Solo 401(K) vs. SEP-IRA: the Self-Employed Retirement Decision

Self-employment's consolation prize is retirement-plan space employees can only envy, but the two main vehicles fill it differently. A SEP-IRA accepts employer contributions of up to 25% of compensation; a solo 401(k) accepts the same employer layer plus a $24,500 employee deferral (2026, before catch-ups). At $100,000 of self-employment profit, that difference is roughly $18,000 versus $43,000 of shelter, same business, same year, different paperwork.

Here is the decision, including the cases where the humble SEP still wins.

The Math at Different Incomes

For unincorporated self-employment, the employer layer works out to about 20% of net profit after the SE-tax adjustment. A SEP gets only that layer. The solo 401(k) stacks the employee deferral first: at $60,000 profit, SEP shelters ~$11,000 while the solo 401(k) shelters ~$35,500; at $150,000, roughly $28,000 versus $52,500; the gap closes only as income approaches the level where the employer layer alone hits the $72,000 overall cap (2026), roughly $350,000+, where the vehicles converge. Below that line, which is most of self-employment, the solo 401(k) simply shelters more; run your own figures against irs.gov worksheets or with your CPA.

Features Beyond the Limits

The solo 401(k) adds a Roth option for the employee deferral (and, under SECURE 2.0, potentially employer Roth), loan provisions up to $50,000, and, crucially for high earners, it does not poison the backdoor Roth: SEP balances are IRA money that triggers pro-rata taxation on backdoor conversions, while 401(k) balances stand outside that calculation, and can even absorb old IRA balances via roll-in to clean the slate. Mega-backdoor after-tax contributions are possible in solo plans whose documents allow them. The SEP's advantages: near-zero paperwork, no annual filing, and it can be opened and funded as late as your extended filing deadline, the classic rescue for a big year discovered in March.

Administration and the Fine Print

Solo 401(k)s require a plan document (free at major custodians), employee deferrals timed with the year (elections by year-end; deadlines vary by entity type), and a Form 5500-EZ once assets pass $250,000, ten minutes annually. The solo plan works only while your business has no non-spouse employees; hire staff and the plan must cover them under testing rules, at which point plan design becomes a real project, see the owner tax stack. Both vehicles allow a spouse who legitimately works in the business to double the household's space on their own compensation.

Choosing, and Leveling Up

Default to the solo 401(k) unless you specifically need the SEP's retroactive setup or minimal paperwork. Fund the employee layer first (Roth or pre-tax by bracket), then the employer layer as profits confirm. Owners in their late 40s+ with sustained high profits should evaluate the next tier, a cash-balance plan stacked on the 401(k), sheltering $100,000-$300,000+ annually depending on age. That stack, and the entity questions around it, is standard fare in our tax planning for owners.

Tony Colunga · Founder, Attend Wealth

Tony leads Attend Wealth, a fee-based wealth management firm in Atlanta serving professionals, families, and business owners. Advisory services are held to a fiduciary standard. More about Attend