Solo 401(k) Contribution Split Planner

Introduction

Solo 401(k) plans are attractive because one person can save in two roles at the same time. You can make an employee salary deferral, and your business can also make an employer profit-sharing contribution. That flexibility is exactly why planning gets tricky. The IRS limits are generous, but they do not all work the same way. Some limits apply to employee deferrals across every workplace plan you participate in. Others depend on compensation, business structure, and whether you are old enough for catch-up contributions. This planner helps you sort those moving parts into one clear year-end picture.

If you are self-employed, the question usually is not simply, “What is the maximum contribution?” The more practical question is, “How should I split my remaining contribution room between regular deferrals, catch-up amounts, and employer deposits without crossing a limit?” That is what this page is built to answer. It converts the annual rules into a readable breakdown so you can see where the bottleneck sits, whether you still have regular deferral room, and how much employer profit sharing remains after all the relevant caps are applied.

How to use

Start with your age and business structure. Age matters because catch-up contributions only become available once you are at least fifty by year end. Business structure matters because sole proprietors and partners calculate contribution compensation from net earnings after half of self-employment tax, while S-corporation or C-corporation owners typically use W-2 wages. That single choice changes the employer contribution formula, so it is worth selecting carefully.

Next, enter the earnings figure that fits your business type. If you are a sole proprietor or partner, use net business profit before retirement contributions. If you are paid through corporate payroll, use your annual W-2 wages. Then add any elective deferrals already made to other plans this year, any solo 401(k) deferrals already deposited, and any employer contributions already made to this plan. When you click Evaluate contribution headroom, the results box summarizes what still fits under the 402(g), catch-up, and section 415 rules. The comparison table turns that summary into line items you can export as CSV for your records or a conversation with your CPA.

In plain terms, the output answers three questions. First, how much regular employee deferral room is left? Second, if you qualify, how much catch-up room remains? Third, after accounting for those items and any prior employer deposits, how much employer profit sharing can still be added without creating an excess contribution? Read the results as planning guidance rather than legal advice, especially if your compensation changes during the year or you participate in multiple retirement plans.

Understanding the formulas that govern solo 401(k) deposits

Solo 401(k) plans follow the same basic IRS framework that applies to larger employer plans, but the calculations feel more technical because the business owner stands on both sides of the transaction. For 2024, the maximum regular employee salary deferral under Internal Revenue Code section 402(g) is $23,000. Participants who are age fifty or older can also make an additional $7,500 catch-up deferral, bringing the employee-side ceiling to $30,500. Separately, section 415 generally caps the sum of employer contributions and regular employee deferrals at the lesser of $69,000 or 100% of compensation. Catch-up contributions sit on top of that annual additions cap, but they still require enough compensation to support them.

Compensation itself depends on how the business is organized. Sole proprietors and partners do not usually use raw Schedule C profit as the final contribution base. Instead, they use net earnings after subtracting half of the self-employment tax. Corporate owner-employees use W-2 wages. Because the self-employment tax deduction affects the contribution base and the contribution base affects the contribution amount, sole proprietor calculations can look circular at first. The IRS shortcut resolves that loop. Rather than using 25% and backing out the deduction repeatedly, the equivalent effective rate for the employer piece is 20% of adjusted net earnings. The MathML formula below expresses that relationship directly.

In that expression, N is the business profit and S is the self-employment tax. The fraction 0.25 divided by 1.25 simplifies to 0.2, which is why planners often refer to a 20% employer contribution rate for sole proprietors. For corporations, the employer formula is more direct: profit sharing is generally limited to 25% of W-2 wages. The self-employment tax estimate used here applies the 12.4% Social Security portion only up to the 2024 wage base of $168,600 and applies the 2.9% Medicare portion to all self-employment earnings included in the calculation.

How the planner processes your inputs

When you submit the form, the calculator cleans the numbers, converts blanks or invalid entries to zero, and refuses to build a recommendation from negative inputs. For a sole proprietor, it computes self-employment tax on 92.35% of net profit, respects the Social Security wage base, subtracts half of that tax, and treats the remainder as compensation. For corporate wages, compensation is simply the W-2 amount you enter. From there, the tool checks your age to determine whether catch-up contributions should be available.

The next step is allocation logic. Deferrals already made to other employer plans reduce your regular 402(g) space first. Deferrals already made to the solo 401(k) then fill whatever regular space remains before counting as catch-up, which mirrors the way payroll systems and year-end plan reviews usually think about the limit. After that, the planner measures remaining employer headroom under both the business-type percentage rule and the section 415 annual additions limit. The result is a practical answer instead of a theoretical maximum: the amount you can still add this year without crossing the most restrictive rule.

Worked example: a consultant balancing multiple income streams

Imagine a 52-year-old consultant who nets $180,000 on Schedule C and has already contributed $10,000 in elective deferrals through a part-time W-2 job. She has not yet contributed anything to her solo 401(k) for the year. The planner begins by computing self-employment tax on 92.35% of profit, or $166,230. The Social Security portion applies at 12.4% and the Medicare portion applies at 2.9%, producing an estimated self-employment tax of $25,434.67. Half of that amount, or $12,717.34, reduces compensation for contribution purposes, leaving adjusted compensation of $167,282.66.

From there, the employer side is straightforward. Because she is a sole proprietor, the employer profit-sharing limit is 20% of adjusted compensation, or $33,456.53. On the employee side, she has a $23,000 regular deferral limit plus a $7,500 catch-up allowance because she is over fifty. Her other-plan deferrals use $10,000 of the regular limit first, leaving $13,000 of regular headroom. None of the catch-up allowance has been used, so the full $7,500 remains. The planner then checks section 415. Since the annual additions cap is the lesser of $69,000 or compensation, and her compensation exceeds $69,000, the relevant statutory cap is $69,000. Once the remaining regular deferral is accounted for, the employer contribution still fits comfortably. The practical year-end split becomes $13,000 regular deferral, $7,500 catch-up, and $33,456.53 employer contribution.

Comparison of contribution strategies by business structure

The comparison below shows why the same economic income can produce different contribution room depending on whether the owner reports self-employment profit directly or receives W-2 wages from a corporation. It does not mean one structure is always better. Instead, it shows that compensation definitions and payroll mechanics change the retirement savings ceiling in ways that are easy to miss when you only look at gross business income.

Notice that the sole proprietor example has a lower employer contribution limit because self-employment tax reduces the compensation base. The corporation example can support a larger employer contribution, but payroll administration, reasonable compensation rules, and employment tax considerations still matter. That is why this planner is most useful when it is paired with judgment. It shows the retirement-limit consequences of a compensation choice, but it does not replace the legal and tax analysis behind that choice.

Interpreting the results panel and CSV export

The results panel tells you what is binding first. If compensation is low, the annual additions limit may effectively shrink to compensation even though the published dollar cap looks much higher. If you already used your employee deferral elsewhere, the planner shifts the conversation toward employer contributions and, if eligible, catch-up room. The exportable CSV lists the same components shown in the table so you can save a dated planning record, compare scenarios, or bring a clean summary into a meeting with a tax preparer or plan administrator.

Limitations, compliance reminders, and assumptions

This calculator assumes 2024 limits: a $23,000 regular elective deferral cap, a $7,500 catch-up amount for age fifty and older, and a $69,000 section 415 annual additions limit. It uses the 2024 Social Security wage base of $168,600 when estimating self-employment tax. It does not model every tax nuance. For example, it ignores the additional 0.9% Medicare surtax because that surtax affects tax liability rather than the plan contribution formula being illustrated here. It also assumes that deferrals already made to other plans first consume regular 402(g) space rather than catch-up space.

Most important, the tool assumes your compensation figure is reasonably known. In real life, year-end bonuses, late invoices, amended bookkeeping, and payroll adjustments can change the final result. Revisit the calculation whenever there is a material change. If you sponsor more than one plan, use a defined benefit plan, or are coordinating large deductions with estimated taxes and Roth strategies, confirm the result with a qualified advisor before money moves. The goal of this page is clarity: a faster, more transparent way to understand the moving parts before you commit to a funding decision.