What is the pro-rata rule for backdoor Roth IRA conversions?

It's the IRS rule (IRC §408(d)(2), reported on Form 8606) that determines how much of any Roth conversion is taxable. Instead of looking only at the account you're converting, it uses the ratio of your total after-tax basis to your total balance across ALL traditional, SEP, and SIMPLE IRAs combined.

Why does an old 401(k) rollover IRA affect a new backdoor Roth conversion?

Because the IRS treats all your traditional IRAs as one combined pool for the pro-rata calculation, regardless of which specific account holds which dollars. A large pre-tax balance in an old rollover IRA dilutes the after-tax percentage of any new conversion, even if that new contribution itself was made correctly and reported as nondeductible.

How can I avoid the pro-rata rule when doing a backdoor Roth?

The standard fix is a 'reverse rollover' — moving your pre-tax traditional/SEP/SIMPLE IRA balance into a 401(k) or other employer plan that accepts incoming rollovers, before year-end. Once that pre-tax money is out of your IRA, it no longer counts toward the pro-rata calculation, and a subsequent backdoor conversion can be clean.

Does my spouse's IRA balance affect my backdoor Roth pro-rata calculation?

No. The pro-rata rule is calculated per individual, not per household. Your spouse's traditional IRA balance doesn't affect your calculation, and yours doesn't affect theirs.

A Forgotten $93K Rollover IRA Just Made Your 'Clean' Backdoor Roth 93% Taxable

We ran the textbook case through the calculator: a $7,000 nondeductible IRA contribution, converted to Roth the same year — done correctly, reported on Form 8606. In the background, a $93,000 balance sits in an old 401(k) rollover IRA from a previous job. The contribution itself wasn’t the problem. The pro-rata rule taxes 93% of the conversion anyway, because it doesn’t look at the new contribution in isolation — it looks at your total traditional IRA balance across every account you own.

The math, worked out

	Value
New nondeductible contribution	$7,000
Old rollover IRA balance (pre-tax)	$93,000
Total traditional IRA balance	$100,000
Non-taxable percentage	7% ($7,000 ÷ $100,000)
Taxable portion of the $7,000 conversion	$6,510 (93%)

The contribution was nondeductible and correctly reported — that part of the process worked exactly as intended. But the IRS pro-rata rule doesn’t ask “was this specific $7,000 already taxed?” It asks “what fraction of your total traditional IRA money, combined, has already been taxed?” With $93,000 of pre-tax money sitting in the mix, the answer is 7% — and that 7% applies to every dollar converted, not just the new contribution.

Why this is the most common backdoor Roth mistake

Most people executing a backdoor Roth for the first time focus entirely on the new contribution — making it nondeductible, converting it promptly, filing Form 8606 correctly. All of that can be done perfectly and still produce a mostly-taxable conversion, because the trap isn’t in the new steps — it’s in old accounts people forget are even relevant. A 401(k) rolled into an IRA years ago, a SEP-IRA from a stretch of self-employment, a SIMPLE IRA from an old employer — none of these feel connected to “doing a backdoor Roth this year,” but all of them count toward the same pro-rata pool.

Where this framework doesn’t apply

You have no other pre-tax IRA balance. If your only traditional IRA money is the new nondeductible contribution itself, the conversion is genuinely clean — 100% tax-free, exactly as the backdoor Roth strategy is supposed to work.
You’ve already done a reverse rollover. Moving pre-tax IRA money into a 401(k) or other employer plan that accepts incoming rollovers removes it from the pro-rata calculation entirely — this is the standard fix, not an edge case, and many people executing backdoor Roths do this specifically to enable a clean conversion.
Your pre-tax balance is in a Roth IRA or 401(k) already. Neither counts toward the pro-rata calculation — only traditional, SEP, and SIMPLE IRA balances matter.
You’re filing jointly but the balance is your spouse’s. The calculation is per individual, not per household — your spouse’s pre-tax IRA doesn’t affect your own backdoor conversion.

What to actually do

Before doing a backdoor Roth, check every traditional, SEP, and SIMPLE IRA you hold — including accounts you may have forgotten about from old jobs.
If you find a meaningful pre-tax balance, check whether your current employer’s 401(k) accepts incoming rollovers.
If it does, complete the reverse rollover before December 31 of the year you plan to convert — the pro-rata calculation uses your year-end balance.
Once the pre-tax money is out of your IRA, the backdoor conversion can proceed cleanly.
File Form 8606 accurately every year, and keep a running record of your basis — losing track of it over multiple years is its own common, costly mistake.

For the broader Roth-conversion strategy this maneuver is a special case of, see the Roth conversion ladder in gap years.

Open the Backdoor Roth IRA Calculator → and check your own IRA balances before converting.