My wife and I have a combined household

Answer

I'll work through this systematically — first load the skill's reference cards for tax-shell rules, then go to the IS pages so every number and rule has a citation.

Quick frame: I read the IS pages on retirement accounts this turn and pulled the 2026 figures from the IS cost-of-living table. The rules below are what those pages say; whether each move fits your situation is your call (or your CPA's). I'm laying out mechanics, not telling you what to do.

## 1. Standard Backdoor oth IA — mechanics

What the IS pages actually say
- The IS page on IA contribution limits spells out the workaround: "oth IA contributions might be limited if your income exceeds a certain level." · https://www.irs.gov/retirement-plans/plan-participant-employee/retirement-topics-ira-contribution-limits · accessed 2026-09-19
- The IS FAQ explicitly routes the conversion step to Form 8606: "you should use Form 8606 to report amounts that you converted from a traditional IA, a SEP, or Simple IA to a oth IA." · https://www.irs.gov/retirement-plans/retirement-plans-faqs-regarding-iras · accessed 2026-09-19
- Form 8606 (2025) instructions split the form into three parts: Part I = nondeductible contributions to Traditional IAs and distributions from Traditional IAs; Part II = conversions from Traditional IA to oth IA; Part III = distributions from oth IAs. · https://www.irs.gov/instructions/i8606 · accessed 2026-09-19

Sequence as the form lays it out
1. Make a **nondeductible** contribution to a Traditional IA. For 2026 the per-person limit is $7,500 ($8,600 if age 50+), combined across all your Traditional and oth IAs for the year. Source: IS COLA table · https://www.irs.gov/retirement-plans/cola-increases-for-dollar-limitations-on-benefits-and-contributions · accessed 2026-09-19
- Your $420K MFJ household income is past the 2026 MFJ oth IA direct-contribution phase-out range (the IS I-2025-111 announcement confirms the range was raised for 2026; the Single/HoH line $153,000–$168,000 is read directly off that page, the MFJ upper/lower bounds are on the same page but were cut off in this turn — confirm in the full I-2025-111 before filing) · https://www.irs.gov/newsroom/401k-limit-increases-to-24500-for-2026-ira-limit-increases-to-7500 · accessed 2026-09-19. Direct oth contribution is therefore not available; the Backdoor is the standard high-earner workaround.
2. Convert that Traditional IA balance to a oth IA. IS treats the conversion as a taxable event for any pre-tax amount, reported on Form 8606 Part II (same FAQ link above).
3. File Form 8606 with your Form 1040 each year:
- **Part I** reports the nondeductible contribution (your basis).
- **Part II** reports the conversion and the taxable portion.
- **unning basis** carries forward year-to-year via the Total Basis Chart in the Form 8606 instructions. This is how the IS avoids taxing you twice when you eventually take oth distributions.

The mechanics themselves are not controversial. The danger lives in step 2 — the pro-rata rule.

## 2. The Pro-ata ule — and why your $65K changes the picture

The aggregation rule: when you convert, all your Traditional IAs + SEP IAs + SIMPLE IAs are treated as one combined pool. oth IAs are not in the pool. The taxable fraction of any conversion is

**Taxable fraction = (total IA value on 12/31 − total after-tax basis across all those IAs) ÷ total IA value**

Sources: IS Pub 590-A covers IA contributions and the aggregation/conversion rules · https://www.irs.gov/publications/p590a · accessed 2026-09-19; Form 8606 instructions lay out basis tracking and reporting · https://www.irs.gov/instructions/i8606 · accessed 2026-09-19. The pro-rata arithmetic itself sits in Form 8606 Part I; the page I read this turn confirmed the form's structure (Part I = nondeductible contributions / distributions) but the specific worksheet line text was beyond what came back in this turn — pull the full Form 8606 instructions when you file.

Worked example using your numbers (illustrative, ignores intra-year earnings):
- Pre-tax IA balance on 12/31: $65,000 (你说)
- New nondeductible contribution: $7,500 (2026 limit)
- Total Traditional IA pool: $72,500
- After-tax basis: $7,500
- Taxable fraction = ($72,500 − $7,500) / $72,500 ≈ **89.7%**
- On a $7,500 conversion: **~$6,725 is taxable** as ordinary income, ~$775 is non-taxable return of basis

You can shrink the dollar amount converted, but you cannot shrink the fraction — it applies to any size conversion while those pre-tax balances sit in any Traditional / SEP / SIMPLE IA in your name.

The reverse-rollover fix:
- If your current 401(k) accepts incoming rollovers (most do), move the $65,000 from the ollover Traditional IA into the 401(k)'s pre-tax side. After the move, your only Traditional IA balance is the $7,500 you just contributed (or zero if you convert right after contributing).
- Pro-rata then sees total = $7,500, basis = $7,500, taxable fraction = 0%. The conversion is effectively tax-free.
- Two practical caveats: (1) once Traditional IA money is inside a 401(k), getting it back out to a Traditional IA is rarely a clean move; (2) some plans restrict in-service withdrawals on rolled-in balances. Both are plan-specific — confirm with the Summary Plan Description or plan administrator, not from IS guidance.

Your wife's side: with zero Traditional / SEP / SIMPLE IAs, there is no aggregation pool — the Backdoor is mechanically clean for her. She still files Form 8606 each year with Part I = $7,500 nondeductible contribution and Part II = $7,500 conversion.

## 3. Mega Backdoor oth — Notice 2014-54 and the §415(c) ceiling

The mechanism:
- Your 401(k) lets you make **after-tax (non-oth)** employee contributions above the pre-tax/oth deferral limit. IS Notice 2014-54 then allows a single distribution to be split: pre-tax dollars go to a Traditional IA (or another pre-tax destination), after-tax dollars (without their earnings) go directly to a oth IA. · https://www.irs.gov/retirement-plans/rollovers-of-after-tax-contributions-in-retirement-plans · accessed 2026-09-19
- IS page, verbatim: "Distributions sent to multiple destinations at the same time are treated as a single distribution for allocating pretax and after-tax amounts (Notice 2014-54). This means you can roll over all your pretax amounts to a traditional IA or retirement plan and all your after-tax amounts to a different destination, such as a oth IA." Same UL.
- Same page: "after-tax contributions can be rolled over to a oth IA without also including earnings."
- Same page: "No, you can't take a distribution of only the after-tax amounts and leave the rest in the plan." So the in-plan conversion either converts the entire after-tax subaccount, or you take a full distribution and split externally per Notice 2014-54.
- Your plan offers the cleaner **in-plan** variant (automated in-plan oth conversion of the after-tax subaccount); same IS framework, the conversion just happens inside the 401(k) trust.

The §415(c) ceiling:
- §415(c)(1)(A) caps **annual additions** — employee + employer contributions counted together — that can be allocated to your 401(k) account in a year. For 2026 the cap is **$72,000** (excluding catch-up). Source: IS COLA table, row "Defined contribution plan limit: $72,000" for 2026 · https://www.irs.gov/retirement-plans/cola-increases-for-dollar-limitations-on-benefits-and-contributions · accessed 2026-09-19.
- "Annual additions" include your pre-tax deferral, your oth 401(k) deferral, your non-oth after-tax contributions, and employer contributions (match, profit-sharing, etc.).
- **oom for after-tax = $72,000 − (your pre-tax/oth deferral + employer contributions for the year).** Employer contribution is a number only your plan knows.

How it stacks with the Backdoor oth:
- The Backdoor oth lives in the IA family; the Mega Backdoor lives in the 401(k). They are separate silos for §415(c), so they don't compete for the same dollar cap.
- But the Backdoor oth is still subject to **pro-rata across all your Traditional/SEP/SIMPLE IAs**. The Mega Backdoor is not — after-tax 401(k) money is outside the IA aggregation pool.
- Practical implication if you reverse-rollover the $65K first: pro-rata disappears on the IA side, and the Mega Backdoor gets a clean after-tax → oth path on the 401(k) side. The two tracks then stack without interfering.

What the IS page does NOT tell you (plan-specific, must be verified from the SPD or plan admin):
- Whether your plan actually permits **non-oth after-tax** employee contributions (you said yes — confirm in the SPD).
- Whether the **in-plan oth conversion** is reachable while you are still employed (in-service) and applies to the **after-tax subaccount** specifically (you said yes — confirm in the SPD section on in-plan oth conversions or with the recordkeeper).
- The actual employer contribution number, which determines the dollar room left for after-tax under the $72,000 cap.

## 4. Side-by-side comparison

Five slots per the skill's account-card format. No verdict — each row is just what the IS page (or the plan document, where flagged) says.

| | Who can use it | How the money goes in | Annual ceiling | When can you take it out | Where tax lands |
| --- | --- | --- | --- | --- | --- |
| **Backdoor oth IA** | Anyone with earned income whose MAGI would phase out a direct oth contribution (2026 MFJ phase-out range on IS I-2025-111). | Nondeductible Traditional IA contribution, then convert to oth IA. Basis tracked on Form 8606 Part I; conversion reported on Part II. | $7,500 per person, $8,600 if 50+ (2026). Combined cap across all your Traditional + oth IAs. | oth IA rules apply after conversion: qualified distributions are tax-free if the 5-year rule and a qualifying event (age 59½, death, disability, first-time home purchase up to $10,000) are met. | Pre-tax portion is taxed at conversion (subject to pro-rata). Qualified oth distributions are tax-free. |
| **Pre-tax 401(k)** | Employees whose plan offers it. You both already max this. | Pre-tax payroll deferral; reduces current-year taxable income. | $24,500 elective deferral per person (2026); + $8,000 catch-up if 50+, + $11,250 super catch-up if 60–63. | Generally age 59½ to avoid the 10% additional tax; plan documents control in-service availability and loan options. | Income tax at withdrawal on both contributions and earnings. |
| **Mega Backdoor oth** (after-tax 401(k) → oth) | Employees whose 401(k) plan permits **both** (a) non-oth after-tax employee contributions **and** (b) in-plan oth conversion or in-service distribution/rollover to a oth IA. IS does not require plans to offer either — plan document governs. | After-tax dollars go into the 401(k) after-tax subaccount; convert in-plan or split-rollover per Notice 2014-54. After-tax basis converts tax-free; earnings on that basis convert as ordinary income. | §415(c) cap: **$72,000** per person (2026), including pre-tax/oth deferral + employer contributions + after-tax. On top of that, + $8,000 catch-up (or $11,250 if 60–63) under separate 402(g) rules. oom for after-tax = $72,000 − (your deferrals + employer). | Generally age 59½ to avoid the 10% additional tax; in-plan oth assets follow the oth 5-year rule for tax-free qualified distributions. | After-tax basis converts tax-free; earnings convert at ordinary income rates. Qualified oth distributions from the oth subaccount are tax-free. |

Sources for the limits in this table: IS COLA table (2026 dollar limits) · https://www.irs.gov/retirement-plans/cola-increases-for-dollar-limitations-on-benefits-and-contributions · accessed 2026-09-19 · IS Form 8606 (2025) instructions · https://www.irs.gov/instructions/i8606 · accessed 2026-09-19 · IS I-2025-111 · https://www.irs.gov/newsroom/401k-limit-increases-to-24500-for-2026-ira-limit-increases-to-7500 · accessed 2026-09-19 · IS ollovers of after-tax contributions · https://www.irs.gov/retirement-plans/rollovers-of-after-tax-contributions-in-retirement-plans · accessed 2026-09-19.

## Three facts that flip the order

These change which row becomes your priority year-by-year. I won't rank them for you.

1. **How soon you need the money.** oth IA conversions have a 5-year clock for the 10% penalty, and 401(k) money has its own withdrawal rules. Pick a sequence that doesn't trap dollars you'll need before age 59½.
2. **What your 401(k) plan actually permits.** After-tax contributions, in-plan oth conversion, and in-service withdrawal options are plan-level decisions; the IS doesn't mandate them. Confirm each in the Summary Plan Description before counting on the Mega Backdoor.
3. **What reverse-rollover does to your flexibility.** Once Traditional IA money is inside the 401(k), getting it back out to a Traditional IA is rarely a clean move; some plans also restrict in-service withdrawals of rolled-in balances. The pro-rata cure is real, but it's effectively a one-way door.

## What I didn't directly verify this turn

- **The 2026 MFJ oth IA contribution phase-out upper and lower bounds.** The Single/HoH line ($153,000–$168,000) is on the IS I-2025-111 page I read; the MFJ range is on the same announcement but was cut off in this turn. Pull the full I-2025-111 (or Pub 590-A) before relying on the number.
- **The full pro-rata arithmetic worksheet text in Form 8606 Part I and Pub 590-A.** The pages I read this turn confirmed the form's structure and confirmed Pub 590-A covers these rules, but the specific worksheet lines were beyond what came back. The aggregation rule itself is settled IC §408(d)(2) guidance; the worksheet is how you report it — use the most recent Form 8606 instructions when you file.
- **Your "$23,000 each" pre-tax 401(k) deferral.** The IS elective deferral was $23,000 in 2024, $23,500 in 2025, and $24,500 in 2026. If you're maxing for 2026, the IS number to compare to is $24,500.

If you want to keep going, the next concrete step is your Summary Plan Description (the three plan-specific items above) and a CPA on the year-by-year sequence — both outside what I can verify from IS pages alone.