US freelancer: Solo 401(k) or SEP IRA for retirement?

If your income is uneven, retirement saving often fails for a boring reason: you pick a plan that doesn't match how money actually arrives.

This scenario pack compares two realistic behaviors for a single US freelancer (starting at age 38 in January 2026 with $35,000 already invested):

• A Solo 401(k) style plan: monthly autopilot contributions plus a smaller tax-time top-up.
• A SEP IRA style plan: one tax-time contribution each year.

In this pack, both paths invest about $24,000/year in total, but the timing differs:

• Solo 401(k): $1,500/month + $6,000/year top-up.
• SEP IRA: $24,000/year at tax time.

Both paths model the same retirement target (retire at 67, horizon to 92) and the same Social Security planning anchor ($2,600/month). The base and pessimistic variants use $6,200/month retirement spending; the optimistic variants use $7,600/month to keep late-life surplus in a realistic range. They also include a client-gap shock, an equipment refresh, a family-support expense, and a late-life care reserve.

What the numbers show

All amounts are in today’s dollars (the simulator uses a real return). “Safe retirement budget” is the cushion-based estimate that keeps about a 5-year reserve in this model.

What to take away:

• With broadly similar annual saving totals, the wrapper choice matters less than the saving behavior (monthly autopilot vs yearly execution).
• In the Base return case, both plans land near ~$7.1k/mo as an estimated safe retirement budget, versus the planned $6,200/mo spend.
• Compounding is the quiet driver: in Base · Solo 401(k), the model earns ≈$483k of interest by retirement and ≈$1.35M by age 92.

What this comparison evaluates

1. Does an autopilot monthly plan create more margin than a tax-time lump sum when income is uneven?
2. How sensitive is the plan to real returns over a 25-year retirement?
3. How much of your retirement budget is covered by Social Security versus your portfolio?

How the costs are planned

This page does not model taxes, contribution limits, or the detailed legal mechanics of each plan. Instead, it treats "Solo 401(k)" and "SEP IRA" as two behavior + timing patterns:

• Solo 401(k): more frequent contributions (easier to keep on track) but a bit more admin friction.
• SEP IRA: simpler, but it relies on you executing once per year.

These are not legal maximums. They're a realistic "can you actually fund it?" target that sits inside the research ranges:

• The research brief’s ordinary-year savings capacity is roughly $300-$3,500/month, and it notes a wide swing between weak and strong years.
• At the same income level, a Solo 401(k) can have higher legal contribution room than a SEP IRA because it can layer employee deferrals on top of employer-style contributions.

The strategy

Solo 401(k): build the habit, then top up

The Solo 401(k) presets contribute monthly throughout the year and add a smaller yearly top-up at tax time. This mirrors the idea that you save continuously when cashflow allows, then use a strong year’s profit to increase the final total.

SEP IRA: decide at tax time

The SEP IRA presets contribute once per year at tax time. That mirrors a real freelancer pattern: you wait until you know your profit, your tax bill, and whether cash needs to stay liquid for the next client gap.

One nuance: yearly contributions happen later in the year than monthly autopilot deposits, so you get slightly less time in the market. In this pack, the difference is small next to the return assumptions, but it’s one reason “same dollars, different timing” can still show slightly different outcomes.

Personalise it

• Replace the yearly/monthly contribution amounts with what you can actually sustain in weak and strong years.
• Update the Social Security amount (and claiming age) once you have an estimate.
• Increase the client-gap shock if you often have multi-month breaks between contracts.
• Adjust retirement spending to your real target budget (especially rent and health costs).

US-specific notes

• SEP IRA vs Solo 401(k) is not just about maximums. The most realistic failure mode is behavioral: a plan you fund late or inconsistently can lose to a “smaller” plan you actually stick with.
• Administrative friction is real. Solo 401(k) plans can have extra paperwork (for example, Form 5500-EZ once assets are large enough), while SEP IRAs are generally simpler to maintain.
• Social Security depends on reported earnings. Treat the $2,600/month as a placeholder until you can replace it with your own estimate.

This scenario is an educational model, not personal financial advice. It simplifies taxes, benefits, and investment implementation so you can compare ranges and trade-offs.