Post-1986 after-tax contributions are dollars you put into a qualified retirement plan or a traditional IRA after 1986 using money that was already taxed. The principal comes back to you tax-free when you take distributions, but the earnings on it are taxable as ordinary income, and with limited exceptions each withdrawal must return a proportional slice of both. That pro-rata treatment, and the tracking that goes with it, is what makes “post-86 after-tax” a category worth understanding on its own.
Why 1986 Is the Dividing Line
The Tax Reform Act of 1986 changed how after-tax money in a qualified plan comes out. Before the law took effect on January 1, 1987, after-tax contributions in a qualified plan could often be recovered first under more favorable ordering rules. TRA ’86 required post-1986 after-tax amounts to be distributed proportionally alongside pre-tax money and earnings, so you can no longer pull out just your basis and leave the taxable dollars behind.
The distinction still shows up on Form 1099-R, where plan administrators report post-86 after-tax contributions separately from any pre-1987 amounts. Your after-tax basis is your cost in the account, and the IRS expects you to track it precisely. Lose track of it and you risk paying tax a second time on money you already paid tax on once.
How the Pro-Rata Rule Works in a 401(k) or 403(b)
If you made after-tax contributions to a 401(k), 403(b), or similar qualified plan after 1986, every distribution you take contains a proportional mix of tax-free basis and taxable earnings. You cannot cherry-pick just the after-tax dollars.
The math is straightforward. Divide your total after-tax contributions by the total account balance at the time of distribution. That percentage of any withdrawal comes out tax-free. If you have $20,000 in after-tax contributions and your account balance is $100,000, exactly 20% of any distribution is a tax-free return of basis. The other 80% is taxable as ordinary income.1Internal Revenue Service. Rollovers of After-Tax Contributions in Retirement Plans
The rule applies to partial distributions the same way it applies to full ones. A partial distribution must include a portion of the pre-tax amounts as well; you cannot isolate the after-tax slice within a single payout.
Splitting a Distribution to Isolate the After-Tax Money
There is a workaround worth knowing. Under IRS Notice 2014-54, when you take a distribution and direct it to more than one destination at the same time, the IRS treats the whole amount as a single distribution for purposes of allocating pre-tax and after-tax dollars. That lets you route the pre-tax money into a traditional IRA and the after-tax money into a Roth IRA, separating the two cleanly. You have to tell the plan administrator how to allocate the amounts before the rollovers are processed.2Internal Revenue Service. Guidance on Allocation of After-Tax Amounts to Rollovers
Say your account holds $80,000 in pre-tax amounts and $20,000 in after-tax contributions. You can direct the $80,000 to a traditional IRA and the $20,000 to a Roth IRA. The after-tax contributions entering the Roth are not taxed again since you already paid tax on them. Only the earnings tied to those after-tax contributions would be taxable in the conversion.1Internal Revenue Service. Rollovers of After-Tax Contributions in Retirement Plans
Nondeductible IRA Contributions and Form 8606
If you contributed to a traditional IRA but didn’t qualify for a deduction, those nondeductible contributions form your after-tax basis in the IRA. The same principle holds: basis comes back tax-free, earnings are taxable. Unlike a 401(k), though, the tracking burden sits entirely on you.
You report nondeductible IRA contributions each year on Form 8606 (Nondeductible IRAs). The form creates a cumulative record of your after-tax basis so you can prove which portion of future distributions is tax-free. Skip the filing and you face a $50 penalty per missed year, plus the real problem of not being able to prove your basis when you take distributions. Overstating nondeductible contributions triggers a $100 penalty.3Internal Revenue Service. 2025 Instructions for Form 8606 – Nondeductible IRAs
If you missed the form in past years, the basis can still be reconstructed from Form 1040 copies, any previously filed Forms 8606, Forms 5498 showing contributions, and Forms 1099-R for distribution years. IRA custodians can often supply lost Forms 5498, and the IRS provides tax return transcripts on request.4Internal Revenue Service. Instructions for Form 8606
The IRA Aggregation Rule
Here’s where IRA after-tax basis gets tricky, and where many people stumble when they try a backdoor Roth conversion. The tax code requires you to treat all your traditional IRAs, SEP IRAs, and SIMPLE IRAs as a single combined account when calculating the taxable portion of any distribution or conversion. It does not matter that the accounts sit at different financial institutions or were funded in different years.5Office of the Law Revision Counsel. 26 U.S. Code 408 – Individual Retirement Accounts
The calculation uses your December 31 balances for the year you take the distribution. If you have $7,000 in nondeductible contributions and $93,000 in pre-tax IRA money from old rollovers, your tax-free percentage is 7% ($7,000 divided by $100,000). Convert $7,000 to a Roth IRA and only $490 of that conversion is tax-free. The remaining $6,510 is taxable income. The result is the same regardless of which IRA account the money physically came from.
Aggregation is the main reason a backdoor Roth conversion can backfire for anyone holding pre-tax IRA balances. One way to sidestep it is rolling pre-tax IRA funds into a 401(k) before converting, because employer plan balances are not aggregated with IRAs. Not every plan accepts incoming rollovers, so confirm with your plan administrator before you count on the option.
The Mega Backdoor Roth Using 401(k) After-Tax Contributions
After-tax contributions in a 401(k) open the door to what’s called the mega backdoor Roth. You make after-tax contributions to your 401(k) beyond the normal elective deferral limit, then convert those contributions to a Roth account where future growth is tax-free.
For 2026, the total annual additions limit for defined contribution plans is $72,000 ($80,000 with the standard age-50 catch-up, and $83,250 for those aged 60 through 63 under the SECURE 2.0 enhanced catch-up).6Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500 The regular employee deferral limit is $24,500. Once you subtract your own deferrals and any employer match, the remaining room under the $72,000 cap can be filled with after-tax contributions, if your plan allows them.
Not every 401(k) permits after-tax contributions, so the strategy only works when the plan document includes the option. Where it does, the conversion usually runs one of two ways. An in-plan Roth conversion moves your after-tax contributions to a Roth 401(k) inside the same plan; you owe tax on any earnings that accumulated between the contribution and the conversion, but the contributions themselves are not taxed again. Some plans offer an automatic conversion feature that shifts after-tax money to Roth at regular intervals, which keeps the taxable earnings buildup small. Alternatively, when you separate from your employer or if your plan allows in-service distributions, you can roll the after-tax contributions to a Roth IRA using the Notice 2014-54 allocation described above.2Internal Revenue Service. Guidance on Allocation of After-Tax Amounts to Rollovers
The strategy lets high earners move tens of thousands of dollars per year into Roth accounts, well beyond the standard $7,500 Roth IRA contribution limit for 2026, and without the income phase-outs that cap direct Roth IRA contributions at $168,000 for single filers and $252,000 for joint filers.6Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500
The 10% Early Withdrawal Penalty on the Taxable Share
Withdrawals taken before age 59½ generally carry a 10% additional tax, but only on the taxable portion of the distribution. The tax-free return of your after-tax basis is never subject to the penalty.7Internal Revenue Service. Publication 575 (2025), Pension and Annuity Income
Because the pro-rata rule still applies, a pre-59½ withdrawal from a plan that contains after-tax money will produce a partial exemption from the penalty proportional to your basis, not full protection. The usual Section 72(t) exceptions (death, disability, substantially equal periodic payments, qualified medical expenses, and others) apply to the taxable share on their own terms.8Internal Revenue Service. Retirement Topics – Exceptions to Tax on Early Distributions
One boundary worth noting: non-qualified annuity contracts also involve after-tax money, but they follow a different regime (earnings-first withdrawals, an exclusion ratio after annuitization, and their own aggregation rules), and “post-86 after-tax” as the term is used on Form 1099-R and in plan documents refers to retirement plan basis, not annuity basis.