Federal Backdoor Roth Conversion Calculator

How this calculator works

Section 408A of the Internal Revenue Code creates the Roth IRA: contribute after-tax, grow tax-free, withdraw tax-free in retirement (subject to ordering and 5-year rules). The catch is § 408A(c)(3), which phases out direct Roth eligibility above moderate income thresholds — by design, Roth IRAs were not meant for high earners. The "Backdoor Roth" is the legal workaround Congress declined to ban in the 2017 TCJA and again in the 2021 BBB reconciliation bill. It works by contributing nondeductibly to a Traditional IRA (no income limit applies to the nondeductible contribution itself) and immediately converting to Roth under § 408A(d)(3). Done correctly with a clean Traditional IRA pot, the conversion is fully tax-free and the high earner captures the same Roth treatment as a non-phased-out direct contributor.

The trap is IRC § 408(d)(2) — the pro-rata aggregation rule. This calculator surfaces the trap, computes the cleanout strategy that escapes it, and quantifies the parallel mega backdoor Roth path under § 401(a) that is not subject to IRA pro-rata at all.

The 2026 Roth IRA income limits

2026 estimated MAGI phaseout bands under § 408A(c)(3) (inflation-projected from the 2025 baseline; the IRS releases final figures in late 2025):

• Single / Head of Household: $150,000 - $165,000. Full direct Roth contribution below $150K, partial in the band, $0 above $165K.
• Married Filing Jointly: $236,000 - $246,000.
• Married Filing Separately: $0 - $10,000 — the most punitive phaseout in the Code. MFS taxpayers with any meaningful income are fully shut out of direct Roth contribution.

The income limit applies only to the Roth contribution itself. The nondeductible Traditional IRA contribution that starts the Backdoor Roth has no income limit (the deduction phaseout under § 219(g) is a separate test that only matters if you wanted the deduction — which you do not in a Backdoor Roth).

The four-step Backdoor Roth

1. Contribute nondeductibly to a Traditional IRA. Up to the § 219(b) annual limit — $7,500 base plus $1,000 catch-up if age 50+ — in 2026 estimated. No income limit applies to the nondeductible contribution. The contribution is "nondeductible" because you choose not to claim the deduction (or because the § 219(g) workplace-plan phaseout already eliminated it).

2. Immediately convert the Traditional IRA balance to a Roth IRA. Under § 408A(d)(3), any IRA-to-Roth conversion is permitted regardless of income. Same-day conversion is the textbook approach because it minimizes any taxable gains between contribution and conversion.

3. Report on Form 8606. Part I tracks the cumulative basis from your nondeductible contribution; Part II reports the conversion and computes the taxable portion under § 408(d)(2). Failure to file forfeits basis tracking.

4. Pay any tax owed. The taxable portion is zero if your Traditional IRA pot is clean (no prior pretax balance). If you have pretax IRA balance, the § 408(d)(2) pro-rata rule taxes a proportional share of the conversion at ordinary rates.

The pro-rata rule — the most-missed Backdoor Roth pitfall

IRC § 408(d)(2) aggregates all your Traditional, SEP, and SIMPLE IRA balances into a single pot for purposes of taxing any IRA-to-Roth conversion. The taxable portion is:

taxable portion = conversion amount × (pretax balance / total balance)

Where total balance = pretax + basis + any new nondeductible contribution. The IRS uses the December 31 balance of the conversion year, so timing matters — a rollover or other balance change late in the year still counts.

Critical aggregation rules:

• Aggregated: Traditional IRA, SEP-IRA, SIMPLE IRA across all custodians and accounts the taxpayer owns.
• NOT aggregated: 401(k), 403(b), 457(b), Roth IRA, inherited IRA in a beneficiary's name.
• NOT aggregated: Spousal IRAs. Each spouse has their own pro-rata pot.

Worked example 1 — clean pot, $0 conversion tax

A 42-year-old MFJ taxpayer with $400,000 MAGI (above the $236K phaseout start) has $0 in any Traditional/SEP/SIMPLE IRA. They contribute $7,500 nondeductibly to a Traditional IRA and convert same-day to Roth.

• Total Traditional IRA pot: $0 pretax + $0 basis + $7,500 contribution = $7,500.
• Pro-rata taxable percentage: $0 / $7,500 = 0%.
• Taxable portion of conversion: $7,500 × 0% = $0.
• Tax owed at 32% marginal rate: $0.
• Net Roth amount captured: $7,500.

This is the textbook Backdoor Roth. No tax, full $7,500 in the Roth IRA, repeat annually until Congress changes the law or income drops below the phaseout.

Worked example 2 — $93,000 pretax pot poisons the conversion

Same 42-year-old MFJ taxpayer, but they have $93,000 in a Traditional IRA from a prior 401(k) rollover (deductible contributions plus growth). They contribute $7,500 nondeductibly and convert same-day.

• Total Traditional IRA pot: $93,000 pretax + $0 basis + $7,500 contribution = $100,500.
• Pro-rata taxable percentage: $93,000 / $100,500 = 92.5%.
• Taxable portion of conversion: $7,500 × 92.5% = $6,940 (approximately).
• Tax owed at 32% marginal rate: $6,940 × 32% = $2,221.
• Net Roth amount captured: $7,500 − $2,221 = $5,279.

The pretax pot poisoned the conversion. Of the $7,500 they thought they were moving into Roth tax-free, $2,221 disappeared in tax. The taxpayer captured only ~70% of the intended Roth amount. Most Backdoor Roth surprises look like this — the taxpayer reads about the strategy, contributes and converts, and then sees the 1099-R at tax time showing 92.5% of the conversion is taxable.

Worked example 3 — cleanout eliminates the pro-rata trap

Same taxpayer, same $93,000 pretax Traditional IRA balance — but before contributing nondeductibly, they roll the entire $93,000 into their workplace 401(k) (the "cleanout"). The 401(k) accepts incoming IRA rollovers; the rollover completes November 15 of the conversion year. Then they contribute $7,500 nondeductibly to the now-empty Traditional IRA and convert same-day.

• Total Traditional IRA pot: $0 pretax (cleanout removed it) + $0 basis + $7,500 contribution = $7,500.
• Pro-rata taxable percentage: $0 / $7,500 = 0%.
• Taxable portion of conversion: $0.
• Tax owed at 32% marginal rate: $0.
• Net Roth amount captured: $7,500.

The cleanout strategy converted Example 2 into Example 1. The $93,000 pretax balance is now in the 401(k) (still tax-deferred, just at a different custodian) and is no longer in the § 408(d)(2) pro-rata pot because 401(k) balances are not aggregated. The Backdoor Roth converts fully tax-free.

Cleanout requires (a) a workplace 401(k) or 403(b) plan that accepts incoming rollovers from IRAs (most do; verify with the plan administrator) and (b) timing — the rollover must complete before December 31 of the conversion year because the pro-rata calculation uses the December 31 balance. For self-employed taxpayers without a separate 401(k), establishing an individual 401(k) (solo 401(k)) that accepts SEP-IRA rollovers is the equivalent move.

Worked example 4 — mega backdoor Roth captures ~$30,000

Same 42-year-old, MFJ, $400,000 MAGI. Instead of (or in addition to) the standard Backdoor Roth, they explore the mega backdoor via their workplace 401(k). The plan allows after-tax contributions and in-plan Roth conversions (verify with the plan administrator — not all plans do). In 2026 estimated:

• § 415(c) total annual additions limit: $70,000.
• Employee deferral (§ 402(g)): ~$23,500.
• Employer match: assume $10,000.
• Remaining headroom for after-tax contributions: $70,000 − $23,500 − $10,000 = $36,500.

The taxpayer contributes $36,500 after-tax to the 401(k) and immediately converts the after-tax balance to Roth (in-plan or via rollover to a Roth IRA). The conversion is tax-free because the contribution was already after-tax (no pretax basis to allocate). The § 408(d)(2) IRA pro-rata rule does not apply — the after-tax 401(k) balance is segregated from any Traditional IRA pot.

• Total Roth captured: $36,500.
• Conversion tax: $0 (or minimal, depending on intra-period growth).

The mega backdoor captures ~5x the standard backdoor in a single year. The catch is plan support — many 401(k) plans do not allow after-tax contributions, and of those that do, fewer support in-plan Roth conversion or in-service distributions. Verify both features with the plan administrator before relying on the strategy. IRS Notice 2014-54 is the controlling authority for the after-tax-to-Roth allocation.

The cleanout strategy in depth

The cleanout is the single most important Backdoor Roth planning move for any taxpayer with material pretax Traditional IRA balance. It works by exploiting the § 408(d)(2) carve-out for workplace plans: 401(k), 403(b), and 457(b) balances are not aggregated with IRA balances for conversion taxation. Rolling pretax IRA into a 401(k) moves it out of the pro-rata pot.

Execution checklist:

1. Verify the workplace 401(k) plan accepts incoming IRA rollovers. Most do, but some restrict to rollovers from prior 401(k)s. The plan's Summary Plan Description (SPD) or the recordkeeper's portal will state the rules.

2. Initiate a direct rollover (custodian-to-custodian, never check in hand) from the Traditional/SEP/SIMPLE IRA to the 401(k). The 60-day indirect rollover rule under § 408(d)(3) creates needless risk; always use the direct method.

3. Time the rollover to complete before December 31 of the conversion year. The pro-rata calculation uses the December 31 balance.

4. SEP-IRA and SIMPLE IRA balances can also be cleaned out, but SIMPLE balances face an additional restriction: they cannot be rolled into a 401(k) until two years after the first SIMPLE contribution, under § 72(t)(6).

After the cleanout, your Traditional IRA is empty, you contribute nondeductibly, and you convert. The cycle repeats annually until either (a) you exit the Roth phaseout band naturally (income drops below the threshold) or (b) Congress changes the law.

Form 8606 reporting — non-negotiable

Form 8606 (Nondeductible IRAs) is mandatory for any nondeductible Traditional IRA contribution AND for any IRA-to-Roth conversion. Part I tracks the cumulative basis from your nondeductible contributions; Part II reports the current-year conversion and computes the taxable portion under § 408(d)(2).

Without filed 8606s, the IRS has no record that you ever made nondeductible (already-taxed) contributions — and they will treat your entire IRA balance as pretax, taxing the full Backdoor Roth conversion at ordinary rates. The statutory penalty for failure to file is $50 per year, but the far larger cost is the lost basis: years of nondeductible contributions you can never recover the basis for at distribution. File Form 8606 with your annual Form 1040 every year you make a nondeductible contribution or conversion, and keep copies indefinitely — IRA basis follows the account across decades and you may need to prove the basis 30 years from now.

The conversion 5-year rule

§ 408A(d)(2)(B) imposes a 5-year clock on each Roth conversion — separate from the general 5-year rule on Roth earnings withdrawal. Each conversion year has its own clock starting January 1 of the conversion year. Withdrawals of converted principal before the 5-year clock expires trigger the 10% early-withdrawal penalty under § 72(t), even though the conversion itself was already taxed.

After age 59½, the 10% penalty no longer applies regardless of the 5-year clock — so the conversion 5-year rule only matters for pre-59½ withdrawals. Practical implication: Backdoor Roth converters under 59½ should not plan to spend converted principal within 5 years of each conversion. A Backdoor Roth done in 2026 has a clock through January 1, 2031.

Step-doctrine concerns — largely settled post-2017

Pre-2017, some practitioners worried the IRS could invoke the substance-over-form doctrine to collapse the nondeductible contribution + immediate conversion into a direct Roth contribution (which would be disallowed by the income limit). The 2017 TCJA's Joint Committee on Taxation report language explicitly acknowledged the Backdoor Roth and mega backdoor Roth as legal under current law. Congress declined to ban them despite multiple draft provisions. The IRS has never publicly challenged a Backdoor Roth on step-doctrine grounds.

The 2021 Build Back Better reconciliation bill would have banned Backdoor Roth conversions starting 2022, but BBB was not enacted. As of 2026 the strategy remains explicitly legal; the only material risk is a future Congressional ban.

Common errors

• Forgetting SEP/SIMPLE IRA in the pro-rata pot. Self-employed taxpayers with SEP-IRA balances often miss that SEP is aggregated under § 408(d)(2). A $200,000 SEP-IRA balance combined with a $7,500 nondeductible Traditional IRA contribution makes the conversion 96.4% taxable.
• Partial cleanout. Rolling some but not all of the pretax IRA balance into the 401(k) still leaves a poisoned pot — pro-rata applies to whatever remains.
• Timing the cleanout wrong. The rollover must complete by December 31. A January-of-the-following-year rollover doesn't help the prior year's conversion calculation.
• Not filing Form 8606. Forfeits basis tracking; IRS taxes the conversion as fully pretax. The single most-common Backdoor Roth filing error.
• Mixing conversion years. Each annual conversion has its own 5-year clock. Withdrawing the wrong year's principal before its clock expires triggers the 10% penalty.
• Spousal IRA confusion. Spouses have separate pro-rata pots. One spouse's pretax IRA balance does not poison the other spouse's Backdoor Roth — but each must do their own Form 8606.
• Mega backdoor without plan support. Many 401(k) plans do not allow after-tax contributions, and of those that do, fewer support in-plan conversion. Verify both features before relying on the strategy.

This calculator is a planning tool, not advice. The 2026 contribution limits and income phaseout bands are inflation-projected estimates pending the IRS's final late-2025 release; verify against the published figures before relying on them for tax-filing purposes. The § 408(d)(2) pro-rata math is mechanical, but the strategy interactions — cleanout timing, spousal IRA mechanics, SEP-IRA aggregation, mega backdoor plan support — benefit from review by a CPA or financial planner who handles Backdoor Roth planning as a regular practice. Tools, not advice.