The backdoor Roth is described almost everywhere as a two-step move: contribute to a traditional IRA without taking a deduction, then convert it to Roth. No tax, because you already paid tax on the money.
That description is correct for people who have no other IRA. For everyone else it is wrong, and the error is expensive.
The rule that breaks it is called pro-rata, and it works like this: when you convert, the IRS does not care which dollars you moved. It looks at every traditional, SEP and SIMPLE IRA you own, computes what share of the total is after-tax money, and treats your conversion as that same mixture.
With $50,000 of pre-tax IRA money sitting somewhere, a $7,500 backdoor contribution comes across only 13.04% tax-free. The rest is taxable. You pay $1,565 on money you had already paid tax on once.
A note before you start. This is general education, not tax advice. Every figure here comes from this site's backdoor Roth engine, which implements the pro-rata calculation on a $7,500 non-deductible contribution at a 24% combined marginal rate. The rate is an input, not a claim about your situation — a conversion is ordinary income to both the federal government and, in most states, to the state, so your own combined figure is the one to use. Contribution limits and income thresholds change annually and are not printed here for that reason.
1. The number the whole rule reduces to
One fraction decides everything.
After-tax basis ÷ total value of all your traditional, SEP and SIMPLE IRAs
That percentage is the share of any conversion that comes across tax-free. The rest is taxable. It does not matter which account you converted from, which dollars you moved, or how recently you contributed.
Here is what that fraction does as pre-tax money accumulates, on a $7,500 non-deductible contribution converted in full:
| Pre-tax IRA balance | Tax-free share | Taxable | Tax at 24% |
|---|---|---|---|
| $0 | 100% | $0 | $0 |
| $25,000 | 23.08% | $5,769 | $1,385 |
| $50,000 | 13.04% | $6,522 | $1,565 |
| $150,000 | 4.76% | $7,143 | $1,714 |
| $500,000 | 1.48% | $7,389 | $1,773 |
Read the last column carefully, because it is the surprising part. Going from $0 to $25,000 of pre-tax money costs $1,385. Going from $25,000 to $500,000 — twenty times as much — costs another $388.
The damage plateaus almost immediately. By $150,000 you are within $60 of the worst possible outcome. Which means this is not a question of how much pre-tax IRA money you have. It is a question of whether you have any.
2. What counts, and what does not
Inside the calculation:
- Every traditional IRA you own, at every custodian
- Every SEP IRA
- Every SIMPLE IRA
- Measured on 31 December of the conversion year, not on the conversion date
Outside the calculation:
- Your Roth IRA balance, entirely
- Any 401(k), 403(b) or 457 balance, entirely
- Your spouse's IRAs — this is per person, not per household
Three of those deserve emphasis.
"At every custodian" is the one people trip on. The IRS aggregates. An old rollover IRA at a brokerage you have not logged into for six years is in the denominator exactly as if it were sitting next to the account you converted from. There is no per-account version of this rule.
The 401(k) exclusion is not a footnote, it is the fix. Section 5 is built on it.
And the per-person point matters for couples. One spouse's rollover IRA does not contaminate the other's backdoor conversion. Two people in the same household can be in completely different positions.
Run the pro-rata calculation on your own IRA balances3. Who walks into this without noticing
Almost nobody chooses to hold a pre-tax IRA alongside a backdoor Roth strategy. They arrive at it by ordinary means.
A rollover from an old job. The single most common route. You left an employer, rolled the 401(k) into an IRA because that was the advice, and now hold a pre-tax IRA that had nothing to do with retirement planning and everything to do with tidiness.
A deductible contribution from an earlier, lower-income year. Perfectly sensible at the time. It is pre-tax money now.
A SEP or SIMPLE from self-employment. Freelance income, a side business, a year of contracting. Both count.
An inherited IRA is the exception — an IRA inherited from someone other than a spouse is not aggregated with your own for this purpose, which is one of the few places the rules work in your favour by accident.
The common thread is that none of these feel like IRA decisions. They feel like housekeeping, or like something that happened years ago. The pro-rata rule does not distinguish between money you put there deliberately and money that arrived by rollover.
4. What "you already paid tax on it" actually means
It is worth being precise about why the taxed portion is objectionable, because the objection is not that you owe tax on a conversion — that is normal.
A non-deductible contribution is made with money that has already been through your paycheck. You earned it, it was taxed, and what remains is what you contributed. There is no deduction on the way in.
When pro-rata makes part of that conversion taxable, that portion is taxed a second time on the way out. Not in some abstract sense — the same dollars, taxed on the way in and taxed again on conversion.
The engine gives this a name and a number: the cost of pro-rata. With $50,000 of pre-tax money, it is $1,565 — the tax that a clean conversion would have avoided entirely. That figure is the honest measure of what an untouched rollover IRA is costing you, and it is the number to weigh against the effort of the fix.
5. The fix, and why it works
Roll the pre-tax IRA into an employer 401(k).
That is the whole strategy, and it works for exactly one reason: a 401(k) balance is not in the denominator. Move the money there and the fraction that decides everything goes back to 100%.
Four conditions have to hold.
Your plan has to accept incoming rollovers. Many do; not all. This is a five-minute question to your plan administrator and it is the first thing to check, because everything else is contingent on it.
Only pre-tax money moves. Any after-tax basis stays in the IRA — which is fine, because basis is the numerator, and leaving it there is what produces the clean result.
The timing is measured on 31 December. This is the genuinely useful part: because the balance is measured at year end rather than at conversion, the fix still works if you make it after converting, provided it lands in the same calendar year. Someone who converts in March and discovers the problem in October has not missed anything.
And a 401(k) has its own trade-offs. Its investment menu is narrower than an IRA's, its fees may be higher or lower, and access rules differ. The pro-rata fix is a good reason to move money, not the only consideration.
6. If it is already too late for this year
Nothing is lost permanently. This is the reassuring part and it is under-explained everywhere.
The taxed portion becomes basis. When $6,522 of a conversion is taxed, that $6,522 does not vanish — it becomes after-tax basis remaining in your traditional IRAs, tracked on Form 8606, and it reduces the taxable share of every future conversion.
Which means the cost is a timing cost, not a permanent loss. You paid tax earlier than you needed to on money that will not be taxed again.
Two practical consequences. Form 8606 has to be filed, every year, for every year with a non-deductible contribution or a conversion — it is the only record of your basis, and a missing one means the IRS has no reason to believe any of your conversion was tax-free. And basis carries forward indefinitely, so a year of bad pro-rata treatment improves the arithmetic on the next conversion rather than being written off.
7. Four mistakes that are worse than pro-rata
Pro-rata is the famous problem. These are the ones that cause more trouble per occurrence.
Not filing Form 8606. Your basis exists only because this form says so. Without it, a future conversion is fully taxable on the record, and reconstructing years of contributions after the fact is unpleasant.
Contributing above the income limit for a direct Roth and treating the backdoor as automatic. The backdoor exists precisely because direct Roth contributions phase out — but a non-deductible traditional contribution has its own rules, and "I earn too much" is not by itself a complete analysis.
Leaving the money in cash for months before converting. Any earnings between contribution and conversion are taxable, which is fine and small. The bigger problem is that a long gap invites forgetting, and an unconverted non-deductible contribution sitting in a traditional IRA is the exact balance that ruins next year's attempt.
Assuming a spouse's balance is a problem. It is not. The calculation is per person. Households have converted less than they could because one partner's rollover IRA was treated as everyone's problem.
8. What this looks like as a decision
If you have no pre-tax IRA money, the strategy works as described everywhere, the arithmetic is uninteresting, and the only real requirements are filing Form 8606 and converting promptly.
If you have a small pre-tax balance — a few thousand — the cost is real but modest, and converting the whole thing to Roth in one go is often simpler than rolling it into a 401(k). You pay tax on the pre-tax portion once and the problem is gone permanently.
If you have a large pre-tax balance, section 5 is the answer, and it is worth the paperwork. Converting a $150,000 rollover IRA outright would be a large one-year tax bill; rolling it into a 401(k) costs nothing.
If your plan will not accept a rollover and the balance is large, the honest answer is that the backdoor may not be worth it. A $1,714 tax on a $7,500 contribution is a poor trade, and the money is generally better placed in a taxable brokerage account, where at least the growth is eligible for long-term capital gains treatment.
That last case is under-discussed because most published material on the backdoor Roth is written on the assumption that it is always available. It is not, and recognising when it is not is worth more than executing it badly.
9. The state dimension
A conversion is ordinary income to your state as well as to the federal government, in every state that taxes income at all — and the pro-rata portion is ordinary income twice over.
Which makes the combined rate the one that matters. The 24% used throughout this article is a placeholder for federal plus state. In a state charging around 9% on ordinary income, the same $6,522 taxable portion costs materially more than the table shows; in a state with no income tax, it costs less.
There is no state-level version of the pro-rata rule — states start from the federal taxable figure, so the fraction is computed once and flows through. That is a rare simplification in an area with very few of them.
10. The mega backdoor, and why it is a different thing
Two strategies share a name and almost nothing else.
The backdoor Roth described in this article moves money through a traditional IRA: a non-deductible contribution, then a conversion, governed by the pro-rata rule across all your IRAs.
The mega backdoor happens inside a 401(k). It uses after-tax contributions to the plan — a category distinct from both pre-tax and Roth contributions — which are then converted to Roth, either inside the plan or by rolling them to a Roth IRA.
Three differences worth knowing.
The amounts are not comparable. The IRA route moves a single annual contribution. The 401(k) route can move considerably more, because the plan's overall contribution limit is much larger than the elective deferral limit and after-tax contributions fill the gap between them.
The pro-rata rule in this article does not apply to it. A 401(k) has its own rules, and your IRA balances are irrelevant to them. Someone blocked from the ordinary backdoor by a large rollover IRA may still have the mega route fully available.
But it depends entirely on your plan. The plan must permit after-tax contributions and permit either in-plan conversions or in-service withdrawals. Many plans allow neither. This is a plan-document question, not a tax question, and it is answered by your plan administrator in a single phone call.
One caution. Non-discrimination testing can cause after-tax contributions to be refunded after the fact in some plans, which is disruptive rather than costly. Ask about it if you intend to contribute large amounts.
11. What order to do things in
The sequencing matters more than any single step, and it is where most of the avoidable cost sits.
First, find every IRA you own. Every custodian, every account, including ones you have not logged into in years. This is the denominator, and an account you forgot about is exactly as damaging as one you remembered.
Second, ask whether your 401(k) accepts incoming rollovers. Before doing anything else. If the answer is no, section 8's analysis applies and the whole strategy may be the wrong one for you.
Third, move the pre-tax money out — then contribute. Doing it in this order removes any doubt. Doing it the other way round still works, because the balance is measured on 31 December, but it leaves you dependent on completing the rollover inside the calendar year, and rollovers are not always fast.
Fourth, convert promptly after contributing. Days rather than months. Earnings between contribution and conversion are taxable, and more importantly a forgotten contribution becomes next year's problem.
Fifth, file Form 8606. Every year. It is the only record that your contribution was non-deductible, and without it the tax-free treatment you are entitled to has nothing supporting it.
Sixth, keep the paperwork permanently. Basis carries forward indefinitely, which means a form from twelve years ago can still be doing work on this year's return. This is one of the few tax records with no natural expiry.
12. Why the strategy exists at all
It is worth understanding the shape of the thing, because it explains both why it works and why it feels precarious.
Direct Roth contributions phase out above an income level. Roth conversions do not. There is no income limit on converting. That asymmetry is the whole opening: someone above the contribution limit can still put money into a traditional IRA without a deduction, and can still convert it.
Which means the backdoor is not a loophole in the ordinary sense. Nothing about it is hidden or aggressive. It is two entirely permitted transactions performed in sequence, and the sequence produces a result the income limit appears to prevent.
Two consequences follow from that framing.
The pro-rata rule is not a penalty aimed at the strategy. It exists because the tax code has always needed a way to decide which dollars come out of a mixed IRA, and it long predates the backdoor. The strategy runs into it by accident, not by design — which is why the fix is so mechanical.
And the strategy depends on a structural feature rather than an oversight. The removal of the income limit on conversions was a deliberate legislative change, and the resulting sequence has been openly used and openly described for many years. That is a more stable footing than the word "loophole" suggests, though not a guarantee — legislation can change, and material describing the backdoor has periodically anticipated changes that did not arrive.
The practical reading: treat it as a normal part of a savings plan rather than as something to rush before it disappears, and do the paperwork properly, because the paperwork is what makes the sequence defensible rather than merely convenient.
Frequently asked questions
What is the pro-rata rule in one sentence? When you convert, the taxable share of the conversion is determined by the ratio of after-tax basis to the total value of all your traditional, SEP and SIMPLE IRAs — regardless of which account the converted dollars came from.
Does my Roth IRA count in the calculation? No. Roth balances are entirely outside it. So are 401(k), 403(b) and 457 balances, which is what makes the rollover fix work.
How much does a pre-tax balance actually cost me? On a $7,500 conversion at a 24% combined rate: $1,385 with $25,000 of pre-tax money, $1,565 with $50,000, $1,714 with $150,000. It plateaus quickly — the existence of the balance matters far more than its size.
Can I fix it after I have already converted? Yes, if you act within the same calendar year. The balance is measured on 31 December, not on the conversion date, so a rollover into a 401(k) completed in December still repairs a conversion made in March.
What if my 401(k) will not accept a rollover? Then your options are to convert the pre-tax balance outright and pay the tax once, or to skip the backdoor and use a taxable brokerage account instead. With a large balance, the second is frequently the better answer.
Do my spouse's IRAs affect my conversion? No. The calculation is per person. One spouse's rollover IRA has no effect on the other's conversion.
What happens to the portion I was taxed on? It becomes after-tax basis remaining in your traditional IRAs, tracked on Form 8606, and it reduces the taxable share of every future conversion. The cost is a timing cost, not a permanent loss.
Do I have to file Form 8606? Yes, for every year with a non-deductible contribution or a conversion. It is the only record of your basis. Without it there is nothing establishing that any part of a conversion was tax-free.
Does an inherited IRA count? An IRA inherited from someone other than a spouse is not aggregated with your own for this purpose. It follows its own rules, which are the subject of a separate article.
Should I convert immediately after contributing? Generally yes. Any earnings in between are taxable, and more importantly, a long gap risks the contribution being forgotten — leaving exactly the pre-tax-adjacent balance that complicates the following year.
What to do next
Two facts settle this before any arithmetic: whether you hold any pre-tax IRA money at any custodian, and whether your employer plan accepts incoming rollovers. If the first is no, you are clear. If the first is yes and the second is yes, you have a fix.
- Backdoor Roth calculator — the pro-rata fraction, the taxable amount, and what the pre-tax balance is costing you
- Retirement contribution limits — what you can put in this year, and where the catch-up rules bite
- Roth vs. traditional calculator — the underlying rate comparison every Roth decision reduces to
- The Retirement Withdrawal Order Playbook — what a larger Roth balance is actually worth later