What Is Basis in a Roth Conversion? Form 8606 and the Pro-Rata Rule Explained
The direct answer
The direct answer
In Roth conversion searches, “basis” usually means previously taxed money in Traditional IRAs—not the balance of the Roth account receiving the conversion. It generally comes from nondeductible Traditional IRA contributions and certain after-tax amounts rolled into an IRA, reduced by prior nontaxable distributions. Valid basis can make part of a conversion nontaxable. However, it normally is not assigned only to the particular IRA being converted. Form 8606 generally combines the owner’s applicable Traditional, SEP, and SIMPLE IRA basis, year-end balances, distributions, and conversions to divide the transaction proportionally between taxable and nontaxable amounts. This proportional treatment is commonly called the pro-rata rule. [1] [2] [3]
IRA basis can make part of a Roth conversion nontaxable, but Form 8606 generally applies one proportional calculation across the owner’s applicable Traditional, SEP, and SIMPLE IRAs.
Written by Agent Roth Editorial Team
Publication does not imply tax-professional review. Read the editorial policy and corrections policy.
Introduction
IRA basis is confusing because two dollars in the same account can have different tax histories. A deductible contribution and its earnings generally have not yet been taxed. A properly reported nondeductible contribution has already been taxed, while the investment growth on that contribution generally remains tax-deferred.
The account statement does not perform that tax bookkeeping. One person may also have IRAs at several custodians, including a rollover IRA, Traditional SEP IRA, or Traditional SIMPLE IRA. Form 8606 tracks basis and allocates it across the applicable annual activity. Basis may reduce the taxable portion of a conversion, but it does not automatically make the entire conversion tax-free.
What does “basis” mean in a Traditional IRA?
IRA basis is money inside the applicable Traditional IRAs that has already been subjected to income tax and therefore generally should not be taxed again when it is properly reported. IRS instructions define Traditional IRA basis as nondeductible contributions plus nontaxable amounts rolled into those IRAs, minus prior nontaxable distributions and applicable adjustments. [1] [3]
- Nondeductible Traditional IRA contributions
- Certain after-tax amounts rolled from an eligible retirement plan into a Traditional IRA
- Basis carried forward from prior Forms 8606
- Minus basis already returned through previous nontaxable distributions
Searchers sometimes say “Roth conversion basis,” but this article concerns Traditional IRA basis before conversion. Regular Roth IRA contribution basis, amounts from earlier Roth conversions, Roth five-year rules, employer-plan after-tax money, and cost basis in a taxable brokerage account are separate concepts.
How does basis affect a Roth conversion?
The gross amount transferred to Roth is not always the taxable amount. The portion attributable to valid basis may be nontaxable; previously untaxed contributions, earnings, and gains are generally taxable. Basis does not create a new deduction in the conversion year. It helps prevent qualifying after-tax dollars from being taxed a second time. [2] [3]
| IRA composition | General conversion treatment |
|---|---|
| Only deductible contributions and earnings | The conversion is generally taxable. |
| Nondeductible contributions plus earnings | The conversion is generally partly taxable and partly nontaxable. |
| Only valid after-tax basis and no other applicable IRA value | The conversion may be largely or fully nontaxable, subject to the exact Form 8606 calculation. |
| Mixed accounts at several custodians | Applicable accounts are generally aggregated for the calculation. |
For the broader return-level context, read How Are Roth Conversions Taxed? The Tax-Bracket & IRMAA Calculator accepts an externally estimated taxable conversion amount; it does not replace Form 8606.
What is the pro-rata rule?
The pro-rata rule is the common name for the proportional basis calculation. It generally looks across the owner’s applicable Traditional IRAs rather than treating one selected account as though it contains only basis. The year’s basis is allocated between taxable and nontaxable amounts using Form 8606. [2] [3]
Nontaxable percentage = total applicable IRA basis ÷ (year-end applicable IRA value + applicable distributions and conversions)
Nontaxable conversion portion = conversion amount × nontaxable percentage
Taxable conversion portion = conversion amount − nontaxable conversion portion
This is a conceptual summary, not a substitute for Form 8606. Current-year nondeductible contributions, prior basis, year-end values, distributions, conversions, outstanding rollovers, and adjustments can affect the official calculation.
Which accounts count under the IRA aggregation rule?
| Account | Generally included? |
|---|---|
| Traditional IRA | Yes |
| Traditional SEP IRA | Generally yes |
| Traditional SIMPLE IRA | Generally yes |
| Roth IRA | No |
| Roth SEP or Roth SIMPLE IRA | Not part of the Traditional IRA denominator |
| 401(k) | Generally not while held in the employer plan |
| 403(b) | Generally not while held in the employer plan |
| 457(b) | Generally not while held in the employer plan |
| Inherited IRA | Separate rules may apply; do not casually combine it with the owner’s personal calculation |
The current Form 8606 instructions generally define “Traditional IRA” to include Traditional SEP and Traditional SIMPLE IRAs unless stated otherwise. Roth IRAs are treated separately. [3]
Moving employer-plan money into a Traditional IRA can change the IRA values relevant to a later conversion. That does not make a rollover automatically favorable or unfavorable. Plan acceptance, rollover eligibility, fees, investments, creditor protection, timing, and reporting all require separate review. [5]
Are IRAs at different banks or custodians combined?
Generally, yes, for one owner’s applicable Traditional, SEP, and SIMPLE IRAs. Opening a separate IRA for a nondeductible contribution does not automatically isolate its after-tax dollars from other applicable pre-tax IRA assets.
Can I convert only the after-tax dollars?
Generally, a person cannot simply designate that only basis is being converted while leaving all pre-tax IRA dollars untouched when the aggregation and pro-rata rules apply. The taxable and nontaxable portions are usually allocated proportionally.
Specialized facts can differ. Employer-plan after-tax amounts have their own allocation and rollover rules, inherited accounts require separate analysis, and an employer plan may or may not accept an incoming rollover. This guide explains the general IRA rule; it does not provide individualized transaction instructions.
A partial conversion can still be useful for scenario modeling, but it does not bypass basis rules. See How Partial Roth Conversions Work.
A simple pro-rata example
Assume one owner has $100,000 of total Traditional, SEP, and SIMPLE IRA value before accounting for a $25,000 conversion. The owner has $20,000 of valid IRA basis, ends the year with $75,000 in the applicable IRAs, and has no other distributions or outstanding rollovers.
| Step | Illustrative calculation | Result |
|---|---|---|
| Simplified denominator | $75,000 year-end value + $25,000 conversion | $100,000 |
| Nontaxable percentage | $20,000 basis ÷ $100,000 denominator | 20% |
| Nontaxable conversion portion | $25,000 conversion × 20% | $5,000 |
| Taxable conversion portion | $25,000 conversion − $5,000 | $20,000 |
| Remaining basis | $20,000 starting basis − $5,000 used | $15,000 |
Why does the December 31 IRA balance matter?
Form 8606 generally asks for the total December 31 value of all applicable Traditional IRAs and includes certain outstanding rollovers. The calculation is not based only on the account balance when the conversion occurred. [2] [3]
- A later same-year rollover into a Traditional IRA may matter.
- Year-end account values can affect the percentage.
- Other conversions and distributions during the year can enter the calculation.
- An early-year estimate may change when the year-end facts become known.
That timing point is a reason to verify the complete annual record, not an instruction to move assets solely to manipulate a tax result.
What is Form 8606?
The IRS says Form 8606 is used to report nondeductible Traditional IRA contributions, certain distributions when the owner has basis, conversions from Traditional, SEP, or SIMPLE IRAs to Roth IRAs, and certain Roth IRA distributions. [4]
Starting record
Bring forward prior basis
Prior Forms 8606 and supporting records establish basis carried into the year.
Current year
Add contributions and annual activity
Current nondeductible contributions, distributions, conversions, year-end values, and adjustments enter the applicable calculation.
Year end
Report the split and carry forward basis
The form determines taxable and nontaxable amounts and the basis remaining for a later year.
What happens if I never filed Form 8606?
Nondeductible contributions generally should have been reported. IRS guidance states that distributions may be treated as taxable unless satisfactory evidence establishes the nondeductible contributions; a failure-to-file penalty may also apply unless reasonable cause exists. The remedy depends on the records and years involved, so amended or late-filed forms should be evaluated with a qualified tax professional rather than assumed. [1] [3]
- Prior Forms 8606
- Forms 5498
- Forms 1099-R
- IRA contribution confirmations
- Prior tax returns
- Rollover documentation
- Year-end IRA statements
- Records of prior distributions and conversions
Form 5498 can support contribution and account-value records, but it is not a complete replacement for the taxpayer’s Form 8606 history. The IRS instructs taxpayers to retain Forms 8606 and supporting records until all distributions have been made. [3]
Is IRA basis shared between spouses?
No. IRA ownership and Form 8606 reporting are individual. Spouses may file one joint income-tax return, but each spouse owns separate IRAs and tracks separate basis. If both spouses are required to file Form 8606, the IRS instructs them to file a separate form for each spouse. [3]
Is Traditional IRA basis the same as Roth IRA basis?
No. Today’s article primarily concerns Traditional IRA basis before conversion. Roth IRAs use separate ordering and distribution rules.
| Term | What it generally tracks |
|---|---|
| Traditional IRA basis | Previously taxed amounts in applicable Traditional IRAs used to determine taxable and nontaxable distributions or conversions. |
| Regular Roth contribution basis | Regular Roth IRA contributions made with after-tax dollars. |
| Roth conversion basis | Amounts previously converted or rolled into Roth IRAs, tracked for Roth ordering and potential additional-tax rules. |
| Roth earnings | Growth inside Roth accounts, subject to separate qualified-distribution rules. |
The contribution and conversion rules are compared in Roth Conversion vs. Roth Contribution.
Does basis make a backdoor Roth contribution tax-free?
A nondeductible Traditional IRA contribution creates basis. A later conversion may be largely nontaxable when the owner has no other applicable pre-tax Traditional, SEP, or SIMPLE IRA value and little or no growth. Other applicable pre-tax IRA balances can cause the pro-rata calculation to make part of the conversion taxable.
Contribution eligibility, annual limits, timing, account values, growth, and reporting still matter. The transaction is not automatically tax-free, and this article is not a complete backdoor Roth guide.
Does basis affect Medicare IRMAA or Social Security taxation?
Only the taxable portion of a conversion generally increases gross income from the conversion. Valid basis may reduce that portion, but other income and adjustments still matter. Added taxable income may affect AGI, which may in turn affect Medicare IRMAA and the taxable portion of Social Security benefits.
Review the separate guides to Roth conversions and Medicare IRMAA and Roth conversions and Social Security taxation. Basis can reduce the taxable conversion amount; it does not erase every possible secondary interaction.
Common IRA-basis mistakes
- Assuming basis belongs only to one IRA instead of the owner-level calculation.
- Forgetting a rollover IRA held at another custodian.
- Ignoring Traditional SEP or Traditional SIMPLE IRA balances.
- Using only the conversion-date balance instead of reviewing year-end values.
- Confusing regular Roth contribution basis with Traditional IRA basis.
- Failing to carry forward basis from a prior Form 8606.
- Assuming one nondeductible contribution makes the entire conversion tax-free.
- Forgetting after-tax employer-plan money previously rolled into an IRA.
- Combining one spouse’s basis or IRA value with the other spouse’s.
- Treating Form 5498 as a complete substitute for the Form 8606 record.
Questions to review before converting an IRA with basis
- Have I ever made a nondeductible Traditional IRA contribution?
- Do I have prior Forms 8606?
- Do I own any rollover IRAs?
- Do I own any Traditional SEP or SIMPLE IRAs?
- Did I roll after-tax employer-plan money into an IRA?
- What are the expected December 31 IRA values?
- Will I make any other IRA distributions or conversions this year?
- Are any rollovers outstanding?
- What portion of the proposed conversion is expected to be taxable?
- Could that taxable amount affect my tax bracket?
- Could it affect Medicare IRMAA?
- Could it affect Social Security taxation?
- Are my spouse’s accounts and basis being kept separate from mine?
- Have I reviewed the final tax-year Form 8606 instructions?
Frequently asked questions
What is basis in a Roth conversion?
It usually means previously taxed money in applicable Traditional IRAs before conversion. Valid basis can make a proportional part of the conversion nontaxable.
What creates basis in a Traditional IRA?
Nondeductible contributions and certain after-tax amounts rolled into a Traditional IRA can create basis, reduced by prior nontaxable distributions and adjustments.
Does basis reduce the taxable Roth conversion amount?
Yes, valid and documented basis may reduce the taxable portion. It does not create a new deduction or automatically make the whole conversion tax-free.
What is the pro-rata rule?
It is the proportional allocation that generally divides applicable annual IRA distributions and conversions between taxable pre-tax money and nontaxable basis.
Are all of my IRAs combined for the pro-rata rule?
The owner’s applicable Traditional IRAs are generally combined. Roth IRAs and ordinary employer plans are generally outside this specific IRA denominator.
Do SEP IRAs count in the pro-rata calculation?
Traditional SEP IRAs are generally included because the Form 8606 instructions normally include them in references to Traditional IRAs.
Do SIMPLE IRAs count?
Traditional SIMPLE IRAs are generally included in the applicable Traditional IRA calculation unless an instruction specifically provides otherwise.
Does a 401(k) count in the IRA pro-rata calculation?
Generally not while the assets remain inside the employer plan. A later rollover into a Traditional IRA can change the IRA values relevant to the calculation.
Can I convert only my nondeductible contribution?
Generally, a person cannot simply select only basis when other applicable pre-tax IRA money exists. Form 8606 usually allocates the transaction proportionally.
Does a separate IRA at another custodian isolate the basis?
No. Separate statements or custodians generally do not override the owner-level aggregation of applicable Traditional, SEP, and SIMPLE IRAs.
Why does the December 31 balance matter?
Form 8606 generally uses the year-end value of all applicable Traditional IRAs, along with relevant annual distributions, conversions, and outstanding rollovers.
What form reports Roth conversion basis?
Form 8606 generally reports nondeductible Traditional IRA contributions, basis-related distributions, and conversions from Traditional, SEP, or SIMPLE IRAs to Roth IRAs.
What if I forgot to file Form 8606?
Records may be needed to establish basis, and a late or amended filing may sometimes be appropriate. The correct response depends on the years and facts and deserves tax-professional review.
Do spouses share IRA basis?
No. Each spouse owns separate IRAs, tracks separate basis, and generally files a separate Form 8606 when both are required to file.
Is Roth IRA contribution basis the same thing?
No. Regular Roth contribution basis and prior Roth conversion amounts follow separate Roth distribution-ordering rules from Traditional IRA basis before conversion.
Can a backdoor Roth conversion be partly taxable?
Yes. Other applicable pre-tax Traditional, SEP, or SIMPLE IRA values and any growth can make part of the conversion taxable under the pro-rata calculation.
Is investment growth on a nondeductible contribution basis?
Generally no. The nondeductible contribution may create basis, while its earnings and gains generally remain tax-deferred and may be taxable when distributed or converted.
How much of my Roth conversion is taxable?
That depends on valid basis, all applicable IRA values, distributions, conversions, outstanding rollovers, and other Form 8606 inputs. The gross transfer alone may not be the taxable amount.
Key takeaways
- IRA basis generally represents previously taxed Traditional IRA money.
- Valid basis may make part of a Roth conversion nontaxable.
- Applicable Traditional, SEP, and SIMPLE IRAs are generally aggregated.
- A person usually cannot select only basis from one IRA.
- December 31 balances and other annual transactions can affect the calculation.
- Form 8606 is essential for reporting and carrying basis forward.
Primary sources
Rules and thresholds may change. These official federal sources support the concepts discussed above; their inclusion does not imply government endorsement of Agent Roth.
- [1]IRS Publication 590-A: Contributions to Individual Retirement Arrangements (IRAs)
Internal Revenue Service · Verified July 28, 2026Nondeductible contributions, IRA cost basis, earnings, recordkeeping, conversions, and failure-to-report treatment.
- [2]IRS Publication 590-B: Distributions from Individual Retirement Arrangements (IRAs)
Internal Revenue Service · Verified July 28, 2026Partly taxable distributions, proportional basis allocation, annual values, conversions, and remaining basis.
- [3]IRS Instructions for Form 8606
Internal Revenue Service · Verified July 28, 2026Basis definition, Traditional SEP/SIMPLE inclusion, December 31 values, records, penalties, and separate spouse forms.
- [4]IRS About Form 8606, Nondeductible IRAs
Internal Revenue Service · Verified July 28, 2026Official purposes, current revision, and reporting categories for Form 8606.
- [5]IRS Rollovers of Retirement Plan and IRA Distributions
Internal Revenue Service · Verified July 28, 2026Employer-plan and IRA rollover methods, eligibility, timing, and direct-transfer distinctions.
This article is general education and an illustrative explanation, not tax, legal, investment, or financial advice. Form 8606 results depend on complete records and the final form for the applicable tax year. Verify account values, basis, transactions, and filing requirements with appropriately qualified tax professionals before acting. Read the educational and financial disclosures.