Retirement Saving on Variable Income: SEP, Solo 401(k), and the Math That Picks the Right One
The hardest savings target to hit when your income is variable is retirement. Not because the math is complicated. Because the discipline cost of contributing every month, from inconsistent income, is high.
You miss a few months. You tell yourself you will catch up. The catch-up never quite happens. Year-end comes and your retirement contribution is half what you intended. The tax savings you were counting on is half what it should have been. The compound growth on the missed contributions does not happen at all.
The fix is the same fix that works for everything else with variable income: per-deposit allocation, automatic, before you can spend the money. Plus picking the right account type for your situation. Both pieces matter.
Here is the landscape, the math that decides which account fits you, and the system that funds it deposit by deposit.
This piece sits inside the broader How to Pay Yourself as a Business Owner With Variable Income guide.
Why Self-Employed People Under-Save for Retirement
A few patterns conspire against you.
Pattern 1: No automatic withholding from a paycheck.
W-2 employees often have retirement contributions deducted from their paycheck before they see the money. The discipline is built into the system. Self-employed people see the full deposit, then have to choose to send a chunk to retirement. The choice gets skipped on slow months.
Pattern 2: The contribution feels like missed liquidity.
A retirement contribution locks money up for decades (or pays a 10 percent penalty plus tax to access early). Self-employed people, with their reserve anxiety, often prefer keeping money liquid. Reasonable in the short term. Catastrophic over a 30-year career.
Pattern 3: No employer match to motivate the start.
W-2 workers often start a 401(k) because the employer match is free money. Self-employed people get no match. The motivation to start is purely your own future self, which feels less compelling than free money.
Pattern 4: The plan options are confusing.
SEP IRA. Solo 401(k). Traditional IRA. Roth IRA. SIMPLE IRA. Each has different limits, different rules, different fits. Self-employed people often delay choosing until they understand them all, and then never choose at all.
The combination produces a familiar outcome: 40 percent of self-employed Americans have less than $25,000 saved for retirement. The discipline gap, not the math gap, is the cause.
The Four Account Types
Most self-employed people will use one or two of these. Here is what each does.
Traditional IRA.
The simplest option. Anyone with earned income can contribute. Contributions are tax-deductible (if your income is below certain thresholds, which most self-employed people are). Earnings grow tax-deferred. Withdrawals after 59 1/2 are taxed as ordinary income.
Contribution limit (2024): $7,000 per year ($8,000 if 50 or older).
Best fit: - Side hustlers and part-time self-employed people - Anyone supplementing other retirement accounts - People who want simplicity over maximum contribution
Roth IRA.
Same structure as Traditional but contributions are made with after-tax money. Earnings grow tax-free. Qualified withdrawals are completely tax-free.
Contribution limit (2024): same $7,000/$8,000.
Income limits: phases out starting at $146,000 single, $230,000 married (2024 numbers). Above the phase-out, you can do a backdoor Roth (contribute to Traditional, immediately convert to Roth) if you do not have other traditional IRA balances.
Best fit: - Younger self-employed people in lower current tax brackets - Anyone who expects to be in a higher tax bracket at retirement - People who want tax-free withdrawals later
SEP IRA.
Designed for self-employed people. Higher contribution limits than Traditional/Roth, simpler than Solo 401(k).
Contribution limit (2024): 25 percent of net self-employment income, up to $69,000.
The math: net self-employment income (Schedule C net profit) minus half of self-employment tax, multiplied by 0.2 for self-employed people (not 0.25, because of the math of how the deduction stacks).
For $80,000 of net Schedule C profit, the SEP contribution limit is roughly $14,800.
Best fit: - Solo operators with steady net profit between $40,000 and $200,000 - People who want simple paperwork (no annual filing required for SEPs under certain sizes) - Self-employed people who do not have employees (SEP requires equal-percentage contributions for any employees)
Solo 401(k).
Designed for self-employed people without employees (other than a spouse). Highest contribution limits.
Contribution limits (2024): - Employee elective deferral: $23,000 ($30,500 if 50 or older) - Employer contribution (you as the business): up to 20 percent of net self-employment income - Combined cap: $69,000 ($76,500 if 50 or older)
For $100,000 of net self-employment income, you can contribute roughly $23,000 + $18,500 = $41,500. For $150,000 in net profit, the combined limit is closer to $51,000.
Plus: Solo 401(k)s often allow Roth contributions (Roth Solo 401(k) component), loans (up to $50,000 against the balance), and rollovers from other accounts.
Best fit: - Self-employed people with no employees making over $60,000 of net profit - Anyone wanting to maximize annual retirement contributions - People who want Roth contribution options at higher income levels (no Roth income limits for Solo 401(k) Roth component)
Drawbacks: - Annual Form 5500-EZ filing required once balance exceeds $250,000 - More paperwork than SEP - Usually requires a brokerage that supports Solo 401(k) plans (Fidelity, Schwab, Vanguard, E*TRADE all offer them)
Picking the Right Account
A simple decision tree.
Question 1: Do you have any employees?
If yes, your options change significantly. SIMPLE IRA, Group 401(k), or SEP with proportional contributions for employees. Talk to an accountant.
If no, continue.
Question 2: What is your net self-employment income?
Under $30,000: Roth IRA. Maximum contribution. Maximum tax-free growth runway. Skip the SEP/Solo 401(k) complexity until income justifies.
$30,000 to $60,000: Roth IRA + SEP IRA. The Roth handles tax-free growth; the SEP captures additional pre-tax savings.
$60,000 to $200,000: SEP IRA or Solo 401(k). Solo 401(k) usually wins because of the higher contribution ceiling at the lower end of this range.
Over $200,000: Solo 401(k) with Roth component, fully maxed.
Question 3: Will you have employees later?
If maybe in the next few years: stick with SEP IRA. It is easier to scale to a small team than to terminate a Solo 401(k).
If solo for the foreseeable future: Solo 401(k) gives you the higher ceiling.
For most self-employed people without employees making $50,000 to $150,000 of net profit, the Solo 401(k) is the right answer. The contribution flexibility and Roth option make it the best vehicle.
The Per-Deposit Allocation
The discipline trick that actually makes retirement saving happen on variable income.
The standard advice is "contribute to your retirement account at the end of the year, before the tax deadline, with whatever you have left." This rarely works. "Whatever you have left" usually turns out to be less than you intended.
The better approach: a percentage of every deposit routes to a "retirement holding" account. At the end of the year (or quarterly, if you want), you transfer the accumulated amount into your SEP or Solo 401(k).
Why a holding account first: - Solo 401(k) contributions can be made up to your tax filing deadline (April 15 or October 15 with extension). You do not need to fund it the same day the income lands. - A separate holding account keeps the retirement money out of operating cash flow. - You see the balance build, which reinforces the habit.
Recommended percentage: 10 to 15 percent of every deposit, routed to retirement holding. For a $5,000 deposit, $500 to $750 goes to retirement. For a $1,000 deposit, $100 to $150. The percentage scales naturally with the deposit size.
By year-end, the retirement holding account contains your annual contribution, ready to transfer to the actual SEP or Solo 401(k) plan.
What to Invest In
Once the contribution is in the retirement account, you have to invest it. Otherwise it earns nothing.
The default that works for almost everyone: low-cost broad-market index funds.
Three-fund portfolio (works for 90 percent of self-employed savers): - 60 to 80 percent: U.S. total stock market index fund (Vanguard VTI, Fidelity FZROX, Schwab SCHB) - 10 to 20 percent: International stock index fund (VXUS, FZILX, SCHF) - 10 to 30 percent: Bond index fund (BND, FXNAX, SCHZ)
The exact percentages depend on your age and risk tolerance. Younger people lean heavier into stocks (80/20 or 90/10). Older people lean heavier into bonds (60/40 or 50/50).
Expense ratios should be under 0.10 percent (10 basis points). Anything higher is robbery. Vanguard, Fidelity, and Schwab all offer no-fee index funds that beat 95 percent of actively-managed funds over 20 years.
What to avoid: - Actively-managed mutual funds with 1+ percent expense ratios - Individual stock picking (almost everyone underperforms the index) - Real estate inside the retirement account (complicated, illiquid, often not worth the structure) - Bitcoin / crypto inside the retirement account (volatility kills long-term compounding)
Boring is the strategy. Boring works.
Common Retirement Mistakes for Self-Employed
Mistake 1: Starting late because the account is "too small."
A $50 monthly contribution at age 25 grows to $250,000 by age 65 at 8 percent. The same $50 starting at age 45 grows to $30,000. Time matters more than amount. Start with whatever you can do, even if it feels meaningless.
Mistake 2: Contributing once a year instead of monthly.
End-of-year contributions miss most of the year's growth. A $14,000 SEP contribution in January earns a full year of growth. The same contribution in December earns nothing that year. Spread contributions across the year or contribute as early in the year as possible.
Mistake 3: Picking the wrong account type for your situation.
Self-employed people often default to a Traditional IRA because it is familiar. The SEP or Solo 401(k) lets them contribute 3 to 10 times more. The wrong account type costs you tens of thousands in missed contributions over a career.
Mistake 4: Not adjusting contributions to actual income.
You set a $1,000 monthly contribution at the start of the year, when business was strong. A slow quarter hits. You keep contributing $1,000 because you "committed." The contribution drains your reserve, the reserve runs low, you end up putting an emergency on the credit card.
Per-deposit allocation solves this: 12 percent of $5,000 is $600. 12 percent of $1,000 is $120. The contribution naturally scales to actual income, not to a fixed plan made in January.
Mistake 5: Treating retirement money as accessible.
Early withdrawals (before 59 1/2) trigger a 10 percent penalty plus regular income tax. A $10,000 early withdrawal nets about $6,500 to $7,000 after penalty and tax. The retirement money is not your emergency fund. The emergency fund is your emergency fund.
Mistake 6: Forgetting the tax savings from contributions.
A $14,000 SEP contribution at a 24 percent federal marginal tax rate plus 15.3 percent self-employment tax (the half you owe directly) saves you about $5,500 in tax. The contribution is real money in your retirement account, but the cost to your take-home is only $8,500. Most self-employed people underestimate the tax savings and therefore under-contribute.
The Self-Employment Tax Deduction Note
A wrinkle that matters for the math.
When you calculate your SEP IRA or Solo 401(k) "employer contribution" portion, the 25 percent number gets adjusted because of how self-employment tax interacts with the deduction.
The effective contribution limit for the employer portion is approximately:
Net Schedule C profit minus half of self-employment tax, multiplied by 0.2 (not 0.25).
For most self-employed people, this works out to roughly 18.6 percent of net profit, capped at the annual limit. Your accountant or tax software does this math automatically.
The Solo 401(k) "employee" contribution is on top of the employer portion. Up to $23,000 in 2024.
What Changes When Retirement Saving Happens Automatically
The first thing that changes is your sense of future security.
Before automatic saving, retirement felt theoretical. You knew you should save but did not have a system that made it real. The future was an abstract anxiety.
After automatic saving, the retirement balance grows visibly. You check it once a year, maybe once a quarter, and the number is bigger. The compound growth becomes real. The anxiety quiets.
The second thing that changes is your tax bill.
A maximum-funded SEP or Solo 401(k) for a self-employed person making $100,000 saves $10,000 to $15,000 in tax that year. The contribution is real money in your retirement account; the tax savings is real money in your pocket. Not contributing is leaving both on the table.
The third thing that changes is your relationship with slow months.
Per-deposit retirement allocation scales to actual income. Slow months produce smaller contributions automatically. You stop feeling like the slow months are eating into your retirement, because the contribution is proportional. The contribution does its job at every income level.
You are able to pay down debt, even on slow months.
You are able to save without second-guessing.
You are able to predict what is coming.
You are able to budget inconsistent income.
Use the App
Able routes a percentage of every deposit toward a retirement holding bucket. By year-end, the bucket contains your contribution, ready to transfer into your SEP or Solo 401(k). The per-deposit math handles the variance automatically. You make one decision (the percentage) and the system runs it on every check.
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