So, you’re looking for a solo k contribution calculator. Honestly, I get it. Most of the tools you find online are either ancient or they don't account for the weird nuances of the 2026 tax year. If you’re self-employed, you’ve probably realized that a Solo 401(k) is basically the holy grail of retirement accounts, but calculating the exact number you can shove into it is a nightmare.
One wrong move and you’re overcontributing. That leads to IRS penalties.
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Nobody wants that.
The reality is that a Solo 401(k)—or a "one-participant 401(k)" if you want to be formal—is unique because you wear two hats. You are the boss. You are also the worker. Because of that, the IRS lets you contribute twice. But "twice" doesn't mean "unlimited," and the math changes drastically depending on whether you’re a sole proprietor or an S Corp owner.
The 2026 Numbers You Actually Need
Forget 2024 or 2025. We are in 2026 now, and the IRS has bumped the limits. If you’re under 50, the total limit is $72,000. That’s a jump from $70,000 last year.
But wait.
If you’re 50 or older, you get a "catch-up" contribution. For 2026, that catch-up is $8,000. This brings your total possible contribution to $80,000.
Now, here is where it gets kinda wild. Thanks to the SECURE 2.0 Act, there is a special "super catch-up" for people aged 60, 61, 62, and 63. If you fall into that specific age bracket in 2026, your catch-up isn't $8,000. It’s **$11,250**. That means a 62-year-old consultant could potentially squirrel away $83,250 this year.
That is massive.
Why your business structure ruins everything
You can’t just use a generic solo k contribution calculator without knowing how your business is taxed. It’s the biggest mistake people make.
If you are an S Corp owner, your contribution is based on your W-2 salary. Not your total business profit. Just the W-2. If you pay yourself $100,000 in salary, your employee deferral is $24,500. Your employer "profit-sharing" side is 25% of that $100,000.
Total: $49,500.
But if you are a Sole Proprietor (Schedule C), the math is way more annoying. You don’t get a flat 25%. You have to use a "reduced rate" which ends up being about 20% of your net self-employment income after you subtract half of your self-employment tax.
Basically, the IRS makes you do a loop-the-loop before you can find your number.
The High-Earner Roth Trap in 2026
Something huge changed this year. You need to pay attention to Section 603 of SECURE 2.0.
If you made more than $150,000 in the previous year (2025), any catch-up contributions you make in 2026 must be Roth. You don't get a choice. You can't take a tax deduction on that extra $8,000 or $11,250. It has to go in after-tax.
Most people are going to miss this. They’ll try to deduct the whole thing on their 1040 and the IRS is going to flag it.
If your income is lower than $150,000, you can still choose between Pre-tax (Traditional) or Roth for your catch-up. But for the high-fliers? The government wants their tax cut now.
A quick illustrative example
Let's look at "Sarah." She’s 42 and runs a graphic design S Corp. She pays herself $120,000 in W-2 wages.
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- Employee side: $24,500 (the 2026 max).
- Employer side: 25% of $120,000 = $30,000.
- Total Sarah can contribute: $54,500.
Now, if Sarah was 61? She’d add that $11,250 super catch-up on top.
Common calculator mistakes to watch out for
I’ve seen people use a solo k contribution calculator and get a number that’s way too high because they didn't account for other jobs.
The $24,500 employee limit is per person, not per plan.
If you have a day job at a big tech company and you put $15,000 into their 401(k), you only have $9,500 left for your Solo 401(k) employee deferral. You can still do the 25% employer side for your side hustle, but you can't "double dip" on the employee side.
Another thing? Deadlines.
For 2026, you generally have until your tax filing deadline (including extensions) to actually put the money in the bank. However, you need to have the plan established by December 31st of the tax year if you want to count those employee deferrals.
Actionable Next Steps for You
Don't just trust a random slider on a website. Take these steps to get your 2026 contributions right:
- Check your 2025 W-2 or Schedule C. If you made over $150,000, mark your catch-up contributions as Roth only for 2026.
- Verify your age. If you turn 60, 61, 62, or 63 this year, use the $11,250 catch-up limit instead of the standard $8,000.
- Calculate the "Employer" side based on your structure. Use 25% of W-2 for S Corps, or roughly 20% of adjusted net profit for Sole Proprietors.
- Subtract other 401(k) contributions. If you have a day job with a retirement plan, reduce your Solo 401(k) employee limit by whatever you've already contributed there.
- Fund it before the deadline. Aim to have your employee deferrals decided by December 31, 2026, even if you don't move the cash until April 2027.
Getting the math right on your solo k contribution calculator is the difference between a massive tax break and a massive headache. Double-check your business type, respect the $150k Roth mandate, and max out those 2026 limits while you can.