Backdoor Roth IRA: How It Works and the Pro-Rata Trap
A step-by-step guide to the backdoor Roth IRA — income limits, the pro-rata rule, Form 8606, the mega backdoor Roth, and when it is not worth doing.
What a backdoor Roth actually is
There is no special "backdoor Roth" account. It is two ordinary transactions done in sequence: a non-deductible contribution to a Traditional IRA, followed by a conversion of that money to a Roth IRA. Direct Roth contributions phase out at higher modified adjusted gross income, but conversions have no income limit — so the two-step route gets the same dollars into a Roth for high earners.
The four steps
- Contribute non-deductibly. Put the annual IRA limit into a Traditional IRA and do not claim a deduction. This creates basis.
- Let it sit briefly, then convert. Convert the full balance to your Roth IRA. Any earnings between contribution and conversion are taxable ordinary income, which is why most people convert quickly.
- Invest inside the Roth. The conversion itself does not invest the money; leaving it in cash is the most common execution error.
- File Form 8606. Both the non-deductible contribution and the conversion are reported. Skip it and the IRS treats the whole conversion as taxable.
The pro-rata rule — the part that trips people up
The IRS treats all of your Traditional, SEP and SIMPLE IRAs as one pool on December 31 of the conversion year. You cannot convert "just the after-tax dollars." The taxable share of any conversion equals the pre-tax portion of that combined balance.
Example: you hold $93,000 of pre-tax rollover IRA money and add a $7,000 non-deductible contribution. Your basis is 7% of the $100,000 pool, so converting $7,000 produces roughly $6,510 of taxable income — not zero. The remaining basis stays trapped, pro-rating every future conversion.
The standard fix is to empty the pre-tax IRAs before December 31 by rolling them into an employer 401(k) that accepts roll-ins. 401(k) balances are excluded from the pro-rata calculation. Note that Roth IRAs and your spouse's IRAs are also excluded — the test is per person.
The mega backdoor Roth
A different maneuver with a similar name: if your 401(k) plan allows after-tax (not Roth) contributions plus in-plan Roth conversions or in-service withdrawals, you can move far more than the IRA limit into Roth space each year, up to the overall defined-contribution limit less employee deferrals and employer match. It depends entirely on plan documents — check for "after-tax contributions" and "in-plan Roth rollover" before counting on it.
When it is not worth doing
- You have a large pre-tax IRA you cannot roll into a 401(k) — pro-rata eats the benefit.
- You are in a peak-earning year and a straightforward pre-tax deferral saves more tax than the Roth's future benefit is worth.
- You will need the money within five years; converted amounts have their own five-year clock for penalty-free withdrawal before 59½.
- You are already near an IRMAA tier and the taxable portion would push you over it.
How it fits the larger plan
A backdoor Roth adds tax-free assets at the margin; the far bigger lever for most households is how much of the pre-tax balance gets converted during the low-income years between retirement and RMDs. Model both together rather than in isolation.
Educational modeling only. Bracketwise is not tax, legal or investment advice.