Solo 401(k) contribution calculator
A Solo 401(k) lets you stack two contributions: an employee elective deferral (up to $24,500 in 2026, more with catch-up) and an employer contribution (about 20% of adjusted net self-employment earnings for a sole proprietor, or 25% of W-2 wages for an S-corp owner), together capped at $72,000 ($80,000 at 50+, $83,250 at 60 to 63). The calculator below runs your own numbers through that exact math and shows every step, so you can see where the total comes from rather than taking it on faith. For a $100,000 sole-proprietor net profit at age 45, the maximum works out to about $43,087.
Solo 401(k) contribution calculator (2026)
Enter your business type, income, and age. Nothing you type here is saved or sent anywhere; the math runs entirely in your browser.
| Net profit (Schedule C) | $100,000 |
|---|---|
| Net earnings for SE tax (× 92.35%) | $92,350 |
| Self-employment tax (Schedule SE, up to the SS wage base) | $14,130 |
| Half of SE tax (Schedule 1 deduction) | $7,065 |
| Contribution base (net profit minus half of SE tax) | $92,935 |
| Employer contribution rate (25% ÷ 125%, the reduced rate) | 20% of adjusted net earnings |
| Employer contribution | $18,587 |
| Employee elective deferral (under 50, no catch-up) | $24,500 |
| Total 2026 contribution | $43,087 |
Maximum 2026 contribution: $43,087
Formula source: IRS, "Self-employed individuals: Calculating your own retirement plan contribution and deduction," IRS Publication 560, and IRS Notice 2025-67. This calculator estimates the maximum allowed contribution; it is not tax or investment advice, and it does not replace the actual worksheet in Publication 560 or a preparer's calculation.
How does this calculator work?
It runs the same math the IRS uses in its own worked examples, for whichever business structure you pick:
- Sole proprietor or single-member LLC: your net profit is multiplied by 92.35% to get net earnings from self-employment (IRC §1402(a)), then Schedule SE self-employment tax is calculated on that figure, then half of that tax is deducted to get "adjusted net earnings," and finally the employer contribution rate (20%, not 25%) is applied to that adjusted figure.
- S-corp owner-employee: the employer contribution is simply 25% of your actual W-2 wages. There is no reduced-rate step, because wages already exclude any retirement contribution, so there is no circular math to untangle.
Either way, the employee elective deferral (up to $24,500 in 2026, plus catch-up if you qualify by age) is added on top, and the combined total is capped at the 2026 IRC §415(c) limit for your age bracket: $72,000 under 50, $80,000 at 50 to 59 or 64 and up, or $83,250 at 60 to 63.
Why is the sole-proprietor employer contribution about 20%, not 25%?
Because the employer contribution is itself a deduction from the same earnings figure it's calculated against, which creates a circular problem: the contribution depends on your compensation, and your compensation (for a sole proprietor, there is no W-2, so "compensation" means adjusted net self-employment earnings) depends on the contribution. The IRS resolves this by applying a reduced rate to earnings measured before the contribution is subtracted. For the maximum 25%-of-compensation plan, that reduced rate is 25% ÷ 125% = 20%. An S-corp owner has actual W-2 wages that already exclude the contribution, so no such translation is needed there, which is why S-corp guidance simply says "25% of wages" while sole-proprietor guidance says "about 20% of net earnings." Both describe the same underlying rule, applied to two different bases. The full derivation, with a second worked example at a 10% target rate, is on Solo 401(k) contribution limits for 2026.
Why do I have to wait for year-end net profit to know my employer contribution?
For a sole proprietor, the employer contribution is a percentage of adjusted net earnings from self-employment, a number that only exists once the year's books are closed. Net profit can still move between now and December 31 as invoices come in or expenses land, and both the self-employment tax and the adjusted-net-earnings figure flow directly from that final number. Guessing early and overfunding creates an excess contribution that has to be corrected; guessing low leaves money on the table. That's exactly why the deadline to fund the employer piece stretches well past year-end, all the way to the business's tax-filing deadline including extensions. An S-corp owner doesn't face this problem the same way, because the employer contribution is based on W-2 wages the business already set and paid during the year.
What are the 2026 Solo 401(k) limits this calculator uses?
| Limit | 2026 amount | IRC section |
|---|---|---|
| Employee elective deferral | $24,500 | §402(g)(1) |
| Catch-up, age 50 to 59, or 64 and up | +$8,000 ($32,500 total) | §414(v)(2)(B)(i) |
| "Super catch-up," ages 60 to 63 | +$11,250 ($35,750 total) | §414(v)(2)(E)(i) |
| Combined employer + employee limit | $72,000 ($80,000 with the 50+ catch-up; $83,250 with the 60-63 super catch-up) | §415(c)(1)(A) |
| Annual compensation cap | $360,000 | §401(a)(17) |
Source: IRS Notice 2025-67, "2026 Amounts Relating to Retirement Plans and IRAs, as Adjusted for Changes in Cost-of-Living."
These are the same figures used throughout, and they match the ones this calculator applies: nobody under 50 can exceed $72,000 total, the 50-to-59 and 64-plus catch-up raises that to $80,000, and the 60-to-63 super catch-up raises it further to $83,250 (replacing the regular catch-up rather than stacking on it). The $360,000 compensation cap limits how much income or wages can even be used to calculate a contribution; the calculator applies it automatically if your number is that high.
What does this calculator not cover?
A few things are deliberately left out because they don't change the contribution math itself:
- It doesn't file anything or generate tax forms. Report the actual deduction on Schedule 1 (sole proprietor) or the S-corp's Form 1120-S, following your final numbers, not this estimate.
- It uses a simplified Schedule SE calculation (Social Security portion capped at the $184,500 wage base, Medicare portion uncapped) and skips the additional 0.9% Medicare surtax that applies only above much higher income thresholds, since that surtax doesn't affect the retirement-contribution math.
- For S-corp owners, it doesn't flag the SECURE 2.0 rule that anyone whose prior-year wages from the sponsoring employer exceeded $150,000 must make any catch-up contribution as Roth (after-tax) rather than pre-tax. That's a tax-treatment rule, not a contribution-limit rule, so it doesn't change the dollar totals shown here.
- It assumes you're maximizing the employee deferral for your age bracket. If you plan to defer less, your total contribution room is simply that smaller deferral plus the same employer piece.
This tool estimates contribution room; it is not tax or investment advice and doesn't replace a preparer's calculation from your actual return.
The flat truth
The employee deferral is the easy half of a Solo 401(k): pick a number up to the 2026 limit for your age, and it's done. The employer half is where a plain rate would mislead you, because "20% of net earnings" and "25% of wages" are the same rule, applied to two different bases, and the sole-proprietor version can't be locked in until the year's books close. This calculator exists to make that math visible rather than a black-box number, which is exactly the point of running your own figures through it before you decide how much to fund.
Related reading: what a Solo 401(k) is and who qualifies, the full 2026 limits breakdown with worked examples, and Solo 401(k) vs. SEP-IRA if you're weighing the simpler alternative.
Sources
- 2026 elective deferral, catch-up, super catch-up, 415(c) combined limit, and compensation cap: IRS Notice 2025-67, "2026 Amounts Relating to Retirement Plans and IRAs, as Adjusted for Changes in Cost-of-Living" (PDF).
- The reduced-rate mechanic and the self-employed worked examples: IRS, "Self-employed individuals: Calculating your own retirement plan contribution and deduction" and IRS Publication 560.
- 2026 Social Security wage base ($184,500), used only in the sole-proprietor SE-tax step: Social Security Administration, "Contribution and Benefit Base".