A Roth conversion can look simple until you have both pre-tax and after-tax money in your traditional IRAs. The pro rata Roth conversion rule determines how much of a conversion is taxable when you have basis in your IRA.
Quick Answer: What Is the Pro Rata Rule for Roth Conversion?
The IRS generally treats your traditional, SEP, and SIMPLE IRAs as one pool when calculating a Roth conversion with basis. Your conversion is divided between taxable and nontaxable dollars based on your basis and the applicable IRA amounts included in the Form 8606 calculation.
The important detail is that you generally cannot tell the IRS, “I am converting only my after-tax dollars.” The calculation looks at the applicable IRA balances and activity for the year instead.
What Is the Pro Rata Rule for Roth Conversion?
The what is the pro rata rule for Roth conversion question has a straightforward answer: if you have both pre-tax and after-tax money in traditional IRAs, the IRS generally requires the conversion to contain a proportional share of each.
Your after-tax contributions create what the IRS calls – basis. Basis represents money that has already been taxed and should not be taxed again when it is distributed or converted, subject to the applicable rules.
The pro rata rule for Roth conversion prevents you from choosing only the after-tax dollars for conversion while leaving all the pre-tax money behind. Instead, Form 8606 calculates the nontaxable percentage using your basis and the applicable traditional IRA values, distributions, and Roth conversion amount.
For this purpose, “traditional IRA” includes traditional SEP IRAs and traditional SIMPLE IRAs. Roth IRAs are not included in this calculation.
That distinction matters most for people doing a backdoor Roth, people who have accumulated old traditional IRA balances, and anyone who has made nondeductible IRA contributions over multiple years.
The Roth Conversion Pro Rata Rule Formula
The Roth conversion pro rata rule is easier to understand when you see the calculation.
For a simplified example, the nontaxable percentage can be expressed as:
Nontaxable percentage = Total basis ÷ Applicable IRA calculation amount
On the current Form 8606, the IRS first determines the applicable basis, then considers the value of traditional IRAs at December 31, applicable outstanding rollovers, distributions, and the net amount converted to Roth IRAs. The resulting percentage is then applied to the conversion to determine the nontaxable portion.
Four numbers to gather
|
Input |
What it means |
| Total basis | Your after-tax basis in traditional IRAs, generally tracked through Form 8606 |
| Year-end IRA balance | The value of your traditional IRAs on December 31, plus applicable outstanding rollovers |
| Amount converted | The net amount converted from traditional IRAs to Roth IRAs during the year |
| Amount withdrawn | Other applicable distributions from traditional IRAs during the year |
The simplified formula is useful for understanding the concept, but your actual Form 8606 calculation can include more detail. In particular, the IRS calculation adds the December 31 IRA value, applicable distributions, and net Roth conversion amount in determining the denominator.
How do you calculate the pro rata rule?
Suppose you have $10,000 of basis and a $100,000 applicable IRA calculation amount. Your nontaxable percentage would be 10%.
If you converted $20,000, approximately $2,000 would be nontaxable and approximately $18,000 would be taxable, assuming the simplified facts above and no other adjustments.
That is why knowing your basis before starting a conversion is so important.
Which Accounts Count Toward the Roth IRA Pro Rata Rule?
The Roth IRA pro rata rule generally looks across your traditional IRA accounts rather than treating each account as a separate tax bucket. For Form 8606 purposes, traditional SEP and SIMPLE IRAs are included in the definition of traditional IRA.
|
Counted in the calculation |
Generally not counted |
| Traditional IRAs | 401(k) plans |
| SEP IRAs | 403(b) plans |
| SIMPLE IRAs | 457(b) plans |
| Applicable outstanding rollovers | Thrift Savings Plan (TSP) |
| Roth IRAs | |
| Your spouse’s separate IRAs |
A workplace retirement plan such as a 401(k) does not become part of your IRA calculation simply because you own it. However, if money is rolled from an employer plan into a traditional IRA, that IRA balance can become relevant to the calculation.
Your spouse’s IRA is also not combined with yours. Form 8606 is filed separately for each spouse when required.
Inherited IRAs can involve additional rules, especially when basis is involved. If an inherited IRA is part of your situation, confirm the treatment with a qualified tax professional before relying on a general calculation.
Pro-Rata Rule Backdoor Roth Example
The pro-rata rule backdoor roth issue is one of the most common situations where people encounter this calculation.
Consider this example:
- You make a $7,500 nondeductible contribution to a traditional IRA.
- You already have $100,000 of pre-tax money in traditional IRAs.
- You convert the $7,500 contribution to a Roth IRA.
- Your applicable traditional IRA value at year-end is $100,000, excluding the converted amount.
- Your total applicable amount for the simplified example is therefore $107,500.
The $7,500 contribution does not remain completely isolated for tax purposes. Your basis is $7,500, while the applicable IRA amount is $107,500.
Nontaxable percentage:
$7,500 ÷ $107,500 = approximately 6.98%
That means approximately:
|
Amount |
Approximate treatment |
|
Roth conversion |
$7,500 |
|
Nontaxable portion |
$523 |
| Taxable portion |
$6,977 |
So although you intended to convert the $7,500 of after-tax money, only about $523 of the conversion would be nontaxable under these simplified facts.
What happens in a clean backdoor Roth?
Now change one fact: you make the same $7,500 nondeductible contribution, but you have $0 of other traditional, SEP, or SIMPLE IRA money included in the calculation.
The applicable amount is $7,500, your basis is $7,500, and the nontaxable percentage is 100%.
In that simplified example, the entire $7,500 conversion is nontaxable.
This is why existing pre-tax IRA balances can make a backdoor Roth substantially more complicated.
Pro Rata Example for Pre-Retirees With Old IRA Basis
Now consider someone who has been making IRA contributions for years and has both basis and pre-tax money.
Suppose you have:
- $200,000 in a traditional IRA before the conversion
- $20,000 of documented basis
- A $50,000 Roth conversion
- $150,000 remaining in the traditional IRA at year-end
For this simplified example, the denominator is:
$150,000 remaining + $50,000 converted = $200,000
Your basis is $20,000.
Nontaxable percentage:
$20,000 ÷ $200,000 = 10%
Therefore:
|
Conversion |
Amount |
|
Total Roth conversion |
$50,000 |
|
Nontaxable portion |
$5,000 |
| Taxable portion |
$45,000 |
| Remaining basis |
$15,000 |
If the entire $45,000 taxable amount fell into a hypothetical 24% marginal federal tax bracket, the federal income tax attributable to that taxable portion would be approximately $10,800.
If the entire $50,000 were taxable instead, the hypothetical tax would be $12,000.
The difference illustrates why accurately documenting basis matters.
The remaining $15,000 of basis does not disappear. It generally carries forward and can be used in future distributions or conversions, with Form 8606 tracking the basis calculation.
Do You Even Have Basis? A Quick Check
Before worrying about the Roth pro rata rule, determine whether you actually have an IRA basis.
Use this three-step check:
- Have you ever made a nondeductible contribution to a traditional IRA?
- Did you file Form 8606 for the year in which you reported that nondeductible contribution?
- Do you still have traditional, SEP, or SIMPLE IRA money today?
If you have never made nondeductible contributions, you generally do not have basis from those contributions. If your IRA dollars are all pre-tax, a Roth conversion is generally taxable to the extent it consists of pre-tax amounts.
If you believe you have a basis but cannot find your old Form 8606 filings, do not simply assume the basis is gone. Review prior tax returns and supporting records with your tax professional.
Form 8606 is specifically used to report nondeductible traditional IRA contributions, track basis, and report certain IRA-to-Roth conversions.
How to Avoid the Pro Rata Rule Trap on a Backdoor Roth?
There are several ways people may approach the roth ira conversion pro rata rule, depending on their circumstances.
1. Roll eligible pre-tax IRA money into an employer plan
If your employer’s retirement plan accepts incoming rollovers, you may be able to move eligible traditional IRA money into that plan before December 31.
This can reduce the traditional IRA balance relevant to the year-end calculation. The plan must accept the rollover, the transaction must be eligible, and the rollover needs to be completed properly and on time.
For a self-employed person, a properly established solo 401(k) may sometimes be an option if the plan accepts eligible rollovers.
Potential benefit: You may reduce or eliminate the pre-tax IRA balance that complicates a backdoor Roth.
Potential drawback: Employer-plan rules vary, and moving money into a plan can affect your investment choices, fees, and overall retirement strategy.
2. Convert the pre-tax IRA money
Another option is to convert some or all of the traditional IRA balance to a Roth IRA.
Potential benefit: This can move pre-tax money into a Roth and potentially reduce the traditional IRA balance creating the pro rata issue.
Potential drawback: The taxable portion of the conversion increases your income for the year. That can affect your federal tax bill and potentially other income-based calculations.
3. Accept the partial tax-free share
You can also complete the conversion while accepting that only the applicable percentage will be nontaxable.
This may make sense when rolling money into an employer plan is unavailable or when converting the pre-tax IRA balance would create a larger tax bill than you want to recognize in one year.
The important point is to calculate the tax before completing the transaction—not after.
If you are considering a back door conversion to Roth IRA, the existing traditional IRA balance should be reviewed before the transaction. A backdoor Roth is not automatically tax-free simply because the contribution itself was nondeductible.
Where the Rule Appears on Form 8606
Form 8606 is where the IRS documents the basis and calculates the nontaxable portion of a traditional IRA distribution or Roth conversion. The current 2025 form shows the calculation using the taxpayer’s basis, December 31 traditional IRA value, applicable distributions, and net Roth conversion amount.
Simplified Form 8606 mock-up
FORM 8606 — PART I
Your basis in traditional IRAs
↓
Nondeductible contributions + prior basis
↓
Applicable traditional IRA value at Dec. 31
+
Applicable distributions
+
Net Roth conversion
↓
Total calculation amount
↓
Basis ÷ total calculation amount
↓
Nontaxable percentage
↓
Nontaxable portion of Roth conversion
The actual Form 8606 contains the detailed fields used to perform this calculation. The 2025 form specifically instructs taxpayers to enter the value of all traditional IRAs as of December 31, plus applicable outstanding rollovers, then add the relevant distributions and net Roth conversion amount before determining the nontaxable percentage.
If you are required to file Form 8606 and fail to do so, the IRS generally imposes a $50 penalty, unless reasonable cause applies.
Common Pro Rata Mistakes
The pro rata Roth conversion rule creates problems most often when someone uses an incomplete snapshot of their IRA situation.
Watch for these five mistakes:
- Using the conversion-day balance: The Form 8606 calculation uses the applicable traditional IRA value as of December 31, not simply the balance on the day you convert.
- Ignoring outstanding rollovers: Applicable outstanding rollovers can be included in the year-end IRA value used by Form 8606.
- Combining spouses’ IRAs: Each spouse’s IRA situation is generally calculated separately.
- Skipping Form 8606: Failing to document basis can create problems when you later need to establish that part of a distribution or conversion was already taxed.
- Waiting too long for a year-end rollover: If you are using an employer plan to reduce the IRA balance, the transaction needs to be properly completed by the relevant year-end deadline.
The current IRS form specifically identifies December 31 IRA value, applicable outstanding rollovers, distributions, and Roth conversions as parts of the calculation.
How the Pro Rata Rule Fits a Bigger Roth Strategy
The pro rata Roth conversion rule is only one part of Roth conversion planning. The bigger question is how a conversion affects your taxable income, future tax brackets, required minimum distributions, Social Security taxation, and Medicare costs.
That is why Roth conversion strategies should be considered as part of broader tax planning for retirees, rather than as an isolated transaction. Retire Well Dallas also discusses how Roth conversions can interact with Social Security taxation and Medicare IRMAA.
For example, a conversion that looks attractive from a long-term tax perspective may still require careful income management in the year it occurs. Before converting, consider the tax bracket you will occupy and whether the additional income could affect Medicare-related costs.
If you’re researching 2027 IRMAA brackets, use the applicable year’s official Medicare and IRS information when the figures are available rather than relying on an outdated threshold.
Understand Your Roth Conversion Before You Convert
The pro rata Roth conversion rule is not a penalty for having an IRA with basis. It is a tax calculation designed to determine which portion of your conversion represents already-taxed money and which portion represents taxable IRA dollars.
The key is to look at the entire IRA picture before making the conversion. That means reviewing your basis, traditional IRA balances, SEP and SIMPLE IRAs, applicable rollovers, planned distributions, and the amount you intend to convert.
If you’re comparing Roth conversion strategies, Retire Well Dallas can help you coordinate the conversion with broader tax planning for retirees, income planning, and Medicare considerations.
You can also review the firm’s guidance on Roth conversions and Social Security taxation before deciding how a conversion fits into your broader plan.
Ready to Review Your Roth Conversion?
A Roth conversion can affect more than your current-year tax return. The right analysis may need to account for your IRA basis, tax bracket, future required minimum distributions, Social Security taxation, and Medicare-related income considerations.
Retire Well Dallas uses a CPA team-based approach to retirement planning and tax strategy. If you want to review your numbers before making a conversion, schedule a meeting to discuss your situation and understand the potential tax impact before taking action.
Pro Rata Rule FAQ
What IRA balance does the pro rata rule use?
The IRS generally uses the value of your traditional IRAs on December 31 of the conversion year, plus applicable outstanding rollovers. The Form 8606 calculation also incorporates relevant distributions and Roth conversion amounts.
Does my 401(k) count?
No. A 401(k) is not itself part of the traditional IRA calculation. However, if eligible retirement money is rolled from a 401(k) into a traditional IRA, that IRA balance can become relevant to the calculation.
Does my spouse’s IRA affect my conversion?
Generally, no. Each spouse’s traditional IRA balances and basis are handled separately. The IRS instructions state that a separate Form 8606 is filed for each spouse who is required to file one.
How do I avoid the pro rata rule?
Common approaches include moving eligible pre-tax IRA money into an employer plan that accepts rollovers, converting pre-tax IRA assets and paying the resulting tax, or completing the conversion while accepting the applicable taxable percentage.
What form reports it?
Form 8606 reports nondeductible traditional IRA contributions, tracks basis, and reports Roth conversions. Its calculation determines the nontaxable portion of a conversion when you have basis.
