Solo 401(k) Contribution Calculator

A Solo 401(k) (also called Individual 401(k) or one-participant 401(k)) is a 401(k) plan for self-employed individuals or small business owners with no employees other than themselves and their spouse. Available to sole proprietors, single-member LLCs, S-corps, and C-corps. The owner can contribute as both employee (salary deferral) AND employer (profit-sharing) - substantially higher limits than SEP-IRAs at the same income level.

Total annual additions limit: $70,000 (or higher with catch-up). Employee deferral: $24,500 + $7,500 catch-up at 50+ + $11,250 "super catch" at 60-63 (SECURE 2.0). Employer contribution: 25% of W-2 salary (S-corp) or ~20% effective of net self-employment income (sole prop / SMLLC) due to the deduction interaction. Combined cap: $70,000 + applicable catch-ups. Roth Solo 401(k) variant available since SECURE 2.0.

Solo 401(k) almost always wins for the same income level because: (1) employee deferrals stack on top of employer contributions; (2) age 50+ catch-up of $7,500 available (SEP has none); (3) Roth option; (4) loan provisions up to $50,000 or 50% of balance; (5) easier to roll pre-tax IRAs in (helps backdoor Roth). SEP wins on simplicity - no Form 5500-EZ filing until $250K plan assets, no plan document required, can be opened up until tax-filing deadline.

The plan must be ESTABLISHED by December 31 of the contribution year (SECURE 2.0 allows employer-side contributions to be elected up until the tax-filing deadline including extensions). Employee deferrals must be elected by year-end. Contributions can be made up until the tax-filing deadline (April 15 or October 15 with extension). Form 5500-EZ filing is required once plan assets exceed $250,000.