Solo 401(k) Calculator

A Solo 401(k), also called an individual or one-participant 401(k), is a 401(k) plan for a business owner with no employees other than a spouse. It lets you contribute as both the employee (an elective deferral) and the employer (profit-sharing), which is why it has the highest contribution ceiling of any self-employed plan. It also allows Roth contributions and plan loans, neither of which a SEP IRA offers (IRS: one-participant 401(k) plans).

In 2026 you can contribute up to $72,000 combined, or $80,000 if you are 50 to 59 or 64+, and $83,250 at ages 60 to 63 with the SECURE 2.0 super catch-up. That total is built from two parts: an employee deferral of up to $24,500, plus an employer profit-sharing contribution of up to 25% of compensation. The $72,000 figure is the overall section 415(c) limit; catch-up contributions stack on top of it (IRS Notice 2025-67).

It mirrors a SEP IRA. If you are a sole proprietor, the employer profit-sharing piece is 20% of your net earnings from self-employment (net profit minus half of your self-employment tax). If your business is an S-corp or C-corp, it is 25% of your W-2 wages. On top of that employer piece, the Solo 401(k) adds the employee deferral, which is the part a SEP IRA does not have.

At the same income, a Solo 401(k) almost always wins, because it stacks a $24,500 employee deferral (2026) on top of the same 25% employer contribution a SEP IRA offers, and it allows catch-up contributions at 50 and older. The SEP IRA's advantages are simplicity and timing: no plan document and no Form 5500, and you can open one right up to your extended tax deadline. The calculator above shows both numbers side by side so you can see the gap at your income.

Yes, if your plan document allows it, and most modern Solo 401(k) providers do. The employee deferral portion can go in as Roth (after-tax, tax-free in retirement) instead of Traditional. The employer profit-sharing portion is always pre-tax. Unlike a Roth IRA, a Roth Solo 401(k) has no income limit, so high earners who are shut out of a Roth IRA can still build Roth savings this way. Use the Traditional, Roth, and Split toggle above to model it.

If your plan supports after-tax (non-Roth) contributions and in-plan Roth conversions, you can fill the space between your employee-plus-employer contributions and the $72,000 overall limit with after-tax money, then convert it to Roth. This is the mega backdoor Roth. Not every Solo 401(k) document allows it, so confirm with your provider. The calculator flags how much room you have when your regular contributions fall short of the limit.

Yes, but the $24,500 employee deferral is a single limit shared across every 401(k) and 403(b) you participate in, including your employer's. The employer profit-sharing side of your Solo 401(k) is separate and based on your self-employment income, so you can still make that contribution in full. If you already max the deferral at your day job, your Solo 401(k) is limited to the employer profit-sharing piece.

The plan must generally be established by your business's tax filing deadline, including extensions, to make contributions for that year, and employer contributions can be funded up to that deadline. Employee deferrals are more time-sensitive: you should formally elect them before year-end. This is slightly less flexible than a SEP IRA, which can be both opened and fully funded right up to the extended deadline.