This retirement tax calculator evaluates the taxable percentage on Backdoor Roth IRA conversions subject to IRS Form 8606 pro-rata rules. High-income earners using non-deductible Traditional IRA contributions to fund a Roth IRA often face unexpected tax bills if they maintain pre-tax dollars in Traditional, SEP, or SIMPLE IRAs.

### IRS Pro-Rata Rule Mathematical Formulas The IRS aggregates all non-Roth IRAs as of December 31 of the conversion tax year to calculate the non-taxable basis fraction:

- **Total IRA Pool ($P_{\text{total}}$):** $P_{\text{total}} = \text{preTaxBalance} + \text{conversionAmount}$. - **Non-Taxable Basis Percentage ($F_{\text{basis}}$):** $F_{\text{basis}} = (\text{basisAmount} / P_{\text{total}}) \times 100\%$. - **Non-Taxable Conversion Portion:** $C_{\text{non-tax}} = \text{conversionAmount} \times (F_{\text{basis}} / 100)$. - **Taxable Conversion Portion:** $C_{\text{tax}} = \text{conversionAmount} - C_{\text{non-tax}}$. - **Estimated Income Tax Owed:** $T_{\text{owed}} = C_{\text{tax}} \times (\text{taxBracket} / 100)$.

### Worked Numeric Example Suppose an investor makes a $7,000 non-deductible contribution to a Traditional IRA and converts the full $7,000 to a Roth IRA, but also owns a rollover Traditional IRA with a $28,000 pre-tax balance in a 24% tax bracket:

- **Conversion Amount:** $7,000 - **After-Tax Basis:** $7,000 - **Pre-Tax Year-End Balance:** $28,000 - **Tax Bracket:** 24%

**Step-by-Step Evaluation:** 1. **Total IRA Pool:** $28,000 + 7,000 = \$35,000$. 2. **Non-Taxable Percentage:** $(7,000 / 35,000) \times 100 = 20.0\%$. 3. **Non-Taxable Amount:** $7,000 \times 0.20 = \$1,400$. 4. **Taxable Amount:** $7,000 - 1,400 = \$5,600$ (80% of the conversion is taxable). 5. **Estimated Tax Due:** $5,600 \times 0.24 = \$1,344$.

### Client-Side Processing Note All calculations execute in client JavaScript. No retirement balances, tax records, or portfolio entries are transmitted across any network.