How to Calculate Your 1099-R Taxable Amount Not Determined

When Box 2b of your 1099-R is checked as “taxable amount not determined,” the payer is telling you it doesn’t have the records needed to split your distribution between taxable earnings and your already-taxed contributions. That calculation now falls to you, and you have to do it before you file. The method depends on where the money came from: traditional IRAs use the pro-rata rule on Form 8606, employer pensions and qualified annuities use the Simplified Method, non-qualified annuities use the General Rule, and Roth IRAs follow their own qualification and ordering rules.

Why Box 2b Gets Checked

Custodians check Box 2b when they can’t separate your after-tax money from your pre-tax money. Your after-tax contributions are your basis, and the IRS doesn’t tax the return of basis — only earnings and pre-tax contributions. The problem is that your custodian usually knows only the total it paid out, not a running tally of every after-tax dollar you put in over the years.

This shows up most often with traditional IRA distributions where you made non-deductible contributions, with non-qualified annuities funded with after-tax dollars, and with employer plans that accepted after-tax employee contributions. For Roth IRAs, custodians are specifically instructed to check Box 2b and leave Box 2a blank on virtually every distribution, because they have no way to verify from their end whether you meet the tax-free treatment conditions.¹

Establishing Your Basis Before You Calculate Anything

Without documentation of after-tax contributions, the IRS treats the entire distribution as taxable. Assemble your records first.

For IRAs

The central document is Form 8606, Nondeductible IRAs, which tracks cumulative non-deductible contributions to your traditional IRAs. If you filed it each year you made a non-deductible contribution, your basis trail is already in hand.² Also pull Forms 5498, which your custodian files annually to report IRA contributions, and cross-check them against prior returns to see whether those contributions were deducted.³ Deducted contributions are not basis.

For Pensions and Annuities

Look for plan statements that break out after-tax contributions. Your plan administrator may be able to give you a total after-tax investment figure. For a non-qualified annuity, find the original contract and premium payment records — every premium paid with after-tax dollars is basis.

When the Records Are Gone

If you never filed Form 8606 or your records are incomplete, the IRS can help you reconstruct. Tax return transcripts are available for the current and three prior years, and tax account transcripts go back up to nine years through your IRS Individual Online Account. For older years, submit Form 4506-T.⁴ A Wage and Income Transcript is especially useful because it includes Form 5498 data showing contributions your custodian reported. Pair that with a Return Transcript to see whether those contributions were deducted, and you can reconstruct non-deductible basis even if you never filed Form 8606.⁵

If you made non-deductible contributions but never filed Form 8606, you can still claim that basis. File Form 8606 for the distribution year showing the correct basis. There is a $50 penalty for each year you should have filed Form 8606 and didn’t, but the IRS waives it for reasonable cause.⁶

IRA Distributions: The Pro-Rata Rule on Form 8606

If the distribution came from a traditional, SEP, or SIMPLE IRA and you have basis, you calculate the taxable portion using the pro-rata rule on Form 8606.

The rule treats all of your traditional, SEP, and SIMPLE IRAs as a single pool, calculates what percentage of that pool is basis, and applies that percentage to your distribution to determine the tax-free portion. Line 6 of Form 8606 asks for the total value of all your traditional IRAs as of December 31 of the distribution year, plus any outstanding rollovers. The form then produces the ratio of basis to the total pool, and that ratio determines what comes out tax-free.⁷

A worked example: total basis across all your traditional IRAs is $20,000, combined December 31 value of all your IRAs is $180,000, and you took a $10,000 distribution. The denominator is $190,000 (year-end value plus distributions). Your non-taxable percentage is roughly 10.5% ($20,000 ÷ $190,000), so about $1,050 of the $10,000 distribution is tax-free and the remaining $8,950 is taxable.

Employer-sponsored accounts — 401(k)s, 403(b)s, 457(b)s — are not part of the IRA aggregation. Only traditional, SEP, and SIMPLE IRAs count. That distinction matters if you have both types.

Pensions and Qualified Annuities: The Simplified Method

If your distribution is a series of periodic payments from a qualified employer plan or qualified annuity, use the Simplified Method. Divide your total after-tax investment in the plan by a fixed number of expected monthly payments based on your age when payments began.⁸

For annuities with a start date after November 18, 1996, the expected payment numbers are:

Age 55 or younger: 360 payments
Age 56 to 60: 310 payments
Age 61 to 65: 260 payments
Age 66 to 70: 210 payments
Age 71 or older: 160 payments

The result is the tax-free portion of each monthly payment. If you had $52,000 in after-tax contributions and payments began at age 63, dividing $52,000 by 260 gives $200 tax-free per month. Anything above $200 in a given monthly payment is taxable. Multiply the tax-free monthly amount by the number of payments received during the year and subtract from the gross figure in Box 1 of your 1099-R to get the taxable amount. Once your total tax-free recovery equals your basis, every payment after that is fully taxable.

Non-Qualified Annuities: The General Rule

Non-qualified annuities purchased outside an employer plan use the General Rule in IRS Publication 939. Divide your investment in the contract by the expected return, where expected return is the annual payment multiplied by a life expectancy factor from the Publication 939 actuarial tables.⁹

That result is your exclusion ratio, expressed as a percentage, and it stays the same for the life of the contract. Apply it to each payment to determine the tax-free portion. As with the Simplified Method, once total tax-free payments equal your original investment, everything after is fully taxable.

Roth IRA Distributions

Roth IRA 1099-Rs almost always have Box 2b checked. Custodians are instructed to leave Box 2a blank and mark “Taxable amount not determined” because they cannot verify whether your distribution qualifies for tax-free treatment.¹

A qualified Roth IRA distribution is entirely tax-free. To qualify, your first Roth IRA contribution must have been made at least five tax years before the distribution, and you must be at least 59½ (or the distribution must be due to disability, death, or a first-time home purchase up to $10,000). If your distribution meets those conditions, the taxable amount is zero. Report the gross amount on Form 1040 Line 4a and enter zero on Line 4b.

If the distribution is not qualified, Part III of Form 8606 handles the calculation. Roth distributions follow ordering rules: contributions come out first (always tax-free), then conversions (tax-free if the five-year period for that conversion has passed), and finally earnings, which are taxable and potentially subject to the 10% early withdrawal penalty if you’re under 59½.²

Inherited IRAs With Basis

If you inherited a traditional IRA from someone who made non-deductible contributions, their basis carries over to you. It doesn’t disappear at death, but you’ll need information about someone else’s contribution history to claim it.

File a separate Form 8606 to report the inherited IRA’s basis. The IRS instructions point you to Publication 590-B for the specific steps, and the underlying method is the same pro-rata approach applied to the inherited IRA’s basis and balance.⁷ The practical work is finding the decedent’s basis: check whether they filed Form 8606, look at their prior tax returns and IRA contribution records, and ask the executor. IRS transcripts can fill gaps. Without a basis claim, every dollar of every inherited distribution is taxed even though part of it was already taxed on the way in.

Reporting the Distribution on Form 1040

IRA distributions go on Form 1040 Line 4a (gross distribution from Box 1 of the 1099-R) and Line 4b (taxable amount). Pension and annuity distributions go on Lines 5a and 5b.¹⁰ If your distribution involved non-deductible IRA basis, attach the completed Form 8606. For Simplified Method calculations, keep the completed worksheet with your records; you don’t attach it, but you need to be able to produce it.

Check Box 7 on your 1099-R. The distribution code tells the IRS what type of distribution it was. Code 1 is an early distribution before 59½ with no known exception, which triggers the additional 10% penalty on the taxable portion. Code 7 is a normal distribution. Code G is a direct rollover. If Box 7 shows Code 1 and you qualify for an exception, file Form 5329 to claim it.¹

What Happens If You Get It Wrong

Understating your tax by reporting too little of a distribution as taxable can trigger the 20% accuracy-related penalty under IRC Section 6662 on top of the tax owed.¹¹ Interest also runs on unpaid balances. For the first quarter of 2026, the individual underpayment rate is 7% per year, compounded daily.¹² The $50-per-year Form 8606 penalty is separate.⁶ Both the accuracy penalty and the Form 8606 penalty can be waived for reasonable cause, which is evaluated on all the facts, including the effort you made to get the tax right. Relying on erroneous information from a financial institution or making an honest computational error generally qualifies if you acted in good faith.¹³

The error runs in the other direction, too. If you skip the calculation and report the full distribution as taxable, you’re paying tax on money you already paid tax on. The IRS won’t stop you. Claiming your basis is your job, and Box 2b is the notice that the job hasn’t been done yet.

1
Internal Revenue Service. Instructions for Forms 1099-R and 5498 (2025)
2
Internal Revenue Service. About Form 8606, Nondeductible IRAs
3
Internal Revenue Service. Form 5498 IRA Contribution Information
4
Internal Revenue Service. Transcript Types for Individuals and Ways to Order Them
5
Internal Revenue Service. Request for Transcript of Tax Return
6
Office of the Law Revision Counsel. 26 U.S.C. 6693 – Failure to Provide Reports on Certain Tax-Favored Accounts
7
Internal Revenue Service. Instructions for Form 8606 (2025)
8
Internal Revenue Service. Publication 575 (2025), Pension and Annuity Income
9
Internal Revenue Service. Publication 939 (12/2025), General Rule for Pensions and Annuities
10
Internal Revenue Service. 1040 (2025) General Instructions
11
Office of the Law Revision Counsel. 26 U.S.C. 6662 – Imposition of Accuracy-Related Penalty on Underpayments
12
Internal Revenue Service. Interest Rates Remain the Same for the First Quarter of 2026
13
eCFR. 26 CFR 1.6664-4 – Reasonable Cause and Good Faith Exception