Solo 401(k) for the Self-Employed: How Much Tax Can It Actually Save?

If you work for yourself with no employees (a spouse can count), the Solo 401(k) is one of the most powerful tax tools you can open. It’s also one of the most misunderstood. People ask things like, “How can my Solo 401(k) cut my tax bill as much as possible?” and then get ten different answers.

Here’s the plain-English version, no accounting degree required.

The big idea: you’re both the employee and the employer

A Solo 401(k) lets you contribute in two roles at once:

• As the employee, you defer part of your earnings. For 2026 the employee limit is $24,500, with extra catch-up room if you’re 50 or older (and a bigger catch-up for ages 60 to 63).
• As the employer, you make a profit-sharing contribution. For a sole proprietor, this is generally capped at about 20% of your net self-employment earnings after the deduction for half of your self-employment tax. Your pro or plan provider can calculate the exact figure.

Both pieces together are subject to an overall annual limit, which is $72,000 for 2026 before catch-up contributions. Limits change every year, so check current IRS guidance before you contribute.

That two-hat structure is why a Solo 401(k) can shelter much more income than an IRA for someone with decent profits.

How it lowers your tax bill

There are two flavors of the employee contribution, and they work very differently:

Traditional (pre-tax). Your contribution reduces your taxable income this year. If your marginal rate is 24%, every $1,000 you put in lowers your federal income tax by roughly $240, and you pay tax later when you withdraw in retirement.

Roth (after-tax). No deduction now, but qualified withdrawals later are tax-free. Roth is attractive when you expect higher rates in the future, or when this year’s income is unusually low.

The employer profit-sharing piece is typically pre-tax, though some plans allow Roth treatment for that too. Check what your provider supports.

One thing a Solo 401(k) does not do is reduce your self-employment tax. Retirement contributions lower your income tax, but your SE tax is still calculated on your net profit. That’s covered in our self-employment tax explainer.

A simple example

Say you net $90,000 of profit as a freelance designer. Roughly speaking:

1. You calculate SE tax on your net profit, then deduct half of it.
2. You can defer up to the employee limit from your earnings.
3. You can add an employer contribution of about 20% of your adjusted net earnings.

Even if you only contribute, say, $15,000 pre-tax and you’re in the 22% bracket, that’s about $3,300 less federal income tax. The numbers here are illustrative only, since your exact result depends on your filing status, state, and other income.

The deadlines that trip people up

This is where good intentions go to die:

• Opening the plan. For employee deferrals, many providers require the plan to be established by the end of the calendar year in which you want to defer. Waiting until April and then asking to defer last year’s income can be too late.
• Employee deferral timing. In general, the employee-contribution election has to be set up within the year it applies to.
• Employer contribution timing. The employer contribution can often be made up to your tax filing deadline, including extensions.
• Form 5500-EZ. Once your plan’s assets pass a threshold (check current IRS guidance), you have an annual filing requirement.

If it’s already October, you still have time to open a plan and defer for this year, but the window is closing. Ask your pro or provider about their specific cutoffs.

Solo 401(k) vs. SEP IRA

People often compare the two:

• Solo 401(k): higher potential contributions at lower profit levels, a Roth option, and possible loan features. More setup and some paperwork.
• SEP IRA: simple to open, employer contributions only, can be funded up to the filing deadline. Less flexibility, and contributions are capped at a percentage of profit.

If your profit is modest, the Solo 401(k) usually lets you save more because of the employee deferral. If you have employees, the rules change a lot, so talk to a pro.

How it interacts with other deductions

A pre-tax Solo 401(k) contribution lowers your taxable income, which can also affect other things. For example, it may change the size of your qualified business income deduction, so it’s worth reading how the QBI deduction works. It also lowers the income you base your estimates on, which matters for avoiding the kind of April tax surprise that catches new freelancers.

Common mistakes

• Contributing more than you earned. You can’t defer more than your net self-employment earnings.
• Mixing personal and business accounts. Keep contributions clean and traceable.
• Forgetting state rules. Most states follow the federal treatment, but check yours.
• Raiding it early. Withdrawals before retirement age often trigger penalties.
• Spending the cash you’d need for taxes. A Solo 401(k) contribution is not a substitute for setting aside tax money.

This is why Toozi exists

The hardest part of using a Solo 401(k) isn’t opening it. It’s knowing how much you can actually afford to put in, given your real profit, your tax bill, and your cash flow. Most people guess in December and either under-save or run out of money for their tax payment.

Toozi tracks your income and expenses over SMS so you can see your year-to-date net profit and what you should be setting aside. With that in hand, deciding on a retirement contribution becomes a math question instead of a gamble. See your numbers at toozitax.app.

The Toozi team

Toozi isn’t your CPA or financial advisor, and this isn’t formal tax or investment advice. Contribution limits and rules change every year, so verify current figures with the IRS or a tax professional before you contribute.