You have a W-2 with a 401k plan, and you also run a side business that's profitable enough to fund a solo 401k. You want to know how much you can actually contribute to the solo 401k once your W-2 401k contribution is accounted for. Most articles tell you the answer is $72,000 for 2026. Most articles are wrong.
The 401k rules separate the employee side of the contribution from the employer side, and they treat each side under a different limit. Once you understand which limit is per-person and which is per-plan, the math becomes mechanical. Until you understand that distinction, the calculators online will give you wildly different answers depending on which one ignores the W-2 plan entirely.
The mistake the standard explanation makes
The standard solo 401k explanation goes: "You can contribute up to $72,000 in 2026 — that's the $24,500 employee deferral plus up to $47,500 from the employer side." Technically true if the solo 401k is your only plan. If you also have a W-2 401k, that's not what you can contribute. Under §402(g)(1), the elective deferral limit is a per-person ceiling. You don't get to add a fresh $24,500 to the solo plan on top of what you already contributed to the W-2 plan. Whatever you put in the W-2 401k comes out of the same $24,500.
That sounds obvious once you've heard it. The problem is that most general personal-finance coverage — and a lot of the calculators — quote the solo 401k limit as if the employee deferral resets per plan. When you're modeling your retirement contribution capacity, that produces a number that's $15,000–$24,500 too high.
The two buckets — employee deferral and employer contribution
The 2026 limits, the parts that matter (per IRS Notice 2025-67):
Employee deferral. Capped at $24,500 for 2026 across all your 401k plans combined. This is the §402(g) limit and it follows the person, not the plan. If you put $24,500 into your W-2 401k, you've used the entire employee deferral. Nothing left for the solo 401k on this side.
Employer contribution. Capped at $47,500 for 2026 (the §415(c) overall limit of $72,000 minus the $24,500 employee deferral). This limit is per-plan, not per-person. The employer side of the W-2 plan and the employer side of the solo 401k each have their own §415(c) ceiling — so this is the bucket where having two plans actually gives you more contribution capacity than one.
Catch-up. An additional $8,000 for 2026 if you're 50 or older. $11,250 for ages 60–63 (the SECURE 2.0 enhanced catch-up under §414(v)). Per Notice 2025-67, catch-up follows the same per-person rule as the regular employee deferral — you don't get a fresh catch-up for the solo plan if you've already used it on the W-2 side. The SECURE 2.0 mandatory Roth treatment for catch-ups applies to participants whose prior-year FICA wages exceeded $150,000.
The math on the employer side, for a self-employed business owner: roughly 20% of net self-employment earnings (after the SE tax deduction) for a sole proprietor or single-member LLC; 25% of W-2 wages for an S-corp. Whatever method applies to your business structure, the employer-side contribution to the solo 401k is its own calculation, capped at $47,500 for 2026.
Three scenarios with real numbers
Hybrid earner with $280K W-2 income, $90K net side business income, age 42 (no catch-up). Three plausible W-2 deferral choices, and what each does to the solo 401k capacity.
| Scenario | W-2 deferral | Solo 401k employee deferral remaining | Solo 401k employer side (S-corp) | Solo 401k employer side (sole prop) | Total retirement contribution |
|---|---|---|---|---|---|
| A — Max W-2 401k | $24,500 | $0 | $12,500 | $16,623 | $37,000–$41,123 |
| B — Split deferral | $15,000 | $9,500 | $12,500 | $16,623 | $37,000–$41,123 |
| C — All to solo 401k | $0 | $24,500 | $12,500 | $16,623 | $37,000–$41,123 |
Notice what changes and what doesn't. The buckets shift across the three scenarios; the total capacity is the same in all three. The W-2 plan doesn't give you more retirement room — it just shifts where the money sits.
What this table doesn't show: employer matching. In Scenario C, if your W-2 employer matches at 4% of salary, you've walked away from $11,200 in matching contributions (4% × $280K). That's real money lost — typically enough to make Scenario A or B the right answer in practice. Whether that's worth it depends on the rest of the math.
The most useful takeaway from these scenarios isn't "which one to pick" — it's that the total contribution capacity is the same across all three. The W-2 plan doesn't give you more retirement room; it just shifts where the money sits and changes what your employer match looks like. The bottleneck is your business's profitability and entity structure, not your W-2 plan.
How I actually run this: I max the full $24,500 employee deferral through my W-2 employer's 401k, and I use the solo 401k exclusively for the employer-side profit-sharing contribution from my S-corps and LLCs. The W-2 plan is genuinely good — low-fee index funds, low administrative costs — so the per-person employee-deferral capacity belongs there. The solo 401k carries everything that has to live on the employer side anyway. If you have both options, the order I'd suggest is: contribute enough to the W-2 plan to capture the full company match first — that match is free money you don't get back — and then decide where the rest of the deferral goes based on a clean fee comparison and whether either plan gives you Roth capacity you actually want.
Where the mega backdoor Roth fits in
The solo 401k can carry a feature most W-2 401k plans don't have — at least, not the ones at the typical large-employer plan administrator. The "mega backdoor Roth" maneuver works only when your solo 401k plan document permits two specific provisions: after-tax (non-Roth) employee contributions, and in-plan Roth conversions or in-service distributions. This is where the custodian question matters. The off-the-shelf brokerage solo 401k plan documents — Schwab, Fidelity, E*TRADE, Vanguard — generally do not include these provisions in their prototype documents. To access mega backdoor mechanics in a solo 401k you typically need a non-prototype, customized plan document from a specialty third-party administrator (My Solo 401k Financial and Solo 401k Plan are the two most commonly cited; others exist). The big-bank default solo 401k plan documents do not get you there.
Here's how it works once the plan document is right. The §415(c) overall limit is $72,000 of total annual additions to the plan for 2026. That includes employee deferral + employer contribution + after-tax employee contribution. So if you've contributed $24,500 in pre-tax employee deferral and $12,500 in employer contribution, you have $35,000 of room left to fill — but only if your plan document allows after-tax contributions. You fill that $35,000 with after-tax dollars, then immediately convert those after-tax dollars to Roth under the plan's in-plan Roth conversion provision. Result: $35,000 going into Roth annually, on top of the pre-tax contributions, with no income limit on the conversion.
This is the move most solo 401k holders aren't using because their off-the-shelf plan document doesn't allow it. Moving to a customized plan document from a specialty administrator is a 30-day administrative task that opens up $30K–$40K of additional Roth space every year. Plan-document mechanics are what gate the strategy — not the custodian's marketing page.
For this article, the relevant point is: when modeling your solo 401k capacity, the relevant limit isn't the $37K headline number for pre-tax contribution — it's the $72K §415(c) overall limit for 2026, and the difference between them is the mega backdoor Roth opportunity, available only with the right plan document.
How to think about your number
If you're running a solo 401k alongside a W-2 plan, these are the variables that actually determine your contribution capacity:
Confirm exactly how much you've deferred to the W-2 401k year-to-date and whether the W-2 plan will accept additional contributions later in the year. Some plans cut off contributions in November for processing reasons.
Confirm the employer-side calculation method for your business entity. S-corp owners use 25% of W-2 wages (the reasonable salary number). Sole props and single-member LLCs use roughly 20% of net SE earnings. Multi-member LLCs depend on the specific structure.
Confirm whether your solo 401k plan document allows after-tax contributions and in-plan Roth conversions. Off-the-shelf brokerage prototype documents generally don't; a customized document from a specialty third-party administrator generally does. If it does, you have meaningful additional contribution capacity. If it doesn't, the contribution ceiling for practical purposes is the pre-tax limit, not the §415(c) overall limit.
Confirm whether the SECURE 2.0 §603 Roth catch-up rule for S-corp vs. sole-prop operators applies to you, and whether the enhanced catch-up tier does as well (ages 60–63 only, $11,250 for 2026). The standard 50+ catch-up is $8,000 for 2026 per Notice 2025-67.
Coordinate the S-corp election framework and its reasonable-salary number, if applicable, with the retirement contribution target. The salary determines both the FICA exposure and the employer-side retirement contribution ceiling. The optimization is a two-variable problem, and the right answer to "what should my reasonable salary be" depends partly on what you're targeting for the retirement plan.
The headline takeaway from this piece, the one to leave with: the $72,000 number you'll see online for 2026 isn't your number if you also have a W-2 401k. Your number is the $47,500 employer-side limit on the solo plan plus whatever's left of the $24,500 employee deferral after your W-2 contributions — and a meaningfully higher number if the plan document gets you mega backdoor access. Build the calculator from there.