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Mega Backdoor Roth Tax Impact Estimator

Estimate the income tax that may arise when after-tax employer-plan contributions have earnings before they are converted or rolled to Roth. In a common Mega Backdoor Roth workflow, the after-tax contribution itself already has basis, while earnings that accumulate before conversion can be taxable. This calculator applies your entered federal and state marginal rates to those pre-conversion earnings and displays the full conversion amount for context. Plan mechanics and tax reporting can be more complex, so the result is a focused estimate rather than a return-preparation calculation.

Inputs

USD
USD
%
%
Result
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Estimated tax on earnings
Total amount converted—
Modeled taxable earnings—
Combined entered rate—

1. Enter after-tax basis
Use the amount already contributed with after-tax dollars.

2. Enter earnings before conversion
Include only growth that accumulated before the Roth conversion or rollover.

3. Enter marginal tax rates
Use rates that reasonably represent the incremental taxable earnings.

4. Review the estimated tax
The main result taxes the earnings, not the after-tax contribution basis.

5. Test conversion timing
Smaller pre-conversion earnings can illustrate the value of prompt conversion when the plan allows it.

Total conversion amount = after-tax basis + pre-conversion earnings Estimated tax = pre-conversion earnings × (federal marginal rate + state rate)

The model treats the entered after-tax contribution as basis and the entered earnings as taxable at conversion. Actual plan records and reporting govern the taxable amount.

This tool does not calculate annual-additions capacity, withholding, bracket stacking, state-specific exclusions, or taxes after Roth conversion.

What the result means

The main result is the modeled income tax attributable to pre-conversion earnings under the entered marginal rates.

If your plan supports frequent automatic in-plan Roth conversion, actual earnings before conversion may be small; confirm the plan's process.

Given

  • After-tax contribution basis: $30,000
  • Earnings before conversion: $600
  • Federal marginal rate: 24%
  • State rate: 5%

Calculation
Total converted = $30,000 + $600 = $30,600. Estimated tax = $600 × (0.24 + 0.05) = $174.

Result
Estimated tax on pre-conversion earnings: $174.

The contribution basis is not taxed again in this simplified model; the tax estimate comes entirely from the $600 of earnings.

Why are the after-tax contributions not taxed again here?

They are entered as basis, meaning tax has already been paid on those dollars. The calculator focuses on earnings that accrued before conversion.

Are pre-conversion earnings always taxable?

Tax treatment depends on how amounts are converted or rolled and the plan's records. In a straightforward Roth conversion of after-tax basis plus earnings, the earnings are generally the taxable component.

Does the calculator enforce the 2026 annual-additions limit?

No. The 2026 IRS annual-additions limit is $72,000 before applicable catch-up contributions, but your available after-tax space depends on deferrals, employer contributions, compensation, and plan terms.

What if I convert immediately after each contribution?

If there is little or no gain before conversion, the modeled taxable earnings can be close to zero. Actual timing and market movement still matter.

Does this cover state taxes correctly?

It simply applies the rate you enter. States differ in how they tax retirement-plan transactions, so use a state-specific rate only after checking your rules.