Summary of what this blog covers

• A plain-English tour of IRA basis, the aggregation rule, and the December 31 snapshot that decides how much of your Roth conversion is taxable.
• Why that old rollover IRA quietly sabotages a “tax-free” backdoor Roth.
• Practical planning moves: isolate basis, roll pre-tax into a 401(k), time the conversion, and file a clean Form 8606 plus a quick worksheet and decision tree.

First principles: five plain-English definitions

• Basis (after-tax money). These are traditional IRA dollars you already paid income tax on, usually from nondeductible IRA contributions you reported on Form 8606. Basis is valuable because it can move to Roth without more tax if you isolate it.
• Pre-tax dollars. Deductible IRA contributions, employer plan rollovers, and all IRA earnings you haven’t paid tax on.
• Aggregation rule. For conversions, the IRS treats all your traditional, SEP, and SIMPLE IRAs as one pot. It does not matter which account you touched. Employer plans like 401(k)/403(b)/governmental 457(b) live outside this pot.
• Pro rata rule. The tax-free slice of any conversion equals: (Total IRA basis ÷ [Dec 31 value of all IRAs + distributions during the year]) × Amount converted. Everything else in that conversion is taxable.
• December 31 snapshot. The IRS uses your year-end IRA value for the fraction. Empty pre-tax dollars into a 401(k) before December 31 and the denominator shrinks in your favor.

Two “aha” ideas that unlock everything

Aha #1: Aggregation is why your rollover IRA matters

You drop a $6,500 nondeductible contribution into IRA A. Great. But you also have $93,500 pre-tax in rollover IRA B. You try to convert “just the $6,500.” The IRS says: nice try. Your tax-free slice is $6,500 ÷ $100,000 = 6.5% of whatever you convert. The other 93.5% is taxable income.

Aha #2: December 31 is the referee

You can contribute in January, convert in May, and still fix your denominator in October if you roll pre-tax IRA dollars into a 401(k) before December 31. Same contribution, different denominator, very different tax result.

The math in 90 seconds (and three realities people miss)

Reality 1: Basis lives and dies on Form 8606. Part I tracks nondeductible contributions and carries basis forward. Part II computes the taxable portion of your conversion. Missing or messy Form 8606 is the number-one reason backdoor Roths get over-taxed.

Reality 2: Which IRA you touch doesn’t matter. Convert from IRA A and ignore IRA B? The IRS still looks at A + B + C at December 31.

Reality 3: 401(k)s are your shelter. Balances in 401(k)/403(b)/457(b) do not count in the pro rata denominator. That is your lever.

Napkin example

• Basis on Form 8606: $6,500
• Total value across all IRAs at year-end: $100,000
• Convert $6,500 to Roth in June

Tax-free = $6,500 ÷ $100,000 × $6,500 = $422
Taxable = $6,078
Not the vibe. Fixable, though.

Why the rollover IRA changes everything and how to disarm it

The problem

Your rollover IRA is mostly pre-tax. It inflates the denominator and dilutes your basis.

The classic solution: isolate basis before year-end

1. Roll pre-tax IRA dollars into your employer 401(k) (or into a solo 401(k) if you have self-employment income), assuming the plan accepts roll-ins. Plans typically accept pre-tax dollars and do not accept IRA basis.
2. What remains in your IRA is mostly basis.
3. Convert that leftover basis to Roth. With little pre-tax left, the conversion is largely tax-free.

Timing tip: Complete the roll-in by December 31 of the conversion year to shrink the denominator for that year’s fraction.

Edge cases to watch

• SEP and SIMPLE IRAs count in aggregation. SIMPLE IRAs usually must be open two years before rolling to a 401(k).
• If your employer plan doesn’t accept roll-ins, consider a solo 401(k) (if eligible) or delay the backdoor Roth until you can isolate basis.
• Keep an eye on stray balances. A forgotten SEP from two years ago will poison the fraction if it stays in IRA land.

Clean examples you can copy

Example 1: Backdoor Roth without cleanup

You make a $7,000 nondeductible IRA contribution for 2026. You also hold a $93,000 rollover IRA. You convert $7,000 to Roth.
Denominator = $93,000 + $7,000 = $100,000
Tax-free = $7,000 ÷ $100,000 × $7,000 = $490
Taxable = $6,510

Example 2: Isolate basis, then convert

Before year-end, you roll $93,000 pre-tax from the rollover IRA into your 401(k). Your IRA now equals about $7,000 of basis (plus tiny earnings). You convert the $7,000 to Roth.
Denominator ≈ $7,000
Tax-free ≈ $7,000
Taxable ≈ $0 (ignoring small earnings)

Same contribution. Different denominator. Problem solved.

Reporting trail: the paperwork that keeps audits calm

• Form 1099-R (from your custodian) reports the IRA distribution you used for the conversion. It does not decide taxability by itself.
• Form 8606 is the scoreboard. You report nondeductible contributions (Part I) and compute the taxable portion of your conversion (Part II). That is where the pro rata math lives.
• Form 1040 / Schedule 1 matters only if you deducted a traditional IRA contribution (not typical in backdoor Roths). Backdoor Roths use nondeductible contributions; the value is tax-free Roth growth later, not a current deduction.

Five planning moves that actually change outcomes

1. Empty the IRA before Dec 31. Move all pre-tax IRA dollars to a 401(k) if the plan allows. The fraction flips.
2. Coordinate the calendar. Contribution month, conversion month, and the Dec 31 snapshot can differ. The IRS still uses year-end values to compute the fraction for that year’s conversions. Translation: you can convert in March and still fix the denominator in October.
3. Respect earnings drift. Deposit $7,000 in January and it grows to $7,400 by March. If you convert only $7,000, you leave $400 of pre-tax earnings behind that will dilute a future conversion. Many owners convert the entire position promptly to minimize taxable creep.
4. Include SEP/SIMPLE balances in your map. Cleaned up the rollover IRA? Great. Do not forget last year’s SEP. It lives in the denominator until you move it to a 401(k) or wait out the two-year SIMPLE clock.
5. RMD years have an order of operations. Once required minimum distributions apply, you must take the RMD first before any Roth conversion from that IRA. The RMD amount cannot be converted. Order matters.

Advanced notes that separate tidy returns from messy ones

• Recharacterization rules. You can no longer undo a Roth conversion by recharacterizing it back to traditional. That ended years ago. You can still recharacterize contributions (Roth ↔ traditional), but not conversions. Plan before you press convert.
• Married couples track basis separately. Basis is individual. Spouse A’s clean backdoor Roth is unaffected by Spouse B’s rollover IRA. Each spouse files their own Form 8606.
• Community-property states. Community rules affect ownership, but the pro rata math still aggregates only the IRAs owned by that individual.
• “Step transaction” anxiety. There is no published waiting period between a nondeductible contribution and a conversion. Many taxpayers contribute and convert promptly to reduce earnings. The key is consistent 8606 reporting and proof the traditional contribution was nondeductible.
• Mega backdoor alternatives. If your 401(k) supports after-tax contributions and in-plan Roth rollovers or in-service distributions, a Mega Backdoor Roth can bypass IRA pro rata altogether. This is plan-specific and worth asking about.

Decision path you can run in five minutes

1. Do you have any pre-tax money in traditional, SEP, or SIMPLE IRAs?
   • No: Proceed. Your fraction is nearly 100% basis.
   • Yes: Continue.
2. Can your 401(k) accept roll-ins of pre-tax IRA dollars?
   • Yes: Roll pre-tax out before Dec 31. Convert the basis that remains.
   • No: Consider a solo 401(k) if you have self-employment income, or postpone the backdoor Roth.
3. Any SEP/SIMPLE balances?
   • Yes: Include them. Roll after the SIMPLE two-year clock if required.
4. Is your Form 8606 complete for every nondeductible year?
   • No: Reconstruct it now. Missing basis equals paying tax twice.
5. Any other portfolio moves this year?
   • Yes: Coordinate conversion income with broader tax planning so you avoid tripping a phaseout or a higher bracket you care about.

“Show me the numbers” mini-worksheet

1. Prior-year basis (Form 8606, line 14): ______
2. + Current-year nondeductible contribution: ______
3. = Total basis this year: ______
4. Dec 31 value of all traditional/SEP/SIMPLE IRAs: ______
5. + Total distributions/conversions this year: ______
6. Pro rata fraction: Basis ÷ (Line 4 + Line 5) = ______ %
7. Tax-free part of this year’s conversion: Fraction × Amount converted = ______
8. Taxable part: Conversion − Tax-free = ______

Run it twice: once before moving pre-tax to a 401(k), and again after. The gap is your “aha.”

Troubleshooting: when numbers refuse to behave

The 401(k) won’t accept roll-ins.
Options: Open a solo 401(k) if you have even modest 1099 income and set it to accept roll-ins. Delay the backdoor Roth and build taxable savings or use in-plan Roth at work if available. Ask HR about plan amendments; many employers add roll-ins when enough employees ask.

I forgot Form 8606 in prior years.
File the missing forms now to establish basis. Then convert. Do not compound the error by moving ahead without it.

I converted already and just learned about the rollover IRA.
The conversion stands. Pay tax according to the fraction this year, then fix structure for future years.

Earnings popped before I could convert.
Next time, convert the entire position quickly. Or accept a small taxable amount this year and reset the denominator for next year.

You don’t have a Roth problem

You have a bucket problem, too much pre-tax water diluting your clean after-tax dye. Let’s fix the bucket, then take the right-sized scoop.

Book an IRA Structure Check with Insogna. We’ll isolate basis, line up 401(k) roll-ins, clean up Form 8606, and schedule a Roth conversion that fits your cash flow and your bracket. Clear plan. Confident decision.

Frequently asked questions