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Mega Backdoor Roth After Tax Value Estimator

Compare a simplified Mega Backdoor Roth accumulation scenario with holding the same after-tax savings in a taxable account. The estimator assumes after-tax workplace-plan contributions are converted to Roth and then grow without annual tax drag, while the taxable comparison pays tax only on gains at the end. It is useful for illustrating potential long-term tax-value differences, but plan rules determine whether after-tax contributions and in-plan Roth conversions or rollovers are available. Annual plan contribution limits also change over time.

Inputs

USD
years
%
%
Result
—
Estimated Roth value
Taxable-account value after modeled tax—
Modeled Roth advantage—
Total contributions—

1. Enter the annual after-tax amount
Use only an amount your plan permits and that fits within applicable contribution limits.

2. Choose the investment period
Set the number of years the annual amount will be invested.

3. Set a growth rate
Use the same rate for both comparison paths.

4. Enter a tax rate on gains
This rate is applied only to the taxable account's ending gain in this simplified comparison.

5. Compare ending values
Focus on the Roth value, taxable after-tax value, and modeled difference.

Roth value: R(t+1) = (R(t) + annual amount) × (1 + growth rate) Taxable after-tax value = taxable ending value − max(ending value − contributions, 0) × tax rate

The model assumes prompt conversion of after-tax contributions so it does not build in taxable pre-conversion earnings.

For 2026, IRS rules set a $72,000 defined-contribution annual-additions limit before applicable catch-up contributions; employer and employee amounts count toward that limit, and individual plan terms can be more restrictive.

What the result means

The main result is the projected Roth balance under the selected contribution and return assumptions.

This is an illustrative tax-value comparison, not a determination that your plan supports a Mega Backdoor Roth strategy.

Given

  • Annual after-tax amount: $20,000
  • Years: 20
  • Growth: 6%
  • Tax on taxable gain: 20%

Calculation
Compound $20,000 added each year at 6% for 20 years. Then subtract 20% of the taxable comparison's ending gain while leaving the Roth path unreduced.

Result
Projected Roth value is about $779,854. With $400,000 of contributions, the simplified taxable comparison is about $703,883 after tax, a difference of roughly $75,971.

The difference comes from the assumed tax on the taxable-account gain; real taxable accounts may incur taxes throughout the holding period instead.

What makes a Mega Backdoor Roth different from a regular Roth IRA contribution?

The strategy typically uses after-tax contributions to an employer plan followed by a Roth conversion or rollover, rather than a direct annual Roth IRA contribution. Availability depends on the employer plan.

Does this calculator enforce the annual 401(k) limits?

No. Enter an amount that fits your plan. IRS annual-additions limits and elective-deferral limits are separate, and employer contributions also use annual-additions capacity.

Why does the taxable comparison tax gains only once?

That keeps the model transparent. Actual taxable investments may generate annual dividends, interest, or realized gains, which can create more tax drag.

What if after-tax contributions earn money before conversion?

Those earnings can create taxable income when converted. Use the Tax Impact Estimator in this series to model that amount separately.

Is the Roth ending value guaranteed to be tax-free?

Qualified Roth distributions can be tax-free, but distribution rules and account type matter. The tool is not a legal or tax determination.