What Is Tax Basis in Retirement Accounts and Pensions?

Tax basis in retirement accounts is the money you already paid income tax on before it went into the account, and the IRS lets you pull that money back out tax-free — but only if you can prove it’s there and only through the specific recovery method that applies to your account type. Get the tracking wrong and you’ll pay income tax a second time on dollars that were already taxed once.

Where Basis Comes From

Basis builds up any time after-tax money enters a retirement account. The most common source is a nondeductible contribution to a traditional IRA. If your income is too high to deduct the contribution, the dollars still go in, and because you already paid tax on them, they become basis.¹

Employer plans can also carry basis. Some 401(k) and 403(b) plans accept after-tax contributions beyond the standard elective deferral limit, and those dollars are your basis in the plan. Employer matches and investment earnings are not basis, because neither has been taxed yet.

Defined benefit pensions create basis through mandatory or voluntary employee contributions made with after-tax dollars. Federal tax law calls this pool the employee’s “investment in the contract” — the total after-tax money you’ve put in, minus anything you’ve already recovered tax-free through prior distributions.²

Recovering Basis From a Traditional IRA

You might expect you can withdraw just the after-tax portion of a traditional IRA and owe nothing. The IRS won’t allow it. Under 26 U.S.C. § 408(d)(2), all of your traditional IRAs, SEP IRAs, and SIMPLE IRAs are treated as a single contract for distribution purposes.³ Every withdrawal pulls proportionally from both the taxable and non-taxable portions of that combined pool. This is the pro-rata rule.

The math is straightforward. Divide your total basis across all traditional IRAs by the total year-end value of those IRAs (plus any distributions taken during the year). That fraction is the tax-free percentage of each dollar you withdraw. With $20,000 in basis and a combined $200,000 across all traditional IRAs, 10% of each distribution is a non-taxable return of basis and 90% is ordinary income.

Employer plans like 401(k)s and 403(b)s are not swept into this calculation.⁴ The pro-rata rule applies only to IRA-type accounts. That distinction matters when you consider rolling after-tax 401(k) money into an IRA, because once those dollars land there, they get pulled into the aggregation.

How Roth IRA Basis Works

Roth IRAs handle basis differently, and the rules favor you. Every dollar you contributed to a Roth was after-tax money, so your entire contribution history is basis. Roth distributions follow a specific ordering rule that lets you pull contributions out first, before touching any growth.⁵

Regular contributions come out first. These are always tax-free and penalty-free, regardless of your age or how long the account has been open.
Conversion and rollover amounts come out next, on a first-in, first-out basis. The taxable portion of each conversion is distributed before the non-taxable portion.
Earnings come out last. They’re only tax-free if the distribution is “qualified” — you’re at least 59½ and the account has been open for at least five tax years.

The practical effect is that you can always reach your Roth contributions without tax consequences. The five-year clock and age requirement only matter once you’ve drained contributions and conversions and start withdrawing earnings.⁶ Pull earnings early and you’ll owe income tax plus potentially a 10% penalty on that portion.

The Backdoor Roth Pitfall

The backdoor Roth strategy is where traditional IRA basis and Roth basis intersect, and where the pro-rata rule causes the most damage. The play is simple in theory: make a nondeductible contribution to a traditional IRA, then convert it to a Roth. Because the contribution had basis, you’d expect the conversion to be largely tax-free.

It isn’t, if you hold other traditional IRA money. The conversion is taxed based on the ratio of basis to total traditional IRA value across every account.³ Someone with $6,000 in nondeductible contributions and $194,000 in pre-tax IRA money who converts $6,000 finds that only 3% of the conversion is tax-free.

You report the conversion on Part II of Form 8606, which runs the pro-rata calculation for you.⁷ The non-taxable portion of the conversion becomes Roth basis, subject to its own five-year clock for penalty-free access. The strategy works cleanly only when you have no other traditional IRA balances to trigger aggregation.

Recovering Basis From a Pension or Annuity

Pensions use a different method entirely. Instead of a pro-rata calculation against total account value, the IRS uses the Simplified Method: you divide your basis at the annuity starting date by a set number of expected monthly payments, and that fixed dollar amount is excluded from income each month.²

The expected number of payments comes from a table in IRS Publication 575 based on your age when payments begin:⁸

Age 55 or under: 360 months
Age 56–60: 310 months
Age 61–65: 260 months
Age 66–70: 210 months
Age 71 or older: 160 months

Retire at 65 with $52,000 in after-tax contributions and you divide $52,000 by 260. That’s $200 excluded from income each month. Once you’ve received enough payments to recover the full $52,000, every subsequent pension payment is fully taxable. If you die before recovering all your basis, the unrecovered amount can be claimed as a deduction on your final tax return.

The Simplified Method is mandatory for annuity payments from qualified employer plans, 403(b) plans, and government plans when the annuity start date was after November 18, 1996, and certain age and guarantee conditions apply.⁸

Inherited Basis

When someone dies with basis in a traditional IRA, that basis passes to the beneficiary. The tax protection the original owner built up doesn’t disappear, but the tracking requirements are strict and this is where families quietly overpay.

A surviving spouse who inherits a traditional IRA and elects to treat it as their own can combine the inherited basis with any basis in their own traditional IRAs. Every other beneficiary must keep inherited basis separate. Inherit IRAs from more than one person and each maintains its own independent basis pool, with a separate Form 8606 filed for each.⁶

For pensions, a beneficiary entitled to a survivor annuity recovers the deceased employee’s remaining basis using the same method the employee would have used.⁹ Payments continue partially excluded from income until the full investment in the contract is returned.

The hardest part of inheriting basis is proving it exists. You need the deceased person’s last Form 8606 or some other record establishing the cumulative nondeductible contributions. Without it, the basis effectively disappears and every dollar of distribution gets taxed as ordinary income.

Tracking Your Basis: Form 8606 and 1099-R

Form 8606 is the official tool for tracking traditional IRA basis. File it with your Form 1040 any year you make a nondeductible contribution, take a distribution from an IRA that has basis, or convert traditional IRA money to a Roth.⁷ The form carries your cumulative basis forward year to year. The basis at the bottom of this year’s form becomes the starting figure for next year’s calculation.

For employer plans, the key document is Form 1099-R. Box 5 shows the employee’s after-tax contributions or insurance premiums recoverable tax-free.¹⁰ When you take a distribution from a 401(k) or pension that included after-tax contributions, this box tells you and the IRS how much basis is being returned.

Keep these records indefinitely, or at least until the period of limitations closes for the tax year in which you fully deplete the account.¹¹ For retirement accounts, that means holding onto every Form 8606 and 1099-R until well after the final distribution. If you’re 40 and making nondeductible contributions now, you may need those records 50 years from now.

Fixing Missed Filings and Reconstructing Lost Basis

Skipping Form 8606 in a year you made nondeductible contributions triggers a $50 penalty; overstating nondeductible contributions carries a $100 penalty. Both can be waived for reasonable cause.¹ The real cost of not filing is far larger than the penalty: without the form on record, the IRS has no evidence your contributions were nondeductible, and you can end up paying income tax on money that was already taxed.

You can still fix a missed filing. File Form 8606 for each missed year along with a Form 1040-X within the amendment time limit. To correct a mistake on a previously filed 8606, complete a new form with the correct figures and attach it to a 1040-X.⁷

Reconstructing lost basis is tedious but possible. Request wage and income transcripts from the IRS, which show Form 5498 data reporting IRA contributions for each year. Compare those against your old tax returns: if you contributed to a traditional IRA but didn’t claim an IRA deduction, that contribution was nondeductible and counts as basis. Old custodial statements showing contribution deposits can help too. The sooner you do this work, the easier it is. Waiting until required distributions begin at 73 and scrambling for records from decades ago is a problem worth avoiding by filing Form 8606 consistently now.

1
Internal Revenue Service. Instructions for Form 8606
2
Office of the Law Revision Counsel. 26 USC 72 – Annuities; Certain Proceeds of Endowment and Life Insurance Contracts
3
Office of the Law Revision Counsel. 26 USC 408 – Individual Retirement Accounts
4
Internal Revenue Service. Publication 590-A, Contributions to Individual Retirement Arrangements (IRAs)
5
eCFR. 26 CFR 1.408A-6 – Distributions
6
Internal Revenue Service. Publication 590-B, Distributions from Individual Retirement Arrangements (IRAs)
7
Internal Revenue Service. Instructions for Form 8606
8
Internal Revenue Service. Publication 575 – Pension and Annuity Income
9
Internal Revenue Service. Retirement Topics – Beneficiary
10
Internal Revenue Service. Instructions for Forms 1099-R and 5498 – Section: Box 5
11
Internal Revenue Service. How Long Should I Keep Records