Solo 401(k) Contribution Calculator 2026
The Solo 401(k) has two contribution buckets — employee deferral and employer profit-sharing — that most owners don't fully use together. For S-corp owners, the W-2 salary level you set determines how much profit-sharing you can receive. This calculator shows your exact 2026 maximum, the optimal S-corp salary point, and your mega-backdoor Roth capacity.
How the Solo 401(k) two-bucket system works
Bucket 1 — employee elective deferral
As the employee of your own business, you can defer up to $24,500 in 2026 from your W-2 wages (S-corp) or self-employment income (sole prop). This portion can be Roth or pre-tax. The deferral limit applies per person across all plans — if you also participate in a day-job 401(k), your combined employee deferrals cannot exceed $24,500.
Catch-up contributions on top of the base deferral:1
| Age bracket | Additional catch-up | Total employee deferral |
|---|---|---|
| Under 50 | $0 | $24,500 |
| 50–59 or 64+ | $8,000 | $32,500 |
| 60–63 (SECURE 2.0 super catch-up) | $11,250 | $35,750 |
Bucket 2 — employer profit-sharing
You also contribute as the employer. The rate is:
- S-corp or C-corp owners: Up to 25% of your W-2 wages. To reach the §415(c) ceiling, your W-2 salary needs to be at least $190,000 (25% × $190K = $47,500; add $24,500 employee deferral = $72,000).2
- Sole proprietors: Up to 20% of net self-employment income (the IRS Pub. 560 rate-table calculation — effectively 25% of compensation after the SE tax deduction). The effective rate on gross net profit runs 20–23%.3
The §415(c) combined limit
Employee deferral + employer profit-sharing cannot exceed $72,000 in 2026 (plus catch-up, which is in addition to this ceiling). High earners hit this limit whether they use SEP-IRA or Solo 401(k). At lower incomes, the Solo 401(k) beats a SEP-IRA because the $24,500 employee deferral fills the gap where the employer-only SEP falls short.
Mega-backdoor Roth through a Solo 401(k)
If your Solo 401(k) plan document allows after-tax contributions and in-service withdrawals (not all do — Fidelity's Keogh/solo plan does not currently; some third-party plan documents do), you can contribute beyond the pre-tax buckets up to the $72,000 §415(c) ceiling, then immediately convert that after-tax money to Roth.
Example (age 45, W-2 $100,000, 35% bracket):
- Pre-tax employee deferral: $24,500
- Employer profit-sharing (25% × $100K): $25,000
- After-tax mega-backdoor Roth capacity: $72,000 − $24,500 − $25,000 = $22,500
- Tax saved via pre-tax contributions at 35%: $17,150/yr
- Roth conversion eliminates future RMDs on $22,500/yr accumulation
This is in addition to a Roth IRA ($7,500/yr in 2026 if income is below the phaseout) and any Roth catch-up contributions.
2026 Solo 401(k) limits at a glance
| Parameter | 2026 Value | Authority |
|---|---|---|
| Employee deferral (base) | $24,500 | IRC §402(g); IRS Notice 2025-671 |
| Catch-up deferral (age 50–59/64+) | $8,000 | IRC §414(v)(2)(B)(i); IRS Notice 2025-67 |
| Super catch-up (age 60–63) | $11,250 | IRC §414(v)(2)(B)(ii); SECURE 2.0 §109 |
| §415(c) total annual additions limit | $72,000 | IRC §415(c); IRS Notice 2025-67 |
| Employer profit-sharing rate (S-corp) | 25% of W-2 | IRC §401(a)(3) |
| Employer profit-sharing rate (sole prop) | ~20% of net profit | IRS Pub. 560 Rate Table3 |
| W-2 salary to max out §415(c) at 45 | $190,000 | ($72K − $24.5K) ÷ 25% |
| Social Security wage base 2026 | $184,500 | SSA4 |
| Compensation cap for contributions | $360,000 | IRC §401(a)(17); IRS Notice 2025-67 |
| High-earner Roth catch-up mandate | Catch-up must be Roth if FICA wages >$150K in prior year | SECURE 2.0 §603 (eff. 2026)5 |
Sole prop vs S-corp: Solo 401(k) contribution compared
| Net income / W-2 | Sole prop (approx) | S-corp (W-2 salary) | Advantage |
|---|---|---|---|
| $80,000 | ~$37,100 | Same (same math once W-2 is set) | — |
| $120,000 | ~$46,000 | Depends on W-2 salary elected | S-corp if salary optimized |
| $190,000+ | $72,000 (max out) | $72,000 if W-2 ≥ $190K | Tie |
| Age 50+, any income ≥ $190K | $80,000 (with $8K catch-up) | $80,000 (with $8K catch-up) | Tie |
| Age 60–63, any income ≥ $190K | $83,250 (super catch-up) | $83,250 (super catch-up) | Tie |
Solo 401(k) vs SEP-IRA: when the Solo wins
The SEP-IRA and Solo 401(k) share the $72,000 §415(c) ceiling, but the Solo 401(k) wins in almost every situation for owner-only businesses:
- At lower income: Solo 401(k) leads because the $24,500 employee deferral fills contribution space the SEP formula can't reach. A sole prop earning $100K can put $39,300 in a Solo 401(k) vs. ~$19,600 in a SEP-IRA.
- At 50+: Solo 401(k) adds $8,000–$11,250 that SEP can never match.
- Roth access: Solo 401(k) allows Roth deferral at any income. SEP has no Roth option.
- Stacking: Solo 401(k) can be combined with a cash balance plan for an additional $90K–$330K of annual tax-deferred space. SEP-IRA can also stack, but plan aggregation rules are more complex.
The SEP-IRA wins only on simplicity (no plan document, no Form 5500-EZ when assets exceed $250K) and when you have full-time W-2 employees who would qualify for coverage.
Related calculators and guides
- Compare all plans: Solo 401(k) vs SEP-IRA vs Cash Balance side-by-side
- SEP-IRA Calculator — exact 2026 contribution with SE tax step-by-step math
- Cash Balance Plan Calculator — stack an additional $90K–$330K on top of Solo 401(k)
- Solo 401(k) 2026: Roth option, mega-backdoor Roth, and adoption deadline
- SEP-IRA 2026: complete rules, employee coverage trap, and contribution deadline
- All retirement plans for business owners: hub guide
- Quarterly estimated tax calculator — include retirement deductions in your safe harbor
Structure your Solo 401(k) with a specialist
The calculator shows your contribution ceiling — but the optimal Solo 401(k) strategy involves your S-corp salary level, whether to stack a cash balance plan, which plan document allows mega-backdoor Roth, and how contributions interact with your QBI deduction and quarterly estimated taxes. A fee-only financial advisor who specializes in business-owner planning can model all of this for your specific situation. Free match, no obligation.
- Solo 401(k) 2026 employee deferral: $24,500; catch-up age 50–59/64+: $8,000; super catch-up age 60–63: $11,250. IRC §402(g), §414(v)(2)(B). IRS: 401(k) limit increases to $24,500 for 2026. IRS Notice 2025-67 (October 2025).
- §415(c) 2026 total annual additions limit: $72,000. S-corp employer profit-sharing rate: 25% of W-2 wages. IRC §415(c), §401(a)(3). IRS Notice 2025-67. To reach $72K ceiling at age <50: W-2 must be ≥ ($72,000 − $24,500) ÷ 25% = $190,000.
- Sole-prop Solo 401(k) employer contribution calculation and effective ~20% rate. IRS Publication 560: Retirement Plans for Small Business (2026 ed.), Rate Table for Self-Employed Individuals. The SE tax deduction reduces the contribution base: effective rate = plan rate ÷ (1 + plan rate) = 25% ÷ 125% ≈ 20%.
- 2026 Social Security wage base: $184,500. SSA: Contribution and Benefit Base 2026.
- SECURE 2.0 §603: catch-up contributions for participants with FICA wages >$150K in prior year must be Roth effective 2026. IRC §414(v)(2)(B) as amended. IRS: Retirement Topics — Catch-up Contributions.
All tax values verified June 2026 against IRS Notice 2025-67, IRC §402(g), §414(v), §415(c), §401(a)(3), §401(a)(17), and IRS Publication 560.