IRA Rollover Basis Rules After Conversion

When money moves from a 401(k) or another employer-sponsored plan into a Traditional IRA, the transaction is called a rollover. That rollover carries with it a tax characteristic — called basis — that reflects whether the dollars being moved have already been subject to federal income tax. Basis is not a preference or an election; it is a factual record of after-tax contributions embedded in an account, and the Internal Revenue Code requires that record to be maintained and reported separately from pre-tax amounts.

The complexity sharpens when a Roth conversion follows the rollover. A conversion is a taxable event that moves money from a Traditional IRA (or a pre-tax employer plan) into a Roth IRA. After that transaction, the same dollars now carry a different kind of basis — Roth basis — which is governed by a distinct set of ordering and tracking rules. The two basis regimes, Traditional IRA basis and Roth conversion basis, operate under different sections of the tax code and produce different consequences at distribution.

How Basis Accumulates and Moves Through a Rollover and Conversion

Traditional IRA basis arises from nondeductible contributions — amounts that were contributed to a Traditional IRA without a deduction being claimed on the contributor's federal return. Under IRC § 408(o), the account owner is required to file IRS Form 8606 for any year in which a nondeductible contribution is made. Form 8606 is also the mechanism by which cumulative basis is tracked across years; each filing adds to or draws down the running total.

When a 401(k) is rolled over into a Traditional IRA, the mechanics of that transfer are described in detail in how a 401(k) rollover to an IRA works. Pre-tax dollars from a 401(k) — which represent contributions that were excluded from the employee's gross income — arrive in the Traditional IRA without creating basis. They are fully pre-tax. After-tax dollars inside a 401(k), however, do carry basis. If those after-tax 401(k) dollars are rolled into a Traditional IRA, they add to the IRA's existing basis and must be reported on Form 8606 in the year of the rollover. If those same after-tax dollars are instead rolled directly into a Roth IRA, they arrive as Roth basis without creating a taxable conversion event, because they were already taxed.

A Roth conversion occurs when pre-tax or mixed-basis Traditional IRA funds are moved into a Roth IRA. The pre-tax portion of the converted amount is included in gross income in the year of conversion. The after-tax portion — the Traditional IRA basis — is not taxed again. This is the pro-rata rule: when a Traditional IRA contains both pre-tax and after-tax amounts, every conversion (and every distribution) is treated as coming proportionally from both pools. The ratio is calculated on Form 8606 using the aggregate value of all Traditional IRAs the account owner holds at year-end, not just the IRA from which the conversion was taken. A person who holds multiple Traditional IRAs cannot isolate basis in a single account to convert only the after-tax portion tax-free.

Once converted, those dollars become Roth IRA funds. The converted amount is recorded as a Roth conversion contribution, which carries its own five-year holding requirement before earnings can be distributed tax-free. The basis tracking rules at Roth IRA withdrawal apply a specific ordering: regular Roth contributions come out first, then conversion amounts in the order they were made (oldest first), and then earnings last. Each conversion layer carries its own five-year clock for the 10% early distribution penalty, independent of the account's overall age.

Capital gains as a category do not apply to Traditional IRA distributions. Growth inside a Traditional IRA — whether from dividends, interest, or appreciation — is not characterized as capital gain when distributed. It is ordinary income. This is a fundamental feature of the tax-deferred structure: the tax code does not preserve the character of gains inside the account. All pre-tax distributions from a Traditional IRA are taxed as ordinary income in the year received, regardless of what generated the growth.

Who Tracks and Administers Basis at Each Stage

The IRS is the regulatory authority over Form 8606 and the basis-tracking rules under IRC § 408(o). The form is filed annually with the account owner's federal income tax return whenever a nondeductible contribution is made, a conversion occurs, or a distribution is taken from a Traditional IRA that contains basis. Failure to file Form 8606 in a year when it is required can result in a $50 penalty, and the absence of historical filings creates a documentation gap that is the account owner's burden to reconstruct.

The IRA custodian — typically a brokerage or bank acting in that administrative role — is responsible for issuing IRS Form 5498, which reports contributions and rollovers received, and IRS Form 1099-R, which reports distributions and conversions made. The custodian does not track basis on the account owner's behalf. Form 1099-R will show the gross distribution amount and a code indicating whether the transaction was a normal distribution, an early distribution, or a conversion; it will not show the taxable versus nontaxable split. That calculation belongs to the account owner through Form 8606.

For rollovers originating from employer-sponsored plans, the plan administrator — the entity that operates the 401(k) or 403(b) — is required to report the distributable amount and its character on Form 1099-R. The plan administrator must separately identify after-tax contributions in Box 5 of that form. That figure becomes the starting point for determining whether basis arrives in the receiving IRA. The plan administrator's responsibility ends at the point of distribution; the ongoing tracking obligation transfers to the IRA custodian's records and, more precisely, to the account owner's Form 8606 history.

Where Basis Rules Break Down or Produce Unexpected Results

The pro-rata rule is the most common source of unexpected tax results in IRA conversions. An account owner who intends to convert only after-tax (basis) dollars to avoid taxation may not do so selectively if that owner also holds pre-tax Traditional IRA balances. The IRS aggregates all Traditional IRA balances — including SEP IRAs and SIMPLE IRAs — to compute the taxable fraction of any conversion. A large pre-tax IRA balance held at a different custodian will dilute the basis percentage and increase the taxable portion of a conversion, even if the conversion is drawn entirely from the account that holds the basis.

Missing Form 8606 filings create a compounding problem. If basis was established years ago through nondeductible contributions but the forms were not filed, the account owner faces a reconstruction challenge. The IRS does allow late filings of Form 8606, but the documentation burden falls on the account owner to demonstrate the contributions were nondeductible. Without records, the entire IRA balance may be treated as pre-tax at distribution, resulting in taxation of amounts that were already taxed at contribution.

Roth conversion basis and the five-year rule interact in ways that are frequently misread. There is not one five-year clock for a Roth IRA — there are two distinct five-year requirements. The first governs whether earnings are tax-free at distribution and runs from the year of the first Roth IRA contribution of any kind. The second applies separately to each conversion layer and governs whether the converted amount is subject to the 10% early distribution penalty before age 59½. A conversion made at age 57 starts its own five-year penalty clock even if the Roth IRA itself is more than five years old. The taxation mechanics of Roth conversions at distribution follow these layered rules precisely.

Rollovers that mix after-tax and pre-tax 401(k) dollars into the same Traditional IRA can complicate future conversions in ways that were not anticipated at the time of the job change. Once after-tax 401(k) dollars are deposited into a Traditional IRA alongside pre-tax amounts, the pro-rata rule applies to all future conversions and distributions from that IRA. Keeping those amounts separate — by rolling after-tax dollars directly to a Roth IRA at the time of the plan distribution — avoids this aggregation problem, but that decision point exists only at the time of the original rollover.

Medicare has no rollover mechanism of its own. Medicare Part A and Part B are federal health insurance programs, not savings accounts, and there is no concept of a balance that rolls over from year to year in the way IRA funds do. Unused Medicare benefits do not accumulate, transfer between accounts, or carry forward. The term "rollover" does not apply to Medicare coverage in any technical sense.

What Form 8606 and Account Statements Actually Show at This Stage

Form 8606 is the authoritative record of Traditional IRA basis. Part I of the form calculates the nondeductible contribution for the current year and adds it to the prior year's total basis carried forward from the previous filing. Part II handles Roth conversions, computing the taxable and nontaxable portions using the pro-rata formula. Part III covers distributions from Traditional IRAs that contain basis. The cumulative basis figure on line 14 of Part I is carried forward to the next year's filing; it is the running total that must be preserved across decades of account activity.

IRA account statements issued by a custodian show the current market value of the account, transaction history, and contribution and distribution activity for the period. They do not show basis. The statement will not distinguish between pre-tax and after-tax dollars within the account, nor will it display the taxable versus nontaxable fraction of any future distribution. A statement showing a $200,000 Traditional IRA balance provides no information about how much of that balance represents basis and how much is pre-tax.

Form 1099-R, issued after a distribution or conversion, shows the gross amount distributed in Box 1, the taxable amount in Box 2a (which the custodian may leave blank or mark as undetermined if basis is involved), and a distribution code in Box 7 that identifies the nature of the transaction. For conversions to a Roth IRA, the code will indicate a conversion, but the taxable amount calculation still requires the account owner to complete Form 8606. The custodian's 1099-R is an input to that calculation, not its output.

Form 5498 confirms that a rollover was received and records the fair market value of the IRA at year-end. This year-end value is the figure used in the pro-rata denominator on Form 8606, making it a critical number for any year in which a conversion or distribution from a basis-containing IRA occurs.

The basis rules that apply after an IRA rollover and conversion reflect a system designed to prevent double taxation of after-tax dollars — but that system depends entirely on documentation that the account owner, not the custodian or the IRS, is responsible for maintaining across the full life of the account.

Note: This explains how a retirement system works. It is not financial, tax, or legal advice, it is not specific to any individual's retirement, and it is not a substitute for a licensed financial, tax, or legal professional. Rules, ages, and dollar limits change by year — check the cited sources.