The title’s $47,500 is simple arithmetic: the 2026 annual additions limit of $72,000 minus the $24,500 employee deferral limit leaves $47,500 of room for other contributions if your employer adds nothing. A mega backdoor Roth is a way to use that room by making after-tax 401(k) contributions and converting them to Roth money. The conversion timing is the part that decides how much growth gets taxed, and this post shows the numbers.
The 2026 Numbers Behind $47,500
IRS Notice 2025-67, as summarized by the International Foundation of Employee Benefit Plans, sets the 2026 401(k) elective deferral limit at $24,500 and the section 415(c) annual additions limit for defined contribution plans at $72,000 (IFEBP). Schwab uses the same limits and notes that catch-up contributions for those 50 and older are generally treated separately (Schwab).
Employer contributions count toward the $72,000 too. Schwab’s example: $24,500 of deferrals plus $7,500 of employer matching is $32,000, which leaves up to $40,000 for after-tax contributions if the plan allows them (Schwab). Your own room depends on your employer’s contributions, so the $47,500 is the maximum case.
What the Plan Must Allow
- Non-Roth, after-tax 401(k) contributions.
- Either in-plan Roth conversions, which move the money to a Roth 401(k) inside the plan, or in-service distributions to a Roth IRA while you are still employed.
- Without those features, Schwab says the strategy generally isn’t available (Schwab).
The Four Steps
Schwab lays out the process in four steps (Schwab):
- Check the plan rules by requesting the Summary Plan Description to see whether after-tax contributions and in-service distributions are permitted.
- Max out regular deferrals first, with catch-up contributions if eligible, to capture any employer match.
- Make after-tax contributions. They don’t count toward the deferral limit but do count toward the $72,000 limit.
- Convert promptly, either as an in-plan conversion or through an in-service distribution to a Roth IRA.
Why the Timing of the Conversion Matters
The contribution was already taxed, so converting it is generally tax-free. Earnings that build up before conversion may be taxable, and delayed conversions can push those earnings into income in the conversion year (Schwab). The size of the effect is easy to estimate.
Assume $2,000 goes in after-tax at the start of each month for a year, the account grows at an assumed 6% a year (0.5% a month), and the money is converted on one of three schedules. The growth that would be taxable is below.
| Conversion timing | Taxable growth |
|---|---|
| Once, at year-end | $794.48 |
| Every quarter | $240.80 |
| Every month | $120.00 |
The pattern is the point: the sooner each deposit is converted, the less growth builds up before conversion. The 6% rate is an assumption, the monthly schedule converts at each month’s end after one month of growth, and actual results depend on your investments.
Turning the Room Into Per-Paycheck Dollars (Assumed Match)
Here is the room calculation with assumed numbers. Take the 2026 limits above, $24,500 of employee deferrals and an assumed $7,500 of employer matching (the same figure as Schwab’s example). That leaves $72,000 − $24,500 − $7,500 = $40,000.00 of room for after-tax contributions, if your plan allows them.
| If paid over | Per period (assumed $40,000 total) |
|---|---|
| 12 months | $3,333.33 |
| 26 biweekly paychecks | $1,538.46 |
| 24 semimonthly paychecks | $1,666.67 |
The table is arithmetic for planning. Few people will use all of the room, and your plan may have its own limit on after-tax contributions, so the number to start from is whatever the Summary Plan Description allows. In my view, choosing a per-paycheck amount you can sustain is a better start than aiming at the maximum.
How the IRS Splits Pre-Tax and After-Tax Money
The IRS page on rollovers of after-tax contributions says that any distribution generally includes a proportional share of pretax and after-tax amounts, and that you can’t take only the after-tax portion and leave the rest in the plan, because any partial distribution must include some pretax amounts. Its example is a $100,000 balance with $80,000 pretax and $20,000 after-tax, where a $50,000 withdrawal includes $40,000 pretax and $10,000 after-tax (IRS: Rollovers of after-tax contributions in retirement plans).
The same page says distributions sent to several destinations at the same time are treated as one distribution for allocating pretax and after-tax amounts (Notice 2014-54). In its example, from a $100,000 withdrawal you could directly roll $80,000 of pretax money to a traditional IRA, directly roll $10,000 of after-tax money to a Roth IRA and receive $10,000 of after-tax money yourself. It also says earnings on after-tax contributions are pretax amounts, so after-tax contributions can go to a Roth IRA without their earnings, and that pretax amounts rolled into a traditional IRA aren’t included in income until distributed from that IRA. The page says this rule applies to distributions on or after September 18, 2014.
I’d read this as the reason the timing table earlier matters: the earnings are the part that is pretax, so converting contributions soon after they go in leaves less of that part to sort out. Whether your plan lets you convert inside the plan or take an in-service distribution is, as noted above, a plan feature to confirm in the Summary Plan Description.
Two Rules to Know
- Pro rata. If the 401(k) holds both pre-tax and after-tax money, part of a conversion may be taxable in proportion to each. Some plans use separate sub-accounts to allow clean conversion of after-tax dollars (Schwab).
- Nondiscrimination testing. Highly compensated employees may use the strategy, but testing can limit how much they contribute. If a plan fails, the employer may need to correct it, which could limit, return or adjust after-tax contributions (Schwab).
Who It Fits
Schwab describes savers whose plans allow after-tax contributions and conversions, people who have already maxed out regular 401(k) and IRA contributions, and those who want additional Roth assets for tax diversification. It also lists risks: many plans don’t allow it, it takes extra cash on top of regular deferrals, early withdrawals can trigger tax and penalties, and the tax rules can be complicated (Schwab).
My Take on the Mega Backdoor Roth
- I think the first step is the cheapest and most useful one: request the Summary Plan Description and look for after-tax contributions and the conversion option, as Schwab suggests.
- To me, converting soon after each contribution keeps the taxable growth small, which matches Schwab’s advice to convert soon after contributing, and the table above shows the pattern.
- In my suggestion, finish regular deferrals and any employer match before considering this, which follows Schwab’s step order.
Sources
- Schwab: What is a mega backdoor Roth strategy? (July 23, 2026)
- IFEBP: IRS releases 401(k) and pension plan limitations for 2026 (Notice 2025-67)
- IRS: Rollovers of after-tax contributions in retirement plans
Related reading: 401k to IRA Rollover for Crypto: 7 Checks Before You Move and Roth vs Traditional IRA: Your Future Tax Rate Decides, Here Is the Math.
This article offers general information and my own opinions, not financial, tax, or legal advice. The figures in worked examples are assumed. Laws, tax rules, and products change and can differ by location, so check current official sources and talk to a qualified professional about your own situation.