Retirement & investing · free calculator
Mega backdoor Roth capacity calculator
Compute how much you can move into Roth via the mega backdoor in your 401(k) — based on your salary, employer contributions, and the IRC 415(c) annual additions limit.
Mega backdoor Roth capacity
Show the work
- Total contributions so far$32,000
- Annual additions limit$69,000
- Limit as % of salary34.5%
Mega backdoor Roth — the legit $40k+ Roth shortcut
If your 401(k) plan allows after-tax contributions + in-service Roth conversions (also called "in-plan Roth rollovers" or "after-tax-to-Roth"), you can stuff up to $40,000+ extra into Roth annually beyond the standard $23k limit.
The math
The IRC 415(c) annual additions limit (2024: $69,000) is the ceiling on ALL contributions to your 401(k) — yours + employer's. Most people fill it with $23k employee + $10-12k match. The gap between match-cap and $69k is mega backdoor capacity.
How it works
- Max out regular pre-tax / Roth 401(k) ($23,000)
- Receive employer match ($X)
- Make additional after-tax contributions up to (415(c) limit − $23k − match)
- Convert after-tax balance to Roth via in-service rollover (typically same-day in modern plans)
What you need from your plan
- After-tax contributions allowed (different from Roth 401(k))
- In-service rollovers / conversions allowed
- (Ideally) automatic conversion to avoid earnings creating taxable conversions
If your plan doesn't have all three, the mega backdoor isn't available. Ask HR — many plans added it 2020-2024.
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