Solo 401(k) vs SEP: contribution math and the deadline difference

The short answer, and who each plan is for

For a solo practice with no employees, a solo 401(k) usually lets you contribute more at a given income and a SEP-IRA is simpler to run — that is the tradeoff in one line. A one-participant 401(k) covers you, and a spouse who works in the business, but no other employees 1Ref 1Internal Revenue Service (2026).One-participant 401(k) plans.That a solo 401(k) covers an owner and a working spouse with no other employees and allows an employee deferral plus an employer contribution under annual limits.. A SEP is employer-funded only, carries almost no paperwork, and must eventually cover staff on equal terms 2Ref 2Internal Revenue Service (2026).Simplified Employee Pension plan (SEP).That a SEP allows an employer contribution of up to 25% of compensation with minimal administration, can be funded up to the extended filing deadline, and must cover eligible employees on equal terms..

Neither is exotic and both are established at a brokerage in an afternoon. The decision is not really which is "better" — it is which fits your income level, your tolerance for administration, and whether a hire is on the horizon. The rest of this page is the math and the timing behind that fit.

The contribution math: two engines vs one

Both plans let the practice contribute an employer share of up to 25% of compensation, but the solo 401(k) adds a second engine on top: an employee salary deferral you make as the worker 1Ref 1Internal Revenue Service (2026).One-participant 401(k) plans.That a solo 401(k) covers an owner and a working spouse with no other employees and allows an employee deferral plus an employer contribution under annual limits.2Ref 2Internal Revenue Service (2026).Simplified Employee Pension plan (SEP).That a SEP allows an employer contribution of up to 25% of compensation with minimal administration, can be funded up to the extended filing deadline, and must cover eligible employees on equal terms.. That deferral is what lets a solo 401(k) pull ahead at low-to-moderate income — you can defer a large fixed amount regardless of the 25% limit, then add the employer piece 1Ref 1Internal Revenue Service (2026).One-participant 401(k) plans.That a solo 401(k) covers an owner and a working spouse with no other employees and allows an employee deferral plus an employer contribution under annual limits.. A SEP has only the 25% employer piece 2Ref 2Internal Revenue Service (2026).Simplified Employee Pension plan (SEP).That a SEP allows an employer contribution of up to 25% of compensation with minimal administration, can be funded up to the extended filing deadline, and must cover eligible employees on equal terms..

The compensation the 25% is measured against is your net earnings from self-employment — a figure that sits below headline profit 3Ref 3Internal Revenue Service (2026).Self-employed individuals tax center.That a sole proprietor's contribution base is net earnings from self-employment, a figure below headline profit.. There is an overall annual cap on total contributions that the IRS sets and adjusts yearly, so look the current dollar limits up rather than memorizing them 1Ref 1Internal Revenue Service (2026).One-participant 401(k) plans.That a solo 401(k) covers an owner and a working spouse with no other employees and allows an employee deferral plus an employer contribution under annual limits.2Ref 2Internal Revenue Service (2026).Simplified Employee Pension plan (SEP).That a SEP allows an employer contribution of up to 25% of compensation with minimal administration, can be funded up to the extended filing deadline, and must cover eligible employees on equal terms..

The deadline difference the plans hinge on

The two plans are not funded on the same clock, and it catches people every year. A SEP can be both established and funded as late as your tax-filing deadline, including extensions — you can open one the day you file 2Ref 2Internal Revenue Service (2026).Simplified Employee Pension plan (SEP).That a SEP allows an employer contribution of up to 25% of compensation with minimal administration, can be funded up to the extended filing deadline, and must cover eligible employees on equal terms.. A solo 401(k) is split: the employee-deferral election has to be in place during the plan year, while the employer contribution can go in later 1Ref 1Internal Revenue Service (2026).One-participant 401(k) plans.That a solo 401(k) covers an owner and a working spouse with no other employees and allows an employee deferral plus an employer contribution under annual limits..

The practical consequence is real. If you realize in March that you want to shelter last year's income, a SEP is usually the plan still available to you; the solo 401(k) deferral window for that year has generally closed 1Ref 1Internal Revenue Service (2026).One-participant 401(k) plans.That a solo 401(k) covers an owner and a working spouse with no other employees and allows an employee deferral plus an employer contribution under annual limits.2Ref 2Internal Revenue Service (2026).Simplified Employee Pension plan (SEP).That a SEP allows an employer contribution of up to 25% of compensation with minimal administration, can be funded up to the extended filing deadline, and must cover eligible employees on equal terms.. The exact cutoff dates are set by the IRS and have shifted with recent retirement legislation, so confirm the current-year deadline on the plan pages rather than assuming last year's rule still holds 1Ref 1Internal Revenue Service (2026).One-participant 401(k) plans.That a solo 401(k) covers an owner and a working spouse with no other employees and allows an employee deferral plus an employer contribution under annual limits.2Ref 2Internal Revenue Service (2026).Simplified Employee Pension plan (SEP).That a SEP allows an employer contribution of up to 25% of compensation with minimal administration, can be funded up to the extended filing deadline, and must cover eligible employees on equal terms..

What happens when you add your first hire

The plans diverge hard the moment you take on staff. A SEP must cover every eligible employee on the same percentage terms you give yourself, so a 25% contribution for you becomes 25% for them too — a real cost once you have a W-2 hire 2Ref 2Internal Revenue Service (2026).Simplified Employee Pension plan (SEP).That a SEP allows an employer contribution of up to 25% of compensation with minimal administration, can be funded up to the extended filing deadline, and must cover eligible employees on equal terms.. A solo 401(k) stops qualifying as a one-participant plan altogether once you have a non-spouse employee who meets the eligibility rules 1Ref 1Internal Revenue Service (2026).One-participant 401(k) plans.That a solo 401(k) covers an owner and a working spouse with no other employees and allows an employee deferral plus an employer contribution under annual limits..

One distinction saves confusion around the first hire: a worker you pay on a 1099-NEC is a contractor, not an employee, and does not trigger the SEP coverage rule 4Ref 4Internal Revenue Service (2026).About Form 1099-NEC, Nonemployee Compensation.That a worker paid on a 1099-NEC is a contractor rather than an employee, subject to the IRS classification test, and so does not trigger the SEP employee-coverage rule.. But the label is not yours to assign.

• Classification is the IRS's test, not your paperwork. Behavioral control, financial control, and the nature of the relationship decide it 4Ref 4Internal Revenue Service (2026).About Form 1099-NEC, Nonemployee Compensation.That a worker paid on a 1099-NEC is a contractor rather than an employee, subject to the IRS classification test, and so does not trigger the SEP employee-coverage rule..
• Misclassifying to dodge coverage is the employer's exposure. Calling an employee a contractor to avoid contributing for them is a liability, not a strategy 4Ref 4Internal Revenue Service (2026).About Form 1099-NEC, Nonemployee Compensation.That a worker paid on a 1099-NEC is a contractor rather than an employee, subject to the IRS classification test, and so does not trigger the SEP employee-coverage rule..

How your tax election changes the base

The compensation your contribution is measured against depends on how the practice is taxed, and an LLC does not settle that by itself 5Ref 5Internal Revenue Service (2026).Limited liability company (LLC).That an LLC's tax classification — sole proprietor versus S corporation — determines whether the contribution base is net self-employment earnings or W-2 wages.. Taxed as a sole proprietor, you contribute against net earnings from self-employment; taxed as an S corporation, the base is your W-2 wages, and only those wages count 5Ref 5Internal Revenue Service (2026).Limited liability company (LLC).That an LLC's tax classification — sole proprietor versus S corporation — determines whether the contribution base is net self-employment earnings or W-2 wages.3Ref 3Internal Revenue Service (2026).Self-employed individuals tax center.That a sole proprietor's contribution base is net earnings from self-employment, a figure below headline profit.. The same plan can permit very different contributions depending on which election is in place.

This creates a genuine tension worth naming. A low reasonable wage in an S corp trims self-employment tax — the s-corp math — but it also shrinks the W-2 base your retirement contribution is built on, so the two goals pull against each other. For a very high earner who has maxed both a solo 401(k) and the SEP room, a cash-balance plan can layer on top to shelter more, at the cost of more administration. Which combination nets out ahead is a projection for your CPA, not a rule of thumb 5Ref 5Internal Revenue Service (2026).Limited liability company (LLC).That an LLC's tax classification — sole proprietor versus S corporation — determines whether the contribution base is net self-employment earnings or W-2 wages..

Choosing between them

Match the plan to where the practice actually is, not where you hope it goes. If you are solo, want to shelter as much as possible, and can handle a little more paperwork, the solo 401(k)'s deferral usually wins on total contribution 1Ref 1Internal Revenue Service (2026).One-participant 401(k) plans.That a solo 401(k) covers an owner and a working spouse with no other employees and allows an employee deferral plus an employer contribution under annual limits.. If you value near-zero administration, expect to file late, or plan to add staff soon, the SEP's simplicity and later deadline can matter more than the extra room 2Ref 2Internal Revenue Service (2026).Simplified Employee Pension plan (SEP).That a SEP allows an employer contribution of up to 25% of compensation with minimal administration, can be funded up to the extended filing deadline, and must cover eligible employees on equal terms..

• Lean solo 401(k) when you are solo (or solo-plus-spouse), your income is moderate, and you want the largest contribution.
• Lean SEP when you want minimal upkeep, may fund after year-end, or foresee hiring — the equal-coverage cost is easier to reason about than converting a plan later.

One caveat closes the loop: neither plan reduces your self-employment tax. Both cut income tax by lowering taxable income, but the 15.3% is figured before them. Treat them as retirement-and-income-tax tools, and keep the entity question — which is where self-employment tax is actually moved — as a separate decision with your CPA 1Ref 1Internal Revenue Service (2026).One-participant 401(k) plans.That a solo 401(k) covers an owner and a working spouse with no other employees and allows an employee deferral plus an employer contribution under annual limits.2Ref 2Internal Revenue Service (2026).Simplified Employee Pension plan (SEP).That a SEP allows an employer contribution of up to 25% of compensation with minimal administration, can be funded up to the extended filing deadline, and must cover eligible employees on equal terms..