How much mega backdoor Roth headroom do you have left this year?
If your 401(k) plan allows after-tax contributions above the ordinary deferral limit, this calculates the after-tax room you have left under the IRS's overall 415(c) limit for 2025 or 2026 -- and the per-paycheck amount to use all of it before year-end.
After using this, you can now see the exact dollar headroom left in your plan's after-tax bucket this year, the per-paycheck contribution that reaches it by December, and whether your own plan's contribution cap (not just the IRS limit) is what's actually binding. See how conversion frequency changes what you owe or what a hand estimate usually gets wrong.
Your numbers
Everything below is what you already have on your latest paystub or plan portal for this employer's plan. Nothing is uploaded -- the calculation runs entirely in this browser.
Your headroom
- Elective deferral limit (age-adjusted)
- Overall 415(c) limit (age-adjusted)
- Already counted toward that limit
- Headroom under the IRS limit
- Headroom under your plan's own cap
Max additional after-tax you can contribute this year:
Everything you enter stays on this device. Nothing is uploaded. Usage counts contain only feature names and are never sent anywhere.
Common questions
What is the mega backdoor Roth 401(k) strategy?
Some 401(k) plans let you contribute after-tax dollars beyond the ordinary employee deferral limit, up to a much larger IRS ceiling that also covers your employer's contributions. Converting that after-tax money to Roth -- through an in-plan Roth conversion or an in-service withdrawal to a Roth IRA -- moves it into Roth tax treatment: no tax on qualified withdrawals in retirement. Most plans do not offer this; check your plan document or ask your plan administrator before assuming it applies to you.
What's the difference between the $24,500 limit and the $72,000-$83,250 limit?
The $23,500 (2025) / $24,500 (2026) figure is the 402(g) elective deferral limit -- what you control directly through payroll withholding, pre-tax plus Roth 401(k) combined. The much larger $70,000-$81,250 (2025) / $72,000-$83,250 (2026) figure is the 415(c) overall limit -- your elective deferral, your employer's match and profit sharing, and any after-tax contributions, all added together. The gap between what you and your employer have already put in under the smaller limit and the larger limit is your after-tax headroom.
If I changed jobs this year, does the 415(c) limit combine across both employers?
No, and this is the opposite of how the other limit works. The 415(c) overall limit applies per employer plan (or per controlled group of related employers) -- a job change generally gives you a fresh 415(c) ceiling at the new employer. The 402(g) elective deferral limit is the reverse: it is one shared ceiling across every employer you contributed to during the calendar year, related or not, so your pre-tax and Roth paycheck deferrals at both jobs combined still cannot exceed it. This calculator computes the 415(c) headroom for a single employer's plan; if you changed jobs, run it once per employer with each plan's own YTD figures.
Why does converting quickly matter more than converting eventually?
After-tax contributions themselves are not taxed again on withdrawal or conversion -- you already paid tax on that money once. Any investment growth on that after-tax balance before you convert it, though, is taxed as ordinary income at conversion. A plan with automatic or same-day in-plan Roth conversion leaves little or no growth to accumulate, so the conversion is close to tax-free. A plan that only lets you convert periodically (quarterly, annually) lets growth build up in the after-tax bucket first, and that growth is what gets taxed when you finally convert. See the full comparison.
Is this the same as a regular backdoor Roth IRA?
No, and the two get confused constantly. A backdoor Roth IRA is a nondeductible contribution to a traditional IRA (capped at $7,000 for 2025/2026, plus a $1,000 catch-up at 50+) that you then convert to a Roth IRA -- and it can trigger the IRA pro-rata rule if you hold other pre-tax IRA money. The mega backdoor Roth happens entirely inside your 401(k), uses the much larger 415(c) limit instead of the IRA limit, and has no IRA pro-rata-rule exposure at all.
Does my plan have to offer this?
Yes -- and most plans do not. Three separate plan features all have to be present: the plan must allow after-tax (non-Roth) employee contributions above the elective-deferral limit, and it must allow either in-plan Roth conversions or in-service withdrawals so the after-tax money can actually move to Roth. Some plans that allow after-tax contributions still lock the money up until termination, which removes most of the benefit. Check your plan's summary plan description or ask your plan administrator before assuming any of this applies to you.