Backdoor Roth and the Pro-Rata Rule: The Tax You May Not Expect
The backdoor Roth IRA is the standard workaround for high earners who can’t contribute to a Roth directly: put after-tax money in a traditional IRA, then convert it. Done in isolation it is tax-free. But if you already have any pre-tax money in traditional IRAs — including a rolled-over 401(k) — the pro-rata rule makes a slice of every conversion taxable, and that slice is often bigger than people expect.
The rule in one sentence
Every conversion is taxable in proportion to your total traditional IRA balance — you cannot convert only the after-tax dollars, no matter which account they sit in.
The math that surprises people
Say your traditional IRAs hold $100,000 total, of which $20,000 is after-tax basis (the money you put in for the backdoor):
| Item | Amount |
|---|---|
| After-tax basis / total balance | 20% |
| Convert $10,000 | $2,000 tax-free, $8,000 taxable |
| Tax at 24% | $1,920 |
That $1,920 is the hidden cost of the “free” backdoor — and it recurs every year you convert while the pre-tax balance remains. The pro-rata calculator runs your exact numbers.
What counts as pre-tax IRA money?
- Counts: rollover IRAs, SEP IRAs, SIMPLE IRAs, deductible contributions, and the earnings in your accounts.
- Doesn’t count: 401(k)s, 403(b)s, 457(b)s, and TSPs you hold with an employer.
That last line is the fix: roll your pre-tax IRAs into a 401(k), leaving only after-tax basis in the traditional IRA. Now the basis equals the balance, every conversion is tax-free, and the backdoor works as advertised.
Why the pro-rata rule exists
Without it, anyone with pre-tax IRA money could move it to a Roth tax-free by labeling the conversion as “after-tax.” The IRS blocks that by averaging across the whole pool — which is why the paperwork (Form 8606, filed every year you make a non-deductible contribution or conversion) matters so much.
The alternatives when you can’t roll over
If your 401(k) doesn’t accept rollovers, the options are: convert everything now (pay tax once, then all future conversions are clean), stop doing backdoors and use a taxable account instead, or check whether your employer offers after-tax 401(k) contributions with an in-plan Roth conversion — the mega backdoor — which avoids the pro-rata rule entirely.
Run the numbers
See your taxable percentage with the pro-rata calculator, the full backdoor setup with the backdoor Roth calculator, and the conversion tax with the Roth conversion tax calculator. High earners should also check the mega backdoor calculator for the version that sidesteps the rule.