Roth Conversions and the Pro-Rata Rule for IRA Basis

The pro-rata rule matters when an IRA owner has both nondeductible contributions and pretax IRA money. A taxpayer may think, “I will convert only the nondeductible account.” Federal reporting generally looks across the owner’s relevant traditional IRA balances, rather than tracing the conversion to a hand-picked account.

Basis and pretax money are pooled

Nondeductible contributions create basis: amounts that have already been included in income and should not be taxed again when distributed. Deductible contributions and untaxed investment growth are generally pretax. Form 8606 tracks basis and applies a fraction to distributions and conversions using the year-end value of relevant IRAs and other amounts specified in its instructions.

A simplified example

Suppose an owner has $20,000 of documented after-tax basis and $80,000 of pretax traditional IRA value, then converts $10,000 during the year and has no other relevant distributions or year-end adjustments. The conversion is not automatically all basis: a simplified 20% basis ratio would make about $2,000 nontaxable and $8,000 taxable. The actual Form 8606 calculation uses the applicable year’s form and all relevant figures.

Which accounts are generally aggregated

The Form 8606 instructions generally require aggregation of traditional, SEP, and SIMPLE IRAs owned by the same individual for the calculation. Employer plans such as a 401(k) are not simply added to that IRA pool, although separate rollover and distribution rules may apply. Account titling, ownership, inherited status, and transaction timing can change the analysis.

Records make the basis usable

A taxpayer who made nondeductible contributions should retain prior Forms 8606 and supporting contribution records. Basis is not a special cash balance that must remain in one account. If the form was omitted in an earlier year, the taxpayer may need to correct prior reporting rather than assume the basis disappeared or reconstruct it from a current statement alone.

Planning implications

• Estimate the taxable portion before initiating a conversion; a conversion can create current ordinary income.
• Check the year-end IRA value and all distributions that enter the Form 8606 calculation.
• Do not assume transferring pretax IRA assets to an employer plan is available; plan acceptance and rollover rules control.
• Review withholding, estimated tax, and the effect of conversion income on other tax items.

Work through Form 8606 components

For a conversion year, Form 8606 generally determines the nontaxable portion by combining the taxpayer’s basis in traditional IRAs with distributions and conversions, then comparing it with the aggregate year-end value of relevant traditional IRAs plus outstanding rollovers. Traditional, SEP, and SIMPLE IRAs are generally included in the calculation. Roth IRAs and employer 401(k) balances are not part of this IRA aggregation.

Example: a taxpayer has $7,000 of nondeductible basis, converts $7,000, and has $93,000 in pretax traditional/SEP/SIMPLE IRA value on December 31. The conversion is not treated as entirely tax-free; basis is allocated proportionally across the total IRA pool under Form 8606. The exact tax-free amount depends on all distributions and year-end values.

A taxpayer may consider rolling pretax IRA assets into an eligible employer plan before year-end if the plan accepts rollovers and the transaction is permitted. This can change the year-end IRA pool, but it is not a universal fix: plan fees, investment choices, creditor protection, deadlines, and rollover eligibility matter. Confirm the plan accepts the money before moving assets.

Keep prior Forms 8606, nondeductible contribution confirmations, rollover records, and December 31 statements. Missing basis can cause the taxpayer to pay tax twice. A contribution made for a year after December 31 may need separate timing treatment on Form 8606.

The rule applies per taxpayer, not per household. Spouses file separate Forms 8606 and do not combine IRA basis or balances. An inherited IRA has distinct ownership and basis rules; do not include it automatically in the beneficiary’s own conversion calculation.

Check current-year Form 8606 instructions because lines and rules can change. A conversion is generally taxable to the extent pretax amounts are included, and the 10% early distribution tax generally does not apply to a conversion itself, though later Roth withdrawals have their own rules.

Key takeaway

The taxpayer’s IRA basis is allocated across relevant distributions and conversions under Form 8606. A conversion from a separate account is not automatically tax-free just because that account contains nondeductible contributions.