How Does the Pro-Rata Rule Affect a Roth Conversion? (w/Examples) + FAQs

This article reflects federal IRS rules as of June 2026 and covers tax years 2025 and 2026. Tax law changes — confirm current figures with IRS.gov before you file. This is educational, not personal tax advice.

Quick Answer

The pro-rata rule forces the IRS to treat all your traditional, SEP, and SIMPLE IRAs as one account when you do a Roth conversion. You cannot convert only your after-tax dollars. For tax year 2025, the taxable share equals your pre-tax balance divided by your total IRA balance.

What the Pro-Rata Rule Really Means

Many people try a “backdoor Roth” to sneak after-tax money into a Roth IRA. They think they can convert only the after-tax dollars and pay no tax. The pro-rata rule blocks that move, and the surprise tax bill can run into thousands of dollars in a single year.

Here is the deeper problem. The IRS does not look at the one account you converted. It adds up every traditional, SEP, and SIMPLE IRA you own and applies a single after-tax percentage to your conversion. Roughly 60% of U.S. households own a retirement account, and the Investment Company Institute reports that traditional IRAs hold trillions of dollars — much of it pre-tax. That pre-tax money is exactly what triggers the rule.

🧮 How to calculate your exact taxable amount using a simple, copy-the-math formula.
📄 Where the pro-rata math lands on IRS Form 8606, line by line.
🚫 The seven mistakes that turn a tax-free backdoor Roth into a tax bill.
🔄 The legal “401(k) rescue” move that can wipe the rule out before December 31.
🏛️ Whether your state taxes the conversion the same way the IRS does.

Deconstructing the Rule: The Core Parts

A Roth conversion moves money from a pre-tax retirement account into a Roth IRA, where it grows tax-free. You pay income tax now so you owe nothing later. The pro-rata rule decides how much of that conversion is taxable.

The rule rests on one idea: aggregation. The IRS treats all your non-Roth IRAs as a single pool. You cannot pick the after-tax dollars out of that pool. Every dollar you convert carries the same blend of pre-tax and after-tax money.

Three pieces drive the math. First is your basis — the after-tax (nondeductible) money you already paid tax on. Second is your total pre-tax balance — deductible contributions plus all growth. Third is the December 31 value, because the calculation uses your combined IRA balance on the last day of the conversion year, not the day you converted. The 2025 Form 8606 instructions state this plainly on line 6.

Which Accounts Count in the Pool

The pro-rata pool includes traditional IRAs, SEP-IRAs, and SIMPLE-IRAs. The IRS aggregation rule treats them as one. A self-employed person with a fat SEP-IRA often discovers this too late.

What does not count is just as important. Roth IRAs, 401(k)s, 403(b)s, and other workplace plans stay out of the pool. This gap is the basis for the most powerful avoidance move, covered below. The consequence of missing this: you may assume your conversion is tax-free when half of it is taxable.

The Date That Trips People Up

The rule does not use your account balance on the conversion date. It uses the combined value of all your non-Roth IRAs as of December 31 of the conversion year, per Form 8606 line 6. A rollover you do in November still counts if it lands after the conversion.

This timing trap is real. If you convert in March but roll a $50,000 pre-tax IRA back in during December, that $50,000 re-enters the pool and inflates your taxable share. The fix: finish any pre-tax cleanup before year-end, and confirm the money has fully left your IRAs by December 31.

The Formula You Can Copy

Here is the math the IRS uses. It never changes, and you can run it by hand.

Nontaxable percentage = total after-tax basis ÷ total year-end value of all non-Roth IRAs.

Then: amount converted × nontaxable percentage = the tax-free part. The rest is taxable. The official version lives on lines 6 through 13 of the 2025 Form 8606, and the math is the same for 2026.

A Fully Worked Example (Copy the Math)

Meet Dana, age 45, in tax year 2025. She contributes the full $7,000 as a nondeductible (after-tax) contribution, planning a backdoor Roth. She forgot about an old rollover traditional IRA worth $63,000, all pre-tax.

Her total non-Roth IRA value is $7,000 + $63,000 = $70,000. Her after-tax basis is $7,000. So her nontaxable percentage is $7,000 ÷ $70,000 = 10%. When she converts the $7,000, only 10% — $700 — comes out tax-free.

The other $6,300 is taxable income. In the 24% federal bracket, that is about $1,512 in extra tax on a move she thought was tax-free. Worse, her remaining $63,000 IRA still holds the leftover $6,300 of basis, so she must track it on Form 8606 for years. For 2026, the limit rises to $7,500, but the math works identically.

Walking Through Form 8606

Form 8606, “Nondeductible IRAs,” is where you report both the contribution and the conversion. You file one with your Form 1040 by the April deadline (April 15, 2026, for tax year 2025). Skipping it can cost a $50 penalty, and you may end up paying tax twice on the same dollars.

Part I handles the pro-rata math. Line 1 is your nondeductible contribution for the year. Line 2 is your prior basis. Line 6 is the combined December 31 value of all your traditional, SEP, and SIMPLE IRAs — the line most people get wrong.

Line 8 is the amount you converted. Line 10 divides your basis by your total dollars to get the nontaxable percentage. Line 11 multiplies that percentage by the conversion to find the tax-free part, and Line 13 carries it forward. Line 18 shows the taxable amount that flows to your 1040.

A Clean Backdoor Roth on the Form

When you have no other pre-tax IRA money, the form is simple. Lines 6 through 8 effectively zero out the pre-tax pool, so your full conversion is tax-free. The 2025 instructions confirm line 16 should match line 4a of your 1040.

The misconception here is that “no pre-tax IRA” means “skip the form.” Wrong — you must still file Form 8606 to report the nondeductible contribution and lock in your basis. Skip it, and the IRS has no record that you already paid tax on those dollars.

Which Situation Applies to You?

The pro-rata rule hits people differently. Find your case below, then read the matching example.

You earn too much for a direct Roth and have no pre-tax IRA. Your backdoor Roth is clean and fully tax-free. See “Clean Backdoor Roth” above.
You have an old 401(k) rollover or SEP-IRA. The rule will tax most of your conversion. See Marcus and the “401(k) rescue” below.
You are retired and converting in chunks. You will pay tax on the pre-tax share each year, but there is no penalty. See Priya below.
You are self-employed with a SIMPLE or SEP-IRA. That balance is in the pool. Plan around it before December 31.

Three Common Scenarios

Scenario 1: The Clean Backdoor (Marcus, before cleanup)

Marcus, age 38, earns $200,000 and wants a backdoor Roth for 2025. He has a $90,000 traditional IRA from an old 401(k) rollover. Below is what happens if he converts $7,000 without fixing it first.

Marcus’s Move	The Tax Result
Contributes $7,000 after-tax, converts it	IRS pools $7,000 + $90,000 = $97,000
Expects $7,000 tax-free	Only 7.2% ($504) is tax-free
Reality check	$6,496 is taxable income

Scenario 2: The 401(k) Rescue (Marcus, after cleanup)

Marcus rolls the entire $90,000 pre-tax IRA into his employer 401(k) in October, before year-end. Workplace plans are not in the pro-rata pool, so his December 31 IRA pre-tax balance is now zero.

Marcus’s Move	The Tax Result
Rolls $90,000 IRA into 401(k)	Pre-tax IRA pool drops to $0
Contributes $7,000 after-tax, converts it	100% is tax-free
Files Form 8606	$0 taxable on the conversion

Scenario 3: The Retiree Conversion (Priya)

Priya, age 63, has a $400,000 traditional IRA with $20,000 of after-tax basis. She converts $40,000 in 2025 to fill up her lower tax bracket before required distributions begin.

Priya’s Move	The Tax Result
Basis is $20,000 of $400,000 (5%)	5% of the conversion is tax-free
Converts $40,000	$2,000 tax-free, $38,000 taxable
Remaining basis	$19,000 stays in the IRA for next time

How to Avoid the Pro-Rata Rule

The cleanest fix is the “reverse rollover.” You move your pre-tax IRA money into a current employer 401(k) or a solo 401(k), which the IRS rollover rules allow. Because plans sit outside the IRA pool, your pre-tax IRA balance drops to zero.

Timing is everything. The rollover must clear so your combined IRA balance is $0 on December 31. The consequence of bad timing: a stray dividend or a late-arriving rollover can push pre-tax money back into the pool and re-trigger the rule, as one Bogleheads case shows. Convert your new nondeductible contribution promptly, and confirm the year-end statement reads zero.

Federal vs. State: Does Your State Tax the Conversion?

The pro-rata rule is purely federal. But states decide whether to tax the taxable part of your conversion, and they do not all agree.

Federal Treatment	State Treatment
Taxable share is ordinary income, set by Form 8606	Most states follow federal taxable income
Applies to all filers nationwide	No-income-tax states (FL, TX, NV, WA) tax none of it
Tracked on Form 8606	Pennsylvania often excludes conversions for those over 59½

Never assume your state mirrors the IRS. Pennsylvania, for example, treats retirement income unusually, while Florida and Texas levy no state income tax at all. Check your state revenue agency before you convert a large amount.

Mistakes to Avoid

Forgetting an old SEP or rollover IRA. It joins the pool and taxes most of your conversion.
Using the conversion-date balance. The rule uses the December 31 value, so the wrong date gives the wrong tax.
Rolling pre-tax money back in late in the year. It re-enters the pool and re-triggers the rule.
Skipping Form 8606. You lose your basis record and may pay tax twice, plus a possible $50 penalty.
Converting before the cleanup rollover clears. A leftover balance ruins the tax-free result.
Counting your spouse’s IRA in your math. The rule is per person; never combine spouses’ accounts.
Including your 401(k) in the pool. Workplace plans are excluded, and adding them overstates your tax.

Do’s and Don’ts

Do file Form 8606 every year you make a nondeductible contribution, because it is your only proof of basis.
Do check your December 31 balance, since that is the figure the IRS actually uses.
Do consider a 401(k) rollover before converting, because it can make the conversion fully tax-free.
Do keep every Form 8606 you file, because basis tracking spans many years.
Do run the formula yourself, so the conversion holds no surprises at tax time.
Don’t assume a backdoor Roth is automatically tax-free, because hidden pre-tax IRAs change everything.
Don’t mix spouses’ accounts, because each person’s pool is separate.
Don’t convert in December without confirming rollovers cleared, because timing controls the result.
Don’t ignore SEP and SIMPLE IRAs, because they count in the pool.
Don’t rely on your broker to track basis, because that job is yours.

Pros and Cons of Converting Under the Rule

Pro: Once converted, the money grows and is withdrawn tax-free in retirement.
Pro: Roth IRAs have no required minimum distributions for the original owner.
Pro: Paying tax now can beat paying it later if your bracket rises.
Pro: A clean backdoor Roth lets high earners fund a Roth they otherwise cannot.
Pro: Conversions are not capped by income limits, unlike direct Roth contributions.
Con: The taxable share is added to this year’s income and can push you into a higher bracket.
Con: The pro-rata rule makes “isolating basis” impossible without a 401(k) move.
Con: Extra income can raise Medicare premiums (IRMAA) and phase out other tax breaks.
Con: The recordkeeping spans years and is easy to botch.
Con: You generally cannot undo a conversion, since recharacterization was repealed for conversions.

What to Do Next

List every traditional, SEP, and SIMPLE IRA you own and find each balance.
If you hold pre-tax IRA money and want a clean backdoor Roth, ask your 401(k) plan if it accepts incoming rollovers.
Complete any reverse rollover early, so your IRA pre-tax balance is $0 by December 31.
Make your nondeductible contribution and convert it, then save the confirmation.
File Form 8606 with your 1040 by April 15, 2026, for a 2025 conversion.
If your conversion is large or your accounts are complex, hire a CPA or enrolled agent — expect roughly $200 to $600 for the help.

FAQs

Does the pro-rata rule apply to every Roth conversion? Yes — if you hold any pre-tax money in a traditional, SEP, or SIMPLE IRA. If those balances are zero, the rule has nothing to tax and your conversion is fully tax-free.

What IRA balance does the rule use? The December 31 value of all your non-Roth IRAs in the conversion year. It does not use the balance on the day you converted, which surprises many filers.

Do my 401(k) and 403(b) count in the pro-rata pool? No. Workplace plans are excluded. Only traditional, SEP, and SIMPLE IRAs count, which is why rolling IRA money into a 401(k) can erase the rule.

Can I convert only my after-tax contributions? No. Aggregation blocks this. Every converted dollar carries the same pre-tax/after-tax blend across all your IRAs combined.

How much can I contribute before converting in 2026? $7,500, or $8,600 if you are 50 or older, per the IRS 2026 limits. For 2025 the limit was $7,000, or $8,000 with the catch-up.

Does my spouse’s IRA affect my conversion? No. The rule applies per person. You never combine your spouse’s IRA balances with yours when running the math.

What form reports the pro-rata calculation? Form 8606. Part I, lines 6 through 13, runs the math, and line 18 sends the taxable amount to your 1040.

What happens if I skip Form 8606? You risk a $50 penalty and may pay tax twice on the same dollars, because you lose the official record of your after-tax basis.

Can I undo a Roth conversion I regret? No. Recharacterizing a conversion was repealed under the 2017 tax law. Once converted, the conversion and its tax are permanent.

Do all states tax the conversion like the IRS? No. Most follow federal taxable income, but no-income-tax states tax none of it, and states like Pennsylvania treat retirement income differently. Check your state agency.

Is there an income limit to do a Roth conversion? No. Conversions have no income cap, unlike direct Roth contributions, which is why high earners use the backdoor strategy.

When should I hire a professional? When your conversion is large or your IRAs are complex. A CPA or enrolled agent can run the math, time the rollover, and file Form 8606 correctly, usually for $200 to $600.

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