How to Figure the Taxable Amount of an IRA Distribution

To figure the taxable amount of an IRA distribution, start with what is in the account. If your Traditional, SEP, or SIMPLE IRA holds only pre-tax contributions and their earnings, every dollar you withdraw is taxable as ordinary income and no calculation is needed.¹ If the account also holds after-tax (non-deductible) contributions, federal law forces you to treat each withdrawal as a proportional slice of both, using the pro-rata formula on Form 8606.² Roth IRAs follow their own ordering rules and are addressed separately below.

When the Whole Distribution Is Taxable

Most Traditional IRA money is pre-tax: you deducted the contribution in the year you made it, or the money arrived by rollover from a 401(k) or similar plan that had never been taxed. SEP and SIMPLE IRAs are almost always pre-tax as well, funded by employer or salary-reduction contributions.

In that situation the math is trivial. Your custodian issues Form 1099-R, Box 1 shows the gross distribution, and Box 2a shows the taxable amount, which should match Box 1.³ You carry that figure to Form 1040 and stop. The complications begin only when you have made non-deductible contributions at some point, because those create basis in the account.

Basis: The Number You Need Before Anything Else

Your basis is the running total of every non-deductible contribution you have ever made to any Traditional, SEP, or SIMPLE IRA, reduced by any basis you have already recovered in prior distributions.⁴ It is the after-tax money in your IRAs. Without it, you cannot figure the non-taxable share of a withdrawal.

The IRS expects you to report each non-deductible contribution on Form 8606 in the year you make it.⁵ Line 2 carries forward your cumulative basis from prior years. If this is your first Form 8606, that line starts at zero; otherwise you pull it from your most recently filed 8606 using the Total Basis Chart in the instructions.⁴

If you made non-deductible contributions in past years but never filed the form, you have no documented basis. The practical result is that the IRS treats your distribution as fully taxable, and you end up paying income tax on money you already paid tax on once. You can file late Forms 8606 to rebuild the record, but each missed year carries a $50 penalty.⁶ That is cheap compared to the alternative.

The Pro-Rata Rule for Mixed IRAs

When your IRA holds both pre-tax and after-tax money, you cannot choose to pull out just the after-tax portion. Every distribution is deemed to include a proportional share of each, calculated across all your Traditional, SEP, and SIMPLE IRAs combined.²

Aggregate All Your IRAs First

The statute treats every Traditional, SEP, and SIMPLE IRA you own as a single account for this calculation, no matter how many custodians hold them.² You cannot park your after-tax dollars in one IRA and draw from that account for a tax-free withdrawal. The math looks at the combined total.

This trips up people who roll a large 401(k) balance into a Traditional IRA. Those pre-tax rollover dollars enter the combined pool, shrinking your basis percentage and pushing more of any future distribution into the taxable column. Roth IRAs are not part of this aggregation; only Traditional, SEP, and SIMPLE IRAs are.

The Formula

The non-taxable percentage of your distribution equals your total basis divided by the total value of all aggregated IRAs. The denominator is measured as of December 31 of the distribution year, plus every distribution taken during that year.²

A worked example. You have $20,000 of total basis across your Traditional IRAs. On December 31 the combined balance is $180,000, and you took a $10,000 distribution earlier in the year. Your denominator is $180,000 plus $10,000, or $190,000. The non-taxable percentage is $20,000 divided by $190,000, about 10.53%. Of the $10,000 you withdrew, $1,053 is a tax-free return of basis and $8,947 is taxable ordinary income.

The $1,053 you recovered reduces your remaining basis to $18,947, which becomes the Line 2 starting point on next year’s Form 8606. The same math applies whether the withdrawal was voluntary or a required minimum distribution.

Backdoor Roth Conversions Use the Same Formula

The pro-rata rule is what turns many backdoor Roth conversions into partly taxable events. The strategy sounds clean: contribute after-tax money to a Traditional IRA, then convert it to a Roth. Because the contribution was after-tax, the conversion should be nearly tax-free. The rule, though, does not care which dollars you convert. It looks at your whole Traditional IRA universe.

If you hold $5,000 of after-tax money and $95,000 of pre-tax money across all Traditional, SEP, and SIMPLE IRAs, only 5% of any conversion comes out tax-free. Convert $5,000 and $4,750 of it is taxable. A common workaround is to roll pre-tax IRA balances into your current employer’s 401(k) before converting, since employer plans sit outside the IRA aggregation calculation. Not every 401(k) accepts incoming rollovers, so confirm with the plan administrator first.

Roth IRAs: Ordering Layers, Not Pro-Rata

Roth IRA distributions do not use the pro-rata formula at all. The code applies an ordering rule that dictates which category of money is treated as coming out first.⁷

The Three Layers

Regular contributions come out first. Always tax-free and penalty-free, regardless of your age or how long the account has been open, because you already paid tax before contributing.
Conversions and rollovers come out next, on a first-in, first-out basis. The converted principal is not taxed again, but the 10% early withdrawal penalty can apply if you are under 59½ and the conversion has not met its own five-year period.
Earnings come out last. They are taxable and potentially penalized unless the distribution is qualified.

Qualified Distributions

Earnings come out fully tax-free and penalty-free only when the distribution is qualified. That requires your first Roth IRA contribution (to any Roth IRA) to have been made at least five tax years earlier, plus one of these conditions:⁷

You are at least 59½.
You are disabled.
You are a beneficiary withdrawing after the account owner’s death.
The distribution is for a qualified first-time home purchase, up to a $10,000 lifetime cap.

Non-qualified earnings withdrawals are taxed as ordinary income, and the 10% penalty applies if you are under 59½.

Two Separate Five-Year Clocks

Two five-year periods run in parallel, and confusing them is common. The first starts January 1 of the tax year of your very first Roth IRA contribution. Once it runs out it never resets, even if you later open new Roth accounts. This clock is what makes earnings eligible to come out tax-free as part of a qualified distribution.

The second clock is per-conversion. Each conversion begins its own five-year period on January 1 of the year of the conversion. Withdraw the converted amount before that clock finishes and you are under 59½, and the 10% penalty hits the taxable portion of the conversion. After 59½ the conversion clock becomes moot, because qualified distributions are exempt from the penalty anyway.

Form 8606 is used for Roth distributions too, but here it tracks the amounts in each ordering layer rather than running a pro-rata calculation.⁵

Reporting the Taxable Amount on Your Return

After any distribution year, your custodian sends Form 1099-R. Box 1 shows the gross withdrawal, Box 2a shows what the custodian believes is taxable, and Box 7 carries a code indicating the type of distribution.³

If the IRA is entirely pre-tax, Box 1 and Box 2a match, and you report that number on Form 1040. When you have basis, though, the custodian usually cannot compute the correct taxable amount, because they see only their own account and have no view of your lifetime contribution history or your other IRAs. Box 2a in that case may show the full gross amount or be left blank.

Form 8606 is where you do the actual work. Line 2 is your prior basis, Line 6 is the year-end value of all your Traditional IRAs, and Line 7 is your distributions for the year. The form walks you through the pro-rata split.⁴ The taxable figure you calculate there overrides Box 2a of the 1099-R. You are the one on the hook for reporting the right number.

One boundary worth noting: RMDs, which begin at age 73 for Traditional, SEP, and SIMPLE IRAs, are subject to the same pro-rata treatment as any other distribution when your account has basis.⁸ Roth IRAs do not require RMDs during the owner’s lifetime.

Withholding Is an Estimate, Not the Final Answer

Custodians typically withhold 10% of a distribution for federal income tax unless you elect a different rate or opt out. That is an estimate. If your marginal bracket is higher, you will owe more when you file. If basis reduces your taxable amount well below the gross, the 10% withholding may leave you due a refund. Adjusting the withholding election at the time of the distribution is easier than chasing the difference later.

Penalties for Getting the Math Wrong

Two penalties matter here. Failing to file Form 8606 for a year in which you made a non-deductible contribution costs $50 per missed form, waivable for reasonable cause.⁶ The bigger cost is losing the ability to prove your basis, which can turn a partly tax-free withdrawal into a fully taxable one.

Understating income from an IRA distribution can also draw a 20% accuracy-related penalty on the resulting underpayment, applied where the understatement is due to negligence or exceeds the greater of 10% of the tax you should have reported or $5,000.⁹ Between the tax itself, interest on the underpayment, and the 20% penalty, a bad pro-rata calculation is expensive to fix.

1
Internal Revenue Service. Retirement Plans FAQs Regarding IRAs Distributions
2
Office of the Law Revision Counsel. 26 USC 408 – Individual Retirement Accounts
3
Internal Revenue Service. Instructions for Forms 1099-R and 5498
4
Internal Revenue Service. Instructions for Form 8606
5
Internal Revenue Service. About Form 8606, Nondeductible IRAs
6
Office of the Law Revision Counsel. 26 USC 6693 – Failure to Provide Reports on Certain Tax-Favored Accounts or Annuities
7
GovInfo. 26 USC 408A – Roth IRAs
8
Internal Revenue Service. Retirement Topics – Required Minimum Distributions
9
Internal Revenue Service. Accuracy-Related Penalty