A solo 401k allows you to contribute $72,000 annually if you’re under 50. But that number is useless without understanding what goes into it—and what disqualifies you from parts of it. You can deposit $24,500 as an employee deferral, roughly 20% of net self-employment income as employer contribution, and an additional $47,500 as after-tax (mega backdoor), but only if your plan structure allows it and only if you do the math correctly. Worse, if you earned over $150,000 in W-2 wages last year and you’re 50 or older, the IRS forces your catch-up contributions into a Roth bucket you may not want. Most solo 401k owners calculate contributions wrong, discover it months later while filing taxes, and face penalties for overcontribution. This article shows the exact calculation for each contribution type, explains when limits change, and exposes the SECURE 2.0 rules that surprise high-income business owners.
The Three-Bucket Contribution Structure: Employee, Employer, and After-Tax
A solo 401k has three separate contribution mechanisms. They’re not interchangeable, and each has different limits and rules. The total across all three cannot exceed $72,000 (for those under 50). Treating them as a combined pool is a common error that leads to overcontribution penalties.
Employee Deferrals: $24,500 Maximum (Fixed and Simple)
As the employee portion of your solo 401k, you can defer up to $24,500 of your compensation to the plan in 2026. This number is indexed annually and rarely changes more than $500 year-to-year. The key word is “compensation”—your deferral is limited to your compensation (W-2 wages if you’re an S-corp owner, or net self-employment income if you’re a sole proprietor), whichever is lower. A self-employed person earning $20,000 cannot defer $24,500 because $20,000 is their total compensation. They can only defer $20,000.
This is an elective deferral, which is why SECURE 2.0 allowed it to be made retroactively (until April 15) for sole proprietors. But once you reach $24,500, you stop. You cannot exceed this amount as an employee deferral regardless of how much you earn. This is true even if you have a solo 401k with mega backdoor provisions. The $24,500 bucket is capped at $24,500.
Employer Contributions: 20% of Net Self-Employment Income (Sole Proprietor)
This is where most people calculate wrong. Employer contributions for a sole proprietor are not 25% of net profit. They’re approximately 20% of adjusted net self-employment income after subtracting half of self-employment tax. The math is complex but essential to get right.
Start with your Schedule C net profit. Let’s say it’s $150,000. Subtract one-half of your self-employment tax (roughly 7.065% of your net profit). That’s $150,000 minus $10,597.50 equals $139,402.50. Your employer contribution is 20% of that: $27,880. Not $37,500 (which would be 25% of gross). The difference matters, especially at higher income levels. A $200,000 profit yields an employer contribution of roughly $35,845, not $50,000. The 20% figure already accounts for the self-employment tax adjustment.
If you’re an S-corp owner with W-2 wages, the calculation is simpler: 25% of W-2 wages. If you paid yourself $150,000 in W-2 wages, you can contribute 25% of that, which is $37,500. S-corps avoid the self-employment tax adjustment because W-2 wages are already net of FICA taxes paid.
After-Tax Contributions: Up to the Plan Limit Minus Other Contributions
A mega backdoor Roth uses after-tax contributions. These are dollars you’ve already paid income tax on, which you then deposit into your solo 401k and immediately convert to Roth. The tax is paid up front; the conversion is tax-free because the funds were already taxed. After-tax contributions are limited only by the total plan limit ($72,000 for those under 50). If you contributed $24,500 as employee deferrals and $27,880 as employer contribution, you have $19,620 remaining ($72,000 minus $52,380). You can deposit up to $19,620 in after-tax contributions.
But this requires two conditions: your plan documents must allow after-tax contributions (most do, but ask your provider), and your plan must allow in-service conversions (which allows you to convert the after-tax money to Roth immediately after depositing it). If your plan doesn’t support in-service conversions, after-tax contributions sit in the account as after-tax basis, which creates a tax mess down the line. When you withdraw in retirement, you owe tax on the earnings portion only, but tracking that basis over decades is tedious. The mega backdoor is only valuable with in-service conversion language in the plan.
The Catch-Up Contribution Rules: A Normal $8,000 Plus a Secret $11,250
At age 50, you can contribute an additional $8,000 to your employee deferral, raising the limit from $24,500 to $32,500. This is the standard catch-up that most people know about. What few people know is that SECURE 2.0 added a new super catch-up of $11,250 for those aged 50-59 or 64 and above if they earned W-2 wages of $150,000 or more in the prior year.
The Super Catch-Up Rule: $11,250 Extra at Ages 50-59 and 64+
If you’re 50-59 or 64+ and earned at least $150,000 in W-2 wages in 2025, you can make an additional super catch-up contribution of $11,250 in 2026. This stacks on top of the standard $8,000 catch-up. Your total employee deferral becomes $24,500 plus $8,000 plus $11,250 equals $43,750. Few self-employed people qualify because they typically don’t have W-2 wages—they have self-employment income. But a hybrid earner with a W-2 job earning $150k and a side solo 401k business can take advantage. An S-corp owner who pays themselves $160,000 in W-2 wages qualifies. Most sole proprietors don’t.
The $150,000 threshold is indexed for 2026. It might be $160,000 by 2027 or $155,000 by 2025. Check the IRS each year if you’re close to the threshold. The super catch-up is valuable for high-income professionals who didn’t realize it existed. Someone turning 50 with $160,000 in W-2 wages can suddenly contribute $43,750 instead of $32,500, adding $11,250 in retirement savings annually for 10 years—that’s $112,500 in additional tax-deferred or tax-free growth.
The Mandatory Roth Catch-Up: The Surprise That Breaks Plans
Here’s where SECURE 2.0 introduced a rule that few advisors explain clearly: if you’re 50+ and earned $150,000 in W-2 wages in the prior year, you cannot make pre-tax catch-up contributions. Your catch-up contributions must be Roth. You cannot choose to defer the $8,000 or $11,250 as traditional (pre-tax)—the IRS says it must be Roth.
This is problematic for high-income earners who need the current-year tax deduction. Someone earning $200,000 who wants to reduce their current taxable income via a $32,500 deferral ($24,500 base plus $8,000 catch-up) discovers they can only do $24,500 pre-tax and must deposit the $8,000 as Roth. They pay current tax on the $8,000 instead of deferring it. They lose the tax deduction. The rule forces them to lock in current income taxes rather than defer them. Most people don’t understand this until they’ve already structured their plan and then find out from a CPA during tax filing.
The workaround is planning: know the rule in advance, adjust your tax projection, and decide whether you want to accept the Roth catch-up. Some people choose to not take the catch-up. Others accept the Roth designation because they expect higher tax rates in retirement. Some consult with their accountant to find other tax deductions to replace the loss. This rule benefits no one except the IRS and creates immense confusion.
Common Calculation Errors That Trigger Overcontribution Penalties
Overcontribution errors are surprisingly common because the rules are layered and small mistakes compound.
Mixing Up the 20% Sole Proprietor Rate With the 25% S-Corp Rate
A sole proprietor earning $200,000 thinks they can contribute $50,000 as an employer contribution (25% of $200,000). Wrong—they can only contribute $35,845 (20% after self-employment tax adjustment). They overcontribute by $14,155. The excess sits in the account generating earnings, which triggers a 6% excise tax per year on the excess amount. Over five years, that’s a $3,300+ tax on a single mistake. The fix is filing Form 5329 to report and correct the overage, but most people don’t realize they overcontributed until their CPA points it out during tax filing. By then, the deadline to fix it (usually the tax filing deadline) may have passed, and they’re locked into paying the penalty.
Forgetting That After-Tax Contributions Reduce Other Limits
A self-employed person deposits $24,500 in employee deferrals and $30,000 as employer contribution, thinking they still have room for after-tax. They then deposit another $20,000 in after-tax contributions, totaling $74,500. The limit is $72,000. They’ve overcontributed by $2,500. The mistake is treating after-tax as an unlimited pool separate from the total cap. It’s not—all contributions (employee, employer, after-tax) roll up to the single $72,000 cap. This error is common among people who don’t carefully track the running total.
Using Gross Profit Instead of Net Self-Employment Income
A freelancer earns $100,000 in gross income but has $25,000 in business expenses, netting $75,000. They think their employer contribution is 20% of $100,000, which is $20,000. Actually, it’s 20% of roughly $70,000 (after subtracting half of self-employment tax), which is about $14,000. They overcontribute by $6,000. Using the wrong income figure is surprisingly common among people who use “revenue” when they should use “net.” This error compounds year-to-year.
Solo 401k Income Thresholds That Unlock or Block Strategies
Certain income levels create threshold effects where your available strategies shift dramatically.
Under $50,000 Net Income: Contribution Limits Are Proportional
If you earn $50,000 in net self-employment income, your employer contribution is roughly $10,000 (20%), and your total capacity is around $34,500 ($24,500 employee deferral plus $10,000 employer). You cannot max out the $72,000 total limit because your income doesn’t support it. This is fine—contribute what you can. Many part-time freelancers or side business owners are in this bracket.
$150,000-$200,000 Income: The Mandatory Roth Threshold
At $150,000+ in W-2 wages (if you’re 50+), you trigger mandatory Roth catch-up contributions. This income level is the inflection point where the rule applies. For sole proprietors earning $150,000+ in net self-employment income, the threshold doesn’t apply—they don’t have W-2 wages, so mandatory Roth doesn’t affect them. A sole proprietor earning $200,000 can still take the full $8,000 catch-up pre-tax. Only W-2 earners or S-corp owners paying themselves $150k+ in W-2 wages are affected.
$237,500+ Income: Mega Backdoor Becomes Valuable
At very high incomes, the mega backdoor Roth becomes a lever to max out the plan. Someone earning $237,500+ in net self-employment income can contribute nearly the full $72,000: roughly $24,500 employee deferral, $35,000+ employer contribution, and the remainder in after-tax. A sole proprietor earning $300,000 can max out the plan entirely. The after-tax bucket is valuable for high earners because it’s one of the few remaining ways to shelter additional income from current taxation via the conversion to Roth.
Correcting Overcontributions: The Form 5329 Process
If you overcontribute, the IRS penalizes you at 6% of the excess per year. A $5,000 overcontribution left in the account for three years costs $900 in penalties alone (6% times $5,000 times 3 years). The solution is filing Form 5329 to report the excess and requesting a correction. The process is not automatic—you must initiate it.
Identifying an Overcontribution
Most people discover overcontributions during tax filing or when a CPA reviews their 401k contributions. The first step is confirming the error: calculate what you should have contributed vs. what you actually deposited. If the actual exceeds the allowed limit, you have an overcontribution. You need the exact dollar amount.
Filing Form 5329 and Requesting Correction
Form 5329 is filed with your tax return to report the excess contribution and request a correction. You can request that the excess (plus earnings) be returned to you, which removes the overcontribution from the plan and eliminates future 6% penalties. The returned amount is taxed as ordinary income (the principal) and the earnings are also taxed (avoiding double taxation via the 6% excise tax on the excess). If you file the correction timely, the 6% penalty is waived. If you file late or don’t file, the 6% penalty accumulates each year.
Preventing Future Overcontributions
The prevention strategy is tracking contributions throughout the year. Before making your employer contribution in December or April, calculate the exact amount using the correct formula (20% for sole proprietors after self-employment tax adjustment). Use a spreadsheet or tax software to track the running total: employee deferrals plus employer contribution plus any after-tax contributions. Stop when you reach $72,000. If you’re close to the limit, round down slightly rather than up to avoid an overcontribution. Most overcontribution errors occur because someone made rough estimates or skipped the calculation entirely.
FAQ
Can I contribute the same amount to both a solo 401k and a SEP IRA?
No. Contributions to a solo 401k and a SEP IRA are aggregated. If you contribute $50,000 to a solo 401k and $20,000 to a SEP IRA, you’ve contributed $70,000 across both plans, which is within limits. But you can’t contribute $72,000 to each and claim you’re using separate limits—they’re combined. Most people should choose one or the other, not maintain both. The exception is if you have two genuinely separate businesses and want to keep finances separate for accounting purposes, but the IRS still aggregates contributions for limit purposes.
What if my self-employment income changes mid-year and I overcontribute?
This is common if you make a large employer contribution in December based on projected income, but income drops before year-end. You’ve overcontributed against actual income. The solution is filing Form 5329 to request correction. You won’t get penalized if you report it properly and timely. Some people try to avoid this by not making employer contributions until April (during tax filing), when they know final income. This avoids the overcontribution risk and uses the full contribution time window.
Does S-corp salary affect my solo 401k contribution limit?
Your solo 401k contribution is based on the W-2 wages you pay yourself, not on S-corp pass-through income. If you have an S-corp and you pay yourself $100,000 in W-2 and take $50,000 in distributions, your contribution is based on the $100,000 W-2, not the $150,000 total. The S-corp taxation is separate from 401k contributions. Your employer contribution as an S-corp owner is 25% of W-2 wages paid (roughly $25,000 in this example). You cannot boost the contribution by increasing distributions.
Can I contribute $24,500 to a solo 401k and then $8,000 to a traditional IRA?
You can contribute to both, but your traditional IRA contribution is limited based on whether you or your spouse have access to an employer 401k plan. If you have a solo 401k, you’re covered under an employer plan. If your income is above the phase-out range (roughly $77,000-$87,000 single in 2026), you cannot deduct traditional IRA contributions. You could make non-deductible contributions, but that creates a Roth conversion taint issue. Most people with solo 401ks should max the 401k first and skip the traditional IRA, or use Roth IRA contributions if income allows.
What if I have both employee income and self-employment income?
You can contribute to a solo 401k based on your self-employment income. Your W-2 salary is handled by your employer’s 401k. Contributions are not aggregated between your employer plan and your solo 401k—each has its own limit. You can defer up to $24,500 total across both plans, not $24,500 in each. If your employer plan allows you to defer $15,000, you can defer an additional $9,500 in your solo 401k (up to $24,500 total). The employer contribution to your solo 401k is separate and based on self-employment income, which is not aggregated with your W-2 employment.