Solo 401(k) and SEP IRA contribution calculator (2026)

How the maximum is worked out

A sole proprietor's "compensation" for retirement plans is not the profit on Schedule C. Publication 560 has you subtract the deductible half of self-employment tax first, and the contribution itself comes out of the base too. That is why the 25% plan rate works out to 20% of net earnings for a self-employed person. The calculator runs the self-employment tax the same way as the self-employment tax calculator, then applies 20%.

A SEP IRA stops there: the employer contribution is all it allows, up to $72,000 for 2026. A solo 401(k) adds an employee deferral of up to $24,500 on top, and the two together are capped at $72,000. Catch-up deferrals sit above that cap: $8,000 from age 50, or $11,250 at ages 60 to 63. Compensation above $360,000 is ignored.

If you already defer into a 401(k) at a job, those dollars count against the same $24,500, so the solo 401(k)'s extra room shrinks. When the day-job plan takes the full amount, the two plans come out nearly even, and the SEP's simpler paperwork may win. The trade-offs beyond the dollar limit, including deadlines and the Form 5500-EZ filing, are in Solo 401(k) vs SEP IRA.

What this calculator assumes

No employees, a single business, no S corporation salary, and a calendar tax year. It does not model Roth vs pre-tax treatment, the high-earner Roth catch-up rule, or a spouse working in the business. Deadlines to open and fund each plan differ; check them before you count on a number.