1099-R Roth Conversion: Pro-Rata Rule, Form 8606, and Form 1040

When you convert a Traditional, SEP, or SIMPLE IRA to a Roth IRA, your custodian issues Form 1099-R for the transaction, but the form usually won’t tell you how much of the conversion is taxable. For 1099-R Roth conversion reporting, you use Box 1 as the gross amount, calculate the taxable portion yourself on Form 8606 using the IRS pro-rata rule, and then report both figures on lines 4a and 4b of your Form 1040. The taxable portion is ordinary income for the year the conversion occurred.

What Each Box on the 1099-R Means

Your IRA custodian issues one Form 1099-R for each conversion. Four entries on that form do the real work.

Box 1, Gross Distribution, shows the total dollar amount that moved from your Traditional IRA into your Roth IRA. Convert $50,000 and Box 1 reads $50,000. This is the full transfer, not necessarily the taxable amount.¹

Box 2a, Taxable Amount, is where most of the confusion starts. Custodians generally aren’t required to calculate the taxable amount on Traditional IRA distributions, and if you’ve ever made nondeductible contributions to any Traditional IRA, the custodian almost certainly doesn’t know your total basis. Box 2a is usually left blank or filled with zero.²

Box 2b has two checkboxes: “Taxable amount not determined” and “Total distribution.” When the custodian can’t figure the taxable portion, one or both are checked. That’s the form telling you and the IRS that the number still has to be calculated on your return.¹

Box 7 is the distribution code. For a Roth conversion, the custodian enters Code 2 if you were under 59½ when the conversion happened, or Code 7 if you were 59½ or older. The IRA/SEP/SIMPLE checkbox in Box 7 is also marked.³

One boundary worth flagging: Code R is not for conversions. Code R is reserved for recharacterizations of IRA contributions. If your 1099-R shows Code R for what you know was a conversion, contact your custodian for a corrected form before you file.

Calculating the Taxable Amount With the Pro-Rata Rule

If every dollar in your Traditional IRA came from deductible contributions and tax-deferred earnings, the entire conversion is taxable. It’s simple: Box 1 equals your taxable amount.

The math changes when any of your Traditional, SEP, or SIMPLE IRAs holds nondeductible (after-tax) contributions. Internal Revenue Code Section 408(d)(2) requires you to treat every converted dollar as a proportional mix of taxable and non-taxable money. You cannot cherry-pick and convert only the after-tax dollars first. The IRS looks at all of your non-Roth IRA balances as one combined pool, regardless of how many separate accounts you hold.

A Worked Example

Say you hold a Traditional IRA worth $100,000 on December 31 of the conversion year. Of that balance, $20,000 is basis from nondeductible contributions you’ve tracked over the years. The other $80,000 is deductible contributions and accumulated earnings. You convert $50,000.

First, calculate your exclusion ratio by dividing total basis by the total value of all your non-Roth IRAs:

$20,000 ÷ $100,000 = 20%

Apply that ratio to the amount you converted:

20% × $50,000 = $10,000 non-taxable

The remaining $40,000 is the taxable portion you report as ordinary income. Your remaining basis drops from $20,000 to $10,000 and carries forward to next year’s Form 8606.

The December 31 Balance Controls the Calculation

The total value of all your non-Roth IRAs on December 31 of the conversion year drives the pro-rata math, even if you did the conversion in January. Contributing to a Traditional IRA later in the same year, or rolling over a 401(k) into a Traditional IRA, increases the denominator and changes your exclusion ratio. This is the stumbling block that trips up backdoor Roth strategies: there’s no income limit for doing a conversion, but existing pre-tax IRA balances will make part of it taxable.

Reporting the Conversion on Form 8606

Form 8606, “Nondeductible IRAs,” is where you formally calculate the taxable and non-taxable portions of the conversion. You have to file it any year you convert Traditional IRA funds to a Roth IRA or take a distribution when you have basis in your IRAs.⁴

Part I: Tracking Your Basis

Part I establishes your total basis and runs the pro-rata calculation. Using the numbers from the example:

Line 1: any new nondeductible contributions for the current tax year. Zero if you didn’t make any.
Line 2: basis carried forward from the prior year’s Form 8606, Line 14. In the example, $20,000.
Line 3: total basis (Line 1 plus Line 2).
Line 6: total value of all your Traditional, SEP, and SIMPLE IRAs as of December 31, plus any outstanding rollovers. In the example, $100,000.
Line 8: net amount converted to a Roth IRA. In the example, $50,000.

The remaining lines work through the division to determine what fraction of your distributions and conversions is non-taxable. Line 14 calculates your remaining basis after this year’s conversion. In the example, $20,000 minus $10,000 leaves $10,000 of basis carrying forward.⁵

Part II: The Taxable Conversion Amount

Part II isolates the conversion and splits it in two:

Line 16: total conversion amount ($50,000).
Line 17: non-taxable portion from the Part I basis calculation ($10,000).
Line 18: taxable portion, Line 16 minus Line 17 ($40,000).

The Line 18 figure is what you report as ordinary income.⁵

Transferring the Numbers to Form 1040

On Form 1040, the conversion goes on the IRA distributions lines. Line 4a shows the total distribution from Box 1 of the 1099-R (the full $50,000). Line 4b shows the taxable amount from Form 8606, Line 18 ($40,000).⁵

Line 4a should always match Box 1 of the 1099-R for a straight conversion, even when part of the money is non-taxable. The IRS matches 1099-R figures against Line 4a, so entering only the taxable amount on both lines is a common mistake that can generate a notice.

The SIMPLE IRA Two-Year Rule

If the money is coming from a SIMPLE IRA, one extra timing rule applies before you can report a normal conversion. During the first two years of participation in your employer’s SIMPLE IRA plan, a transfer to a Roth IRA (or any non-SIMPLE IRA) is treated as a taxable distribution subject to a 25% penalty, not the standard 10%. After the two-year period, normal conversion rules apply and any early withdrawal penalty drops to the usual 10% if you’re under 59½.⁶

The two-year clock starts on the date your employer first deposited a contribution to your SIMPLE IRA, not the date you opened the account. Confirm the start date with your plan administrator before converting.

Withholding and Estimated Tax on the Conversion

A conversion has to be completed by December 31 to count for that tax year. There’s no option to backdate it into the prior year the way you can with regular IRA contributions.

A large conversion can create a significant tax bill. If you don’t adjust your withholding or make estimated payments during the year, you can face an underpayment penalty. The IRS generally won’t charge the penalty if you pay at least 90% of the current year’s tax or 100% of the prior year’s tax through withholding and estimated payments. If your prior-year adjusted gross income exceeded $150,000 ($75,000 if married filing separately), the 100% threshold rises to 110%.⁷

Because conversion income doesn’t arrive evenly across the year, the annualized income installment method on Form 2210, Schedule AI can reduce or eliminate the penalty by matching estimated payments to the quarter the income actually landed. A fourth-quarter conversion won’t trigger penalties for the first three quarters under that method. You can also ask your custodian to withhold federal tax directly from the IRA, but the withheld amount is itself treated as a taxable distribution and reduces the dollars that actually reach the Roth. Paying the tax from a separate bank account usually works out better.

1
Internal Revenue Service. Instructions for Forms 1099-R and 5498
2
Internal Revenue Service. IRS Form 1099-R
3
Internal Revenue Service. 2025 Instructions for Forms 1099-R and 5498
4
Internal Revenue Service. About Form 8606, Nondeductible IRAs
5
Internal Revenue Service. Instructions for Form 8606 (2025)
6
Internal Revenue Service. SIMPLE IRA Withdrawal and Transfer Rules
7
Office of the Law Revision Counsel. 26 U.S. Code 6654 – Failure by Individual To Pay Estimated Income Tax