Mega Backdoor Roth Calculator
Calculate after-tax 401(k) contribution capacity for a mega backdoor Roth strategy.
- Last reviewed
- August 26, 2026
- Cost
- Free to use
- Data
- Runs in your browser
- Reviewed by
- Anton Jonson
After-Tax 401(k) Capacity
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Total Annual Contribution
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IRS Max Used
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Methodology
How this calculator handles inputs
This calculator uses the values you enter above and applies the formula explained in the guide below. Results update in the browser and are intended for quick planning, comparison, and sanity-checking.
- Use consistent periods, currencies, and units across inputs.
- Review any assumptions before using the result in a decision.
- Recalculate when rates, prices, tax rules, or business terms change.
Important note
This tool provides general planning information only. It is not tax, legal, financial, accounting, or investment advice. Check the current rules for your location and speak with a qualified professional before making a high-stakes decision.
Guide
How it works
Use this calculator to estimate additional Roth space available through after-tax 401(k) contributions.
What this calculator does
The mega backdoor Roth calculator estimates remaining capacity under the total defined contribution plan limit. It subtracts employee pre-tax and employer contributions from the IRS total limit.
It uses:
- annual salary
- employee pre-tax contribution
- employer contribution
- IRS total contribution limit
Mega Backdoor Roth Formula
Capacity = IRS Total Limit - Employee Contribution - Employer Contribution
Where:
- IRS Total Limit = annual defined contribution limit
- Employee Contribution = pre-tax or Roth employee deferral
- Employer Contribution = match or profit-sharing
- Capacity = after-tax contribution room
Example calculation
If:
- IRS total limit = 70,000
- Employee contribution = 24,500
- Employer contribution = 10,000
- Used contributions = 33,500
Then:
- Capacity = 70,000 - 33,500
- Capacity = 36,500
- Total contribution can reach 70,000
- IRS max used = 100%
The mega backdoor Roth capacity is 36,500.
What is mega backdoor Roth?
Mega backdoor Roth is a strategy using after-tax 401(k) contributions and Roth conversion when a plan allows it. It can create additional Roth savings beyond normal IRA limits.
Why mega backdoor Roth matters
- can add large Roth contribution capacity
- uses workplace plan rules
- helps high savers shelter more growth
- requires careful limit tracking
When to use this calculator
- checking after-tax 401(k) capacity
- planning high annual savings
- reviewing employer contribution impact
- comparing Roth strategies
Common mistakes
- assuming every plan allows it
- forgetting employer contributions count
- exceeding the total plan limit
- ignoring conversion timing
Mega backdoor Roth vs backdoor Roth
Backdoor Roth uses IRA contribution rules. Mega backdoor Roth uses after-tax 401(k) contribution space.
The mega version can involve much larger amounts.
FAQs
What is mega backdoor Roth?
It is a strategy using after-tax 401(k) contributions and Roth conversion when permitted by the plan.
How do you calculate mega backdoor Roth capacity?
Subtract employee and employer contributions from the total IRS plan limit.
What is a good mega backdoor Roth amount?
A good amount uses available plan capacity without exceeding IRS limits.
What is the difference between mega backdoor Roth and backdoor Roth?
Mega uses a 401(k). Backdoor Roth uses IRA rules.
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