Contribution math
For sole proprietors, the SEP contribution is 20% of net self-employment earnings (net Schedule C profit minus the deductible half of SE tax). The 20% comes from the algebraic equivalent of "25% of compensation" once the deduction itself reduces compensation. Capped at the annual defined-contribution limit.
Eligibility
Available to any self-employed individual, partner, or eligible employer. Easy to set up — most brokerages offer SEP-IRA accounts with no annual administrative cost. No 5500 filing required.
Deadline
A SEP can be both established and funded as late as the extended due date of the return (October 15 for calendar-year filers). This makes the SEP the most flexible last-minute retirement contribution for self-employed taxpayers.
Coverage of employees
If you have employees, you must contribute the same percentage of compensation to each eligible employee's SEP account as you contribute for yourself. Eligible employees are generally those age 21+ who have worked 3 of the past 5 years and earned at least $750 (2024). The "must include" rule makes SEPs less attractive once you have employees.
Compared to solo 401(k)
A solo 401(k) generally allows a larger contribution at lower income levels because of the employee deferral component. The SEP wins on simplicity. For very high earners with no other employees, contribution capacity is similar at the cap.
Worked example with numbers
Consider a sole prop with $100,000 in gross receipts and $30,000 in legitimate Schedule C deductions, including this category. Each additional $1,000 of qualifying expense reduces Schedule C net profit by $1,000, which reduces self-employment tax by approximately $1,000 × 92.35% × 15.3% ≈ $141, and reduces income tax by $1,000 × marginal rate. At a 22% federal marginal rate, the combined federal tax savings on each additional $1,000 of legitimate deduction is roughly $361, and state savings sit on top of that. The math is why disciplined categorization throughout the year pays for itself.
Common mistakes that disallow the deduction
The recurring ways this deduction gets disallowed in examination cluster in four categories: (1) personal-use expenses bundled with business (the deduction is disallowed entirely or apportioned downward); (2) inadequate substantiation (no receipt, no invoice, no business-purpose note); (3) the wrong line on Schedule C (not fatal, but it weakens audit defense); and (4) double-counting with another line (for example, deducting an expense on Schedule C and also on Form 8829, or as a personal itemized deduction on Schedule A). The fix in every case is contemporaneous bookkeeping and a clean chart of accounts.