How a W-2 401(k) Affects Your Solo 401(k) Contribution

Separate employee deferrals from employer contributions

A 401(k) participant can make elective deferrals as an employee, while an employer may also make a matching or nonelective contribution under its plan. In a one-participant plan for a sole proprietor, the owner wears both hats. At a W-2 job, the employer plan handles its own employee and employer contributions. The side business plan may allow an owner deferral and an employer contribution, but the mechanics differ by tax structure and plan document. Treat each contribution by its legal role, not just by which account receives it.

This distinction prevents the most common planning mistake: taking the full employee deferral limit at the day job and then assuming the side-business plan gets another full employee deferral. It usually does not. The individual employee limit follows the person across eligible plans. By contrast, an employer contribution from a genuinely separate business may be calculated under a separate employer contribution formula, subject to compensation, annual-addition, related-employer, and plan terms. Your exact business relationship and ownership structure can change the result, so do not assume two businesses are separate employers merely because they use different names.

Track the shared employee deferral limit for 2026

For 2026, the basic elective-deferral limit is $24,500 for 401(k) plans, before any applicable catch-up contribution. The IRS says deferrals generally aggregate across 401(k), 403(b), SIMPLE and SARSEP arrangements, regardless of how many plans or sponsors are involved. If your W-2 plan has already received $18,000 of your employee deferrals, the remaining ordinary deferral room across another 401(k) plan is generally $6,500, subject to the plan's own provisions and eligible compensation. Roth and pre-tax deferrals count toward the same individual limit.

The IRS identifies 457(b) plans as having a separate deferral limit. Do not extend that exception to every plan that happens to be offered by a government or nonprofit employer; confirm whether the plan is actually an eligible 457(b) and check its own rules. If you changed jobs, had multiple employers, used a SIMPLE plan, or contributed to a 403(b), obtain year-end statements from each plan. Payroll systems generally cannot see what you deferred to an unrelated employer's plan, so the participant has to reconcile the total.

Year-end coordination checklist

1. Collect final deferral totals from every 401(k), 403(b), SIMPLE/SARSEP, and any 457(b).
2. Compare aggregated employee deferrals with the 2026 individual limit and age-based catch-up rules.
3. Calculate owner-business compensation and employer contributions using the correct entity-specific method.
4. Review employees, plan documents, related-business ownership, annual additions, and Form 5500-EZ filing triggers.
5. Ask administrators for formal correction steps immediately if a shared-limit excess is found.

Calculate the business's contribution using its tax structure

For a sole proprietor, the business contribution is generally based on net earnings from self-employment after the applicable adjustment for one-half of self-employment tax and the plan's contribution formula. The owner deferral is also limited by earned income and the employee deferrals already made at the W-2 job. Publication 560 provides the self-employed worksheets for a qualified plan. Use those worksheets and the plan document; a percent of Schedule C profit is not a substitute for the calculation.

An S-corporation owner generally makes plan contributions based on eligible W-2 compensation from that corporation, not on S-corporation distributions. A side business's contributions may also be affected if employers are related under controlled-group or affiliated-service-group rules. The $72,000 2026 annual-additions limit is generally applied by plan and employer group under the tax rules, and catch-up contributions are treated separately; related businesses can require aggregation. A CPA or plan administrator should review ownership and service relationships before you rely on a two-employer worksheet.

Work an example without promising a personal limit

Assume an employee under 50 defers $18,000 to a W-2 employer's 401(k) during 2026. If a side-business Solo 401(k) accepts employee deferrals and the owner has enough eligible compensation, the ordinary individual deferral room may be no more than $6,500 across the plans combined. That is a ceiling before the side plan's own terms and earned-income limit; it does not guarantee the business can support a $6,500 owner deferral. The employee should confirm the year-to-date W-2 figure with payroll before electing the side-plan amount.

Now consider the employer side. The side business may be able to make a separate employer contribution based on its own eligible compensation, subject to its plan formula and annual limits. Do not combine that estimate with the $6,500 employee room and call the result automatically deductible. Compute sole-proprietor earned income or S-corporation W-2 compensation correctly; check the applicable annual-addition limit and related-employer rules; then reconcile both plans' contributions. The example illustrates the two buckets, not a recommendation or a personalized maximum.

Use a year-to-date reconciliation instead of memory

Create one annual worksheet with a row for every plan and columns for employee pre-tax deferrals, employee Roth deferrals, catch-up deferrals, employer match, employer nonelective contributions, and the final plan-reported total. Update it each quarter and again after the final W-2. The employee-deferral column is the cross-plan check. Employer contributions belong in their own columns because the same personal deferral cap does not govern every contribution type. Add a note for any plan year that differs from the calendar year.

Before making a side-plan election, request a year-to-date contribution summary from the W-2 plan and ask payroll whether the final paycheck will include a true-up or bonus deferral. Record the exact amount the employer has deposited, not just the employee election percentage. After year-end, compare the W-2, final pay statement, plan statement, side-business ledger, and plan administrator's contribution report. If two unrelated payroll systems both allow the full limit, your worksheet—not either payroll portal—must catch the combined excess.

Confirm eligibility, plan terms, and filing duties

A one-participant 401(k) generally covers a business owner with no common-law employees, or the owner and spouse. Once a non-spouse employee meets the plan's eligibility requirements, the plan may need to cover that person and can cease to fit the owner-only description. Eligibility rules changed under recent law and may include long-term part-time employees; read the plan document and current IRS guidance instead of using a blanket hours-of-service assumption. A contractor label does not settle common-law employee status.

A one-participant plan generally must file Form 5500-EZ when plan assets reach $250,000 or more at year-end, and a final return may be required when the plan terminates even below that threshold. The filing is a plan-level reporting duty, distinct from personal contribution limits. Track year-end assets across the plan as required, keep the plan document and adoption date, and calendar the filing deadline. If the W-2 employer and side business share ownership or services, or if an employee becomes eligible, pause before funding and get plan-administrator advice.

Know how to correct an excess before the deadline passes

If the year-end reconciliation shows that your employee deferrals across eligible plans exceeded the individual limit, contact the plan administrators and tax preparer promptly. The correction process depends on the plan type, timing, and the source of the excess. The IRS discusses how excess deferrals should be distributed by April 15 of the following year to avoid potential double taxation. Do not withdraw money informally or assume a rollover solves an excess deferral; request the plan's formal correction procedure and preserve the written response.

A return of excess deferrals is different from a plan contribution that exceeded an employer deduction or annual-addition limit. Identify which limit was breached, which plan accepted the amount, and whether earnings must be included in a corrective distribution. Keep the payroll election, account statement, correction request, distribution confirmation, and tax reporting together. If the mismatch was discovered after a filing deadline or involves multiple businesses under common control, use a tax professional who handles qualified-plan corrections rather than improvising from a general article.

Coordination details that deserve a second look

A paycheck election is not the same as an amount actually deferred. A late-year bonus, payroll correction, or automatic escalation can change the W-2 plan's year-end total. Download the final pay statement and plan statement, and compare pre-tax plus Roth deferrals. Do not count an employer match as employee deferral, and do not omit a prior employer plan just because that job ended. Keep the tax-year boundary visible when a plan uses a non-calendar plan year.

The individual deferral limit is usually about the participant across eligible plans, while the employer contribution calculation is tied to the sponsoring business and applicable plan rules. The terms 'employee contribution' and 'employer contribution' can be easy to mix up in an account portal. Check the plan statement labels and ask the administrator which figure is an elective deferral. If you have more than one business, ownership stakes, or a spouse working in the business, related-employer rules may change how plans are aggregated.

A side-business owner deferral needs eligible compensation. A sole proprietor uses earned income after the self-employment-tax adjustment rather than taking a percentage of gross receipts. An S-corporation shareholder's contribution generally relies on eligible W-2 wages from that corporation, not distributions. These methods are different enough that a spreadsheet cell labeled 'profit times 20%' can mislead. Name the entity and tax treatment beside the formula and save the supporting return schedule.

If you participate in a governmental or nonprofit 457(b), keep it in a separate row because the IRS describes its limit as separate from the aggregated 401(k), 403(b), SIMPLE, and SARSEP deferrals. Still read the 457(b) plan's own terms and catch-up rules. A plan offered by an employer could be a different plan type than its marketing name suggests, so confirm the legal plan type from the summary plan description or administrator before using the exception.

An owner-only plan is not automatically safe from employee eligibility changes. A new hire, a worker whose classification changes, or a long-term part-time worker can require a coverage review. Check eligibility before the first contribution for the year, not only at tax filing. If the business is related to the W-2 employer or provides services through a group of entities, have the plan professional assess controlled-group and affiliated-service-group rules before determining separate limits.

Questions about having two workplace plans