Backdoor Roth IRA Calculator: The Pro-Rata Rule That Taxes 93% of a "Clean" Conversion
A $7,000 backdoor Roth contribution is entirely tax-free — unless you have an old rollover IRA sitting in the background. A $93,000 pre-tax IRA balance turns that same conversion 93% taxable, no matter how carefully you did the contribution itself.
The rule that catches high earners off guard
We ran the classic teaching example through the calculator: a $7,000 nondeductible IRA contribution, converted to Roth the same year, with a $93,000 balance sitting in an old 401(k) rollover IRA in the background. The contribution itself was done correctly — nondeductible, reported on Form 8606. But the pro-rata rule doesn't care which account you're converting from. It taxes the conversion based on the ratio of after-tax basis to your total traditional/SEP/SIMPLE IRA balance across every account you own. Here, that ratio is 7% — meaning 93% of the "backdoor Roth" conversion is fully taxable, the opposite of what most people expect going in.
How the math works
Non-taxable percentage = total after-tax basis (this year's nondeductible contribution plus any carried-forward basis from prior years) ÷ total balance across all your traditional/SEP/SIMPLE IRAs. That percentage applies to any conversion you do, regardless of which specific account the money is converted from — the IRS treats all your traditional IRAs as one combined pool for this calculation.
What this tool doesn't model: the "reverse rollover" strategy (moving pre-tax IRA money INTO a 401(k) that accepts rollovers, which removes it from the pro-rata calculation entirely and is the standard fix for this problem), SEP/SIMPLE IRA-specific timing rules, and state tax treatment, which can differ from federal.
Math runs locally. Inputs never leave your browser.Source on github.
Where this catches people who thought they were fine
- You forgot about an old 401(k) you rolled into an IRA years ago.This is the single most common cause of an unexpectedly taxable backdoor conversion — a rollover IRA from a prior job, sitting untouched, that most people don't think of as relevant to a "new" backdoor Roth strategy.
- You have a SEP-IRA or SIMPLE IRA from self-employment income.These count as traditional IRA balances for the pro-rata calculation too, even though they feel like a separate "business" account.
- Your spouse's IRA doesn't affect your calculation (a common misconception in reverse).The pro-rata rule is calculated per individual, not per household — your spouse's pre-tax IRA balance doesn't pollute your own backdoor conversion, and vice versa.
- A 401(k)-to-IRA rollover mid-year changes your year-end balance.The calculation uses your year-end balance across all traditional IRAs — a rollover completed after your backdoor conversion but before December 31 still counts.
What to actually do with this number
- Check every traditional, SEP, and SIMPLE IRA you hold — including old rollovers you may have forgotten about.
- If you have a meaningful pre-tax balance, check whether your current employer's 401(k) accepts incoming rollovers from IRAs — moving that pre-tax money into the 401(k) removes it from the pro-rata calculation entirely.
- Complete the reverse rollover (if available) before December 31 of the year you plan to do the backdoor conversion — the calculation uses your year-end balance.
- File Form 8606 accurately every year you do this, tracking your cumulative basis — losing track of it is a common, costly mistake.
- If the numbers are complicated (multiple accounts, a recent job change), get a tax professional to verify before converting.