How To Calculate The Maximum Solo 401(k) Contribution As Both Employer And Employee

Your maximum total contribution is the sum of your employee elective deferral (up to $23,000, or $30,500 if 50+) plus an employer profit-sharing contribution of up to 25% of compensation, but the combined total cannot exceed $69,000 (or $76,500 with catch-up) and your compensation base is your net self-employment income minus half of self-employment tax.

The solo 401k contribution two-part breakdown

The Solo 401(k) splits into two distinct contributions with different rules. The employee deferral is a flat dollar cap. For the 2024 tax year, the IRS sets that cap at $23,000. If you are 50 or older, the IRS raises the cap to $30,500. You can choose to contribute any amount up to that cap, including zero, as long as you have enough earned income to cover it. The employer profit-sharing contribution is a percentage-of-compensation cap. You may contribute up to 25% of your plan-eligible pay. The calculation uses a special rate because the contribution itself reduces your pay. You must calculate them separately. The employee piece ignores your income level entirely. It is the same dollar cap for everyone. The employer piece scales with your earnings. The IRS treats the two under different sections of the tax code. Your deferral is an elective salary reduction. The profit-sharing is a non-elective employer contribution. For a self-employed person, the 25% rate applies to your net earnings from self-employment after deducting half of self-employment tax. It does not apply to your Schedule C line 31 net profit directly.

Calculating your pay as a sole proprietor

To find the pay base for your employer contribution, start with your net profit from Schedule C (line 31). Then compute your self-employment tax using Schedule SE. This is the Social Security and Medicare tax on 92.35% of your net profit. It is calculated at the combined 15.3% rate for 2024. The Social Security portion is subject to a wage base. The IRS sets that wage base at $168,600 for the year. Take that self-employment tax amount, divide it by two, and subtract that half from your net profit. For example, if your net profit is $100,000, your self-employment tax is roughly $14,130. That figure comes from taking 92.35% of $100,000 to get $92,350, then applying 15.3%. Half of that tax is $7,065. Your plan-eligible pay is therefore $100,000 minus $7,065, or $92,935. The 25% employer contribution is then 25% of that figure, which equals $23,234. But note the IRS provides a simplified method. You can multiply your net profit by 20% directly to get the same result. The 25% rate applied to net profit minus half SE tax simplifies to 20% of net profit for most taxpayers. That shortcut works only when your net profit is below the Social Security wage base. If you earn more, the calculation gets more complex because the SE tax deduction phases out. Always confirm current wage bases and rates with the official IRS instructions for Schedule SE.

When the overall limit caps your contribution

The failure case occurs when your pay is high enough that the full employee deferral plus the full 25% employer contribution would exceed the annual additions limit. The IRS sets that overall ceiling at $69,000 for 2024. With catch-up, the IRS raises the ceiling to $76,500. For 2024, this threshold hits when your plan-eligible pay exceeds roughly $184,000. The crossover point is lower in practice because the employer contribution reduces the base. Suppose your net profit is $250,000. Your SE tax is capped by the Social Security wage base. Half of it is about $9,767, which is half of the maximum SE tax the IRS sets for 2024. Your pay is $240,233. Twenty-five percent of that is $60,058. Adding the full $23,000 employee deferral gives $83,058. That sum exceeds $69,000 by $14,058. In that scenario, you must reduce the employer contribution to $46,000 to stay within the cap. That figure is $69,000 minus $23,000. The true maximum employer contribution is therefore not 25% of pay. It is the smaller of 25% of pay or $69,000 minus your employee deferral. For someone age 50 or older, the cap rises to $76,500. The same logic applies. Subtract your $30,500 catch-up-inclusive deferral from $76,500 to find the maximum employer contribution. In this example, that would also be $46,000.

Frequently asked questions

Can I make my employee deferral after I file my business tax return?

No, employee deferrals must be elected by December 31 of the tax year. But you can make the actual contribution as late as the tax filing deadline, including extensions. That deadline is typically April 15. The employer profit-sharing contribution has the same deadline. You have until then to finalize both amounts.

What if I also have a traditional 401(k) from a former employer?

Your employee deferral limit of $23,000 is shared across all 401(k) plans you participate in. That includes your Solo 401(k) and any old employer plan. You cannot defer more than $23,000 in total. But the employer profit-sharing contributions from each plan are separate. They do not reduce your Solo 401(k) employer contribution.

Does the 25% employer contribution apply if I have a SEP-IRA instead?

No, a SEP-IRA uses a different calculation. You contribute up to 25% of your net earnings. But the rate is effectively 20% of net profit because the SEP contribution reduces your pay. The annual limit is also $69,000 for 2024. The key difference between a solo 401(k) vs sep-IRA is that the Solo 401(k) lets you add the $23,000 employee deferral on top of the employer piece. A SEP-IRA has no employee deferral component at all. This two-part structure is what makes self-employed retirement plans like the Solo 401(k) uniquely powerful for high savers.

How do I report these contributions on my tax return?

Your employee deferral is reported on Schedule 1, line 16 as “Self-employed SEP, SIMPLE, and qualified plans.” It reduces your adjusted gross income. The employer profit-sharing contribution is deducted on Schedule 1, line 15 as “Self-employed SEP, SIMPLE, and qualified plans.” It also reduces your AGI. You must file Form 5498 from your plan custodian to show the contribution was made.