Does the Pro Rata Rule Apply to 401(k) Plans?

The IRA pro rata rule does not apply to 401(k) plans. A 401(k) is a separate trust from your IRAs, so the IRS does not add your 401(k) balance to your IRA balances when it calculates how much of an IRA distribution or Roth conversion is taxable. The 401(k) has its own proportionality rule for partial withdrawals, but that rule works inside the plan and does not reach across to your IRAs.

What the IRA Pro Rata Rule Actually Does

The pro rata rule sits in Internal Revenue Code Section 408(d)(2). It requires you to treat all of your non-Roth IRAs as a single pool when you figure the tax on any distribution or conversion.¹ That pool includes every Traditional IRA, SEP IRA, and SIMPLE IRA you own. You cannot pick which dollars come out. The IRS forces a proportional split, and every dollar you withdraw or convert carries a mix of pre-tax and after-tax money that mirrors your overall balance.

The math is simple. Divide your total after-tax basis (the nondeductible contributions you already paid tax on) by the total value of all your non-Roth IRAs. That fraction is the non-taxable percentage of any distribution. If you have $10,000 of basis and $90,000 of pre-tax money spread across your IRAs, only 10% of any withdrawal or conversion escapes tax. Converting just the $10,000 you contributed after-tax does not let you dodge tax on the other 90%.

A detail that catches people: the IRS measures your total IRA balance as of December 31 of the year you take the distribution, and adds back any distributions you took during that year.¹ Emptying an IRA in January and converting in February does not change the ratio, because the December 31 snapshot controls. You track your basis on IRS Form 8606 every year you make nondeductible contributions or take distributions that include them.²

Why the 401(k) Sits Outside the Aggregation

A 401(k) is a trust established by your employer under Code Section 401(a). Distributions from qualified employer plans are governed by Section 402, not Section 408. Section 408(d)(2) only aggregates “individual retirement plans,” so your 401(k) balance is invisible to the IRA pro rata calculation.¹ A 401(k) holding $500,000 in pre-tax money does not contaminate a Backdoor Roth conversion from a $7,000 nondeductible Traditional IRA contribution.

The same statutory line explains what does and does not get pulled into the IRA pool:

Traditional, SEP, and SIMPLE IRAs are all “individual retirement plans” under Section 408 and are treated as one contract for distribution purposes. It does not matter how many accounts you hold or at how many brokerages.¹
Roth IRAs are governed by Section 408A and are never aggregated with Traditional, SEP, or SIMPLE IRAs for pro rata purposes.
Inherited IRAs from someone other than a spouse are generally treated separately and excluded from your pro rata calculation under Treasury Regulation 1.408-8. If you inherit a spouse’s IRA and elect to treat it as your own, it joins your IRA pool.
401(k), 403(b), and 457(b) plans are qualified employer trusts, not individual retirement plans, and the IRA aggregation rule does not reach them.

The exemption works in reverse too. If you already have large pre-tax IRA balances making a Roth conversion expensive, rolling those pre-tax IRA dollars into your 401(k) (assuming the plan accepts incoming rollovers) removes them from the IRA aggregation pool. Once the pre-tax money is inside the 401(k), it no longer inflates the denominator in your pro rata fraction.

The 401(k)’s Own Internal Pro Rata Rule

The 401(k) escapes IRA aggregation, but it does have its own proportionality requirement for partial distributions. If your 401(k) holds both pre-tax and after-tax money and you take a partial withdrawal, that withdrawal must include a proportional share of each type.³ You cannot pull only the after-tax dollars out and leave the pre-tax money behind.

If your 401(k) is 80% pre-tax and 20% after-tax, a $10,000 partial distribution must contain roughly $8,000 in pre-tax funds and $2,000 in after-tax funds. IRS Notice 2014-54 did not change this requirement.³ What it changed is where each portion of that distribution can go once it leaves the plan.

Split Rollovers: Where the Separation Pays Off

Notice 2014-54, issued in 2014, established that when a 401(k) distribution includes both pre-tax and after-tax money, you can direct each component to a different destination.⁴ All disbursements scheduled at the same time are treated as a single distribution, and you can then allocate the pre-tax portion to one rollover destination and the after-tax portion to another.

The mechanics: you instruct your plan administrator to process a direct rollover and specify where each component goes. The pre-tax money (elective deferrals, employer matching contributions, and all investment earnings) rolls to a Traditional IRA or another employer plan. The after-tax contribution dollars roll directly to a Roth IRA.⁴ Because those after-tax dollars already had income tax paid on them, the Roth rollover is not a taxable event.

The Notice includes an explicit example. An employee with $80,000 in pre-tax funds and $20,000 in after-tax funds directs $80,000 to a Traditional IRA and $20,000 to a Roth IRA. Pre-tax amounts are assigned first to the direct rollover destinations, so the Traditional IRA receives entirely pre-tax money and the Roth IRA receives entirely after-tax money.³ No tax is owed on the Roth piece.

One important nuance. Earnings generated by your after-tax contributions inside the 401(k) are treated as pre-tax money. Those earnings must go to the Traditional IRA (or stay in an employer plan), not the Roth IRA, unless you are willing to pay income tax on them. The longer after-tax contributions sit in the 401(k), the more earnings accumulate in the pre-tax bucket.

This split rollover, made possible by the fact that the 401(k) is not aggregated with your IRAs, is the mechanical foundation of the Mega Backdoor Roth. It is also why a Backdoor Roth conversion from a small nondeductible IRA contribution can coexist with a large 401(k) balance: the 401(k) money is not sitting in the IRA denominator.

The Trap: Rolling Pre-Tax 401(k) Money Into an IRA

The 401(k)’s exemption from IRA aggregation vanishes the moment pre-tax 401(k) money lands in a Traditional IRA. Once that rollover is complete, those dollars become part of your IRA pool, and the pro rata rule applies to every future distribution or conversion from any of your non-Roth IRAs.¹

This is where most Backdoor Roth conversions fall apart. A taxpayer contributes $7,000 after-tax to a Traditional IRA, planning to convert it to a Roth. Then they remember the $200,000 they rolled from a prior employer’s 401(k) into a Traditional IRA years ago. Their basis is now $7,000 out of $207,000, and only about 3.4% of the conversion is tax-free. The other 96.6% is taxable.

If you want to preserve the Backdoor Roth strategy, keep pre-tax money inside employer plans whenever possible. Roll old 401(k) balances into your current employer’s plan rather than into an IRA. If pre-tax money has already landed in an IRA, many 401(k) plans will accept an incoming rollover of those dollars, effectively clearing the balance that was poisoning your pro rata fraction.

Verify Your After-Tax Basis Before You Move Money

Unlike IRA basis, which you personally track on Form 8606 and could reconstruct from your own records, your 401(k) after-tax basis lives entirely on your plan administrator’s books. When you take a distribution, the administrator reports it on Form 1099-R. Box 1 shows the gross amount, Box 2a shows the taxable portion, and Box 5 shows the employee’s after-tax contributions included in the distribution.⁵ The Box 5 figure is the documented proof that a portion of your distribution was already taxed and can move to a Roth IRA without further tax. If your after-tax basis is not properly recorded, the IRS will treat the entire distribution as taxable.

Confirm with your plan administrator that your after-tax contribution history is accurately reflected before you initiate a split rollover. Fixing errors after a 1099-R has been issued is significantly harder than correcting records beforehand.

The Three Scenarios Side by Side

The distinction between the two rules comes down to three situations:

Distribution or conversion from any non-Roth IRA. The IRA aggregation rule under Section 408(d)(2) applies. Traditional, SEP, and SIMPLE IRAs are one account. Every dollar out carries a proportional mix based on your December 31 balances.
Partial distribution from a 401(k) with mixed money. The plan’s internal pro rata rule applies. Each withdrawal includes a proportional share of pre-tax and after-tax amounts. Notice 2014-54 then lets you route each component to a different destination.
Full distribution or split direct rollover from a 401(k). Pre-tax and after-tax money can be completely separated. Pre-tax goes to a Traditional IRA or another employer plan; after-tax basis goes to a Roth IRA tax-free. IRA aggregation does not enter the picture unless you later mix the pre-tax rollover with existing IRA basis.

Keeping after-tax and pre-tax retirement money inside a 401(k) gives you more control at distribution time than holding the same mix inside IRAs. If your plan allows after-tax contributions and in-service distributions, the building blocks for the Mega Backdoor Roth are in place. If it does not, the next best move is to avoid rolling pre-tax 401(k) money into Traditional IRAs, where it will complicate every future Roth conversion.

1
Office of the Law Revision Counsel. 26 USC 408 – Individual Retirement Accounts
2
Internal Revenue Service. About Form 8606 – Nondeductible IRAs
3
Internal Revenue Service. Rollovers of After-Tax Contributions in Retirement Plans
4
Internal Revenue Service. Notice 2014-54 – Guidance on Allocation of After-Tax Amounts to Rollovers
5
Internal Revenue Service. Instructions for Forms 1099-R and 5498