contribute to a traditional IRA, then convert that money to a Roth IRA. The steps are simple. The part that actually trips people up, and can quietly generate a tax bill nobody warned them about, is a rule most articles mention in passing and few explain with real numbers.
Why This Workaround Exists at All
The IRS caps who can contribute directly to a Roth IRA based on income. For 2026, single filers can contribute the full amount if their modified adjusted gross income (MAGI) is under $153,000, with the amount phasing out completely at $168,000. Married couples filing jointly can contribute fully under $242,000 MAGI, phasing out completely at $252,000. Above those thresholds, a direct Roth contribution isn't allowed at all.
But there's no income limit on converting money already in a traditional IRA to a Roth IRA. That income cap on conversions existed until 2010, when it was repealed permanently. Since then, anyone at any income level can convert traditional IRA money to Roth. That gap, no income limit on contributing to a traditional IRA (just no tax deduction for it above certain income), combined with no income limit on converting, is the entire loophole. It's been standard practice for over a decade and the IRS has never treated it as improper, even though a handful of legislative proposals have tried to close it.
The Actual Steps
- Contribute to a traditional IRA without claiming the deduction. For 2026, that's up to $7,500 if you're under 50, or $8,600 if you're 50 or older (a $1,100 catch-up). You can contribute even above the Roth income limits, you just can't deduct it if your income is too high and you or a spouse has a workplace retirement plan.
- File Form 8606 with your tax return. This records the contribution as basis, meaning after-tax money the IRS already knows shouldn't be taxed again. Skipping this form is the single most common backdoor Roth mistake, and it can cost you real money later if the IRS has no record that you already paid tax on that contribution.
- Convert the traditional IRA balance to a Roth IRA. Ideally do this quickly, before the contribution has time to earn much. Any growth between the contribution and the conversion is taxable at conversion; a same-week or same-month conversion keeps that taxable amount close to zero.
- Repeat annually if you're consistently over the direct contribution limit.
On paper, that's it. In practice, step three is where the pro-rata rule shows up, and it can turn a supposedly tax-free move into a real tax bill.
The Pro-Rata Rule: Where This Quietly Falls Apart
The IRS doesn't let you cherry-pick which dollars you're converting. Under the pro-rata rule (IRC §408(d)(2)), every traditional, SEP, and SIMPLE IRA you own is treated as one combined pool as of December 31 of the conversion year, regardless of which specific account the converted money came from. Any conversion is taxed in proportion to how much of that entire pool is pre-tax money versus after-tax basis. If you have zero other IRA balances, this doesn't matter, your new contribution is 100% basis and the conversion is close to tax-free. If you're carrying an old rollover IRA from a previous 401(k), it matters a lot.
Here's what that actually looks like in dollars. Say you roll over an old 401(k) into a traditional IRA years ago and it's now worth $93,000, all pre-tax money that's never been taxed. This year you contribute $7,500 to a traditional IRA (nondeductible, meant for the backdoor) and convert that $7,500 to Roth.
- Total IRA pool on December 31: $93,000 + $7,500 = $100,500
- Your basis (after-tax money) in that pool: $7,500
- Nontaxable percentage: $7,500 ÷ $100,500 = 7.46%
- Of your $7,500 conversion, only $559.70 comes out tax-free
- The remaining $6,940.30 is taxed as ordinary income
At a 32% marginal tax rate, that's about $2,220.90 in tax on a contribution you may have assumed was tax-free going in. And the basis that didn't get used doesn't disappear, it carries forward on Form 8606 and keeps getting diluted at the same ratio every year you convert again, until the pre-tax balance is gone.
The fix, if your employer's 401(k) plan accepts incoming rollovers, is to move that old pre-tax IRA money into the 401(k) before December 31 of the year you convert. A 401(k) balance isn't counted in the pro-rata pool, only IRAs are. Once that pre-tax IRA balance is at zero, the backdoor Roth goes back to being close to tax-free.
Mega Backdoor Roth: A Different, Much Bigger Strategy
This gets confused with the regular backdoor Roth constantly, but it's a separate strategy with separate mechanics. For 2026, the employee deferral limit into a 401(k) (combined pre-tax and Roth) is $24,500, but the total annual additions limit, everything going into the account including employer contributions, is $72,000. The gap between what you and your employer put in through normal channels and that $72,000 ceiling can potentially be filled with after-tax (not Roth) 401(k) contributions, which can then be converted to Roth.
The catch is that this only works if your specific 401(k) plan allows both after-tax contributions and either in-service withdrawals or in-plan Roth conversions. Plenty of plans don't offer either. This isn't something you opt into by contributing more, it depends entirely on plan design, so check your plan documents or ask HR before assuming it's available to you. It also isn't a bigger version of the regular backdoor Roth, it's a completely separate mechanism running through your workplace 401(k) instead of an IRA.
Who Should Actually Bother With This
If your income is under the direct Roth contribution limits, skip all of this and just contribute to a Roth IRA directly. The backdoor only exists to solve a problem you don't have yet. If you're over the limit, it's genuinely worth doing, but two things determine how clean it is: whether you have existing pre-tax IRA balances (the pro-rata rule) and whether you can actually clear them out via a 401(k) rollover if you do. Get the paperwork right, Form 8606 every year you do this, since a missed form is how people end up paying tax twice on the same dollars when they eventually withdraw.
It's also worth knowing this loophole has been a repeated target of tax legislation, including proposals in 2021 that would have eliminated both the backdoor and mega backdoor Roth entirely for higher earners. None of those proposals became law, and as of 2026 the strategy remains fully legal under current tax code. That could change with future legislation, which is one more reason not to treat it as a permanent fixture of your retirement plan rather than a rule that currently exists.
For the fundamentals on how Roth accounts work and when a Roth makes more sense than a traditional account in the first place, our post on Roth IRA vs. Traditional IRA: How to Actually Decide covers that groundwork. And if the mega backdoor Roth's dependence on plan design has you wondering what else your 401(k) plan controls that you don't see day to day, what happens to your 401(k) match if you leave before you're vested covers another plan-specific detail that catches people off guard.
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Spicy Investing