Solo 401(k) Contribution Calculator

If you earn self-employment income from locums, moonlighting, consulting or your own practice, a solo 401(k) can let you save a lot more for retirement than an IRA, and cut your taxes in the process. This calculator estimates the most you can contribute in 2026.

What this calculator does

Choose whether you're a sole proprietor (1099 or single-member LLC) or paid a W-2 salary by your own S corporation. Enter your net profit or salary, your age, any employee deferrals you've already made to another 401(k) or 403(b) this year, and your marginal tax rate. It shows:

• the maximum total contribution,
• the employee deferral,
• the employer (profit-sharing) contribution, and
• the estimated tax saved if contributions are pre-tax.

The formula in plain English

A solo 401(k) has two parts.

Employee deferral: up to $24,500 in 2026, minus anything you've already deferred to another 401(k) or 403(b) this year. If you're 50 or older, you can add an $8,000 catch-up, or $11,250 if you're 60 to 63.

Employer contribution:

• Sole proprietor: 20% of net profit after subtracting half of your self-employment tax.
• S corporation: 25% of your W-2 salary from the S corporation.

Limits:

• Employee deferrals plus employer contributions can't exceed $72,000, not counting catch-up.
• Contributions can't exceed your earned income.
• Compensation above $360,000 doesn't count.

A worked example

Dr. Patel does locums as a sole proprietor and has $120,000 of net profit. She's 42 and has no other 401(k).

• Employee deferral: $24,500.
• Half her self-employment tax is about $8,480, so her adjusted earnings are about $111,500.
• Employer contribution: 20% × $111,500 = about $22,300.
• Total: about $46,800.
• At a 35% combined tax rate, that saves about $16,400 in income tax.

If she also had a W-2 job where she'd already deferred $24,500, she could still make the employer contribution of about $22,300.

Why it matters at each career stage

If you're: Resident / fellow

If you moonlight as an independent contractor, a solo 401(k) lets you shelter part of that income. Even modest contributions add up, and Roth contributions make sense at residents' lower tax rates.

If you're: Employed

Many employed clinicians with side 1099 income miss the employer contribution room in a solo 401(k). If you have locums, telehealth or expert-witness income, this can be a big tax saver.

If you're: Practice owner

Solo practice owners with no employees can save a lot this way. If you hire staff, you'll likely need a different plan, like a traditional 401(k) with safe harbor contributions.

Setting up a solo 401(k)

Open the plan by year-end. Many providers offer free or low-cost solo 401(k)s.

Pick Roth or pre-tax. Many plans allow Roth employee deferrals, and some allow Roth employer contributions.

Track contributions. Keep records of employee versus employer amounts and when they were made.

File Form 5500-EZ when required. Once plan assets pass $250,000, you file an annual form.

Coordinate with other plans. Remember the shared employee deferral limit across 401(k) and 403(b) plans.

Solo 401(k) and taxes

Pre-tax contributions lower your income tax but not your self-employment tax. Pairing a solo 401(k) with quarterly estimated tax payments helps you avoid underpayment penalties while keeping more of your 1099 income working for you.

Considering an S corporation

At higher profits, some self-employed clinicians form an S corporation to lower self-employment tax. The solo 401(k) math changes: the employer contribution becomes 25% of the salary you pay yourself, not of total profit. A lower salary saves payroll tax but also lowers the employer contribution. A CPA can help you find the balance.

Next, estimate your self-employment tax, plan quarterly estimated taxes and compare Roth vs traditional contributions.