The taxable amount in a Roth conversion

A Roth conversion moves eligible retirement money into a Roth IRA. The tax question is not simply how much cash moved. It is how much of the conversion would have been taxable as an ordinary IRA distribution if the owner had taken the money personally. The taxable part is generally included in income for the calendar year of conversion.

Pretax money is generally taxable on conversion

Traditional IRA deductions and tax-deferred earnings have not yet been included in income. When those amounts are converted to a Roth IRA, the owner generally reports them as taxable income. The conversion itself is not usually an early distribution subject to the 10% additional tax merely because the owner is under 59½, provided the money is transferred under the conversion rules. A separate early-distribution issue can arise if the owner keeps part of the distribution instead of converting it.

After-tax basis is not taxed twice

Nondeductible IRA contributions create basis. When a conversion includes amounts attributable to basis, that portion is not included in gross income again. However, the pro-rata rule generally aggregates all of the individual's traditional, SEP and SIMPLE IRAs for the calculation. A person generally cannot label the after-tax dollars as the converted dollars while leaving all pretax funds behind in another IRA.

A simplified illustration

Suppose an individual has $90,000 of pretax IRA value and $10,000 of documented nondeductible basis, for $100,000 total, and converts $20,000. Ignoring year-end valuation and other Form 8606 adjustments, basis is 10% of the total, so approximately $2,000 of the conversion represents basis and $18,000 is taxable. The actual form calculation uses the applicable year's distributions, conversions, contributions, and December 31 IRA value; this illustration is not a substitute for completing Form 8606.

Form 8606 tracks basis and conversion

Form 8606 is used to report nondeductible IRA contributions and calculate the nontaxable and taxable portions of conversions and distributions when basis applies. Missing records can make it difficult to prove basis and may lead to overtaxing the distribution. A taxpayer should carry prior-year basis forward and reconcile Forms 5498, 1099-R and prior Forms 8606.

Conversion year and payment of tax

The taxable portion is normally reported in the year the conversion occurs. Converting late in the year can leave little time to estimate the tax and make withholding or estimated payments. Withholding tax from the IRA itself can also leave less money in the Roth and may create a separate early-distribution consequence for an owner under 59½ on the amount not converted.

Common exam distinctions

• A conversion is generally a reportable event even when the trustee transfers the money directly.
• Only the taxable portion is included in income; documented basis is not taxed twice.
• The pro-rata calculation considers the taxpayer's aggregate traditional, SEP and SIMPLE IRA balances under Form 8606 rules.
• Tax is generally recognized for the conversion year, not when the Roth later distributes qualified amounts.
• A conversion is different from a Roth contribution, which has separate eligibility and annual limit rules.

Key takeaway

Start with the amount converted, separate pretax value from documented IRA basis under Form 8606, and report the taxable portion in the conversion year. Never assume basis can be isolated to one account.

The pro-rata calculation follows the taxpayer, not the account

The pro-rata rule prevents an owner from converting only the after-tax basis while leaving all pretax dollars behind in a separate traditional IRA. For the calculation, traditional, SEP, and SIMPLE IRA balances are generally aggregated, including year-end balances and distributions or conversions during the year under Form 8606. A workplace 401(k) is not part of that IRA aggregation, although an eligible rollover from an IRA to a plan may change the year-end picture if the plan accepts it.

A simplified illustration: the owner has $20,000 of nondeductible basis across traditional IRAs and $80,000 of total IRA value for the Form 8606 calculation. If $10,000 is converted, approximately 20% of the conversion is nontaxable basis, or $2,000, and about $8,000 is taxable, subject to the exact form computation. The converted account might contain only after-tax contributions; the tax result still follows the aggregated calculation, not the label on that account.

Timing, withholding, and cash to pay the tax

A conversion is generally included in gross income for the calendar year in which the funds move to the Roth IRA. The taxpayer may owe estimated tax or have withholding on the conversion. Paying the tax from the IRA itself reduces the amount that continues growing tax-free and may create a taxable distribution; using outside cash can preserve the full conversion amount but may strain liquidity. Model both options and confirm the client can pay the tax without borrowing or selling an asset at a poor time.

A conversion can affect more than the federal income-tax line. Added income may change state tax, deductions or credits, Medicare income-related premiums in later years, taxation of Social Security, and the net investment income tax. These effects depend on year, household, and law. A conversion should be evaluated with a multi-year projection rather than a single marginal-rate comparison, especially when income is near a phase-in threshold.

When a conversion can help—and when it can hurt

A conversion can be attractive when the client expects a higher future marginal rate, has a long horizon, can pay tax outside the IRA, and wants tax diversification or to reduce future required distributions from the Roth owner’s account. It can be less attractive when the client needs the converted funds soon, is in a high current bracket, must use IRA assets to pay tax, or may qualify for lower tax rates later. No conversion strategy guarantees a lower lifetime tax bill.

Before acting, identify all pretax and after-tax IRA balances, the account’s year-end value, planned distributions, rollover activity, charitable plans, beneficiary goals, and current-year income. Coordinate with a tax preparer because the Form 8606 calculation depends on reported basis and other events. For exam questions, distinguish the taxable portion of the conversion from whether the client should convert; the first is a tax computation, the second is a planning comparison.