Solo 401(k) Contribution Calculator: 2026 Limits Explained
The Solo 401(k) is the highest-capacity retirement shelter available to most freelancers: a self-employed worker with $100,000 of net income can contribute roughly $42,970 for 2026 — about 43% of net profit — by combining an employee deferral with an employer profit-sharing contribution from the same dollars. No other one-person plan comes close at that income level; a SEP-IRA funded from the same $100,000 would top out near $18,470 because it only has the employer bucket. This guide is the companion to our Solo 401(k) contribution calculator: it explains the three contribution buckets in plain English, walks through a complete $100,000 worked example with the self-employment tax adjustment, covers the Roth vs traditional choice, and lists the deadlines that actually disqualify people. All 2026 figures below are estimates — confirm current limits on irs.gov before contributing. If you are still choosing between plan types, start with our freelancer retirement plans guide comparing SEP-IRA vs Solo 401(k) vs IRA, then come back here to size your number.
Who can open a Solo 401(k)
A Solo 401(k) — the IRS calls it a one-participant 401(k) — is a traditional 401(k) plan covering a business owner with no full-time employees other than the owner and optionally a spouse. Freelancers, sole proprietors, single-member LLCs, and owner-only S corporations all qualify; the moment you hire a full-time non-spouse employee (part-time and under-21 workers have special exclusions), the plan generally has to convert to a regular multi-participant 401(k). These plans have exactly the same rules and requirements as any other 401(k) — the "solo" label describes who participates, not a separate legal regime. The IRS spells out the mechanics on its one-participant 401(k) plans page, which is the primary source behind every number in this guide. Note the two-hat structure from the start: as both employer and employee of your own business, you contribute in both capacities, and each capacity has its own limit before the overall cap applies.
How much you can put in: the three buckets
Every Solo 401(k) contribution falls into one of three buckets. Bucket 1 is the employee elective deferral — up to $24,500 for 2026 (estimate), capped at 100% of your compensation. Bucket 2 is the catch-up contribution — an extra $8,000 for 2026 if you are age 50 or older at year-end (estimate). Bucket 3 is the employer profit-sharing contribution — up to 20% of net self-employment income for sole proprietors, or 25% of W-2 wages if your business is an S corporation paying you a salary. The three buckets stack, but a ceiling sits over all of them: total contributions cannot exceed $72,000 for 2026 ($80,000 including the standard catch-up), or 100% of compensation, whichever is less (2026 estimates — confirm on irs.gov). The calculator's default inputs show the most common freelancer case: $80,000 of compensation, $100,000 of net income, age 35, producing $24,500 of employee deferral plus $20,000 of simplified employer share for a $44,500 total.
Bucket 1: the employee elective deferral ($24,500)
The employee deferral is the most powerful bucket dollar-for-dollar because it is available in full even at modest incomes: earn $30,000 of net profit and you can still defer all $24,500 of it (plus nothing more, since the deferral cannot exceed compensation). The 2026 IRS limit on elective deferrals to 401(k) plans is $24,500, aggregating every 401(k), 403(b), SARSEP, and SIMPLE plan you participate in — it is a per-person limit, not a per-plan limit. That aggregation rule is the trap for freelancers with a day job: if your employer's 401(k) already absorbed $15,000 of deferrals, only $9,500 of employee bucket remains for the Solo 401(k), though the employer profit-sharing bucket is unaffected and is usually what still makes the plan worthwhile. Your plan document may impose a lower ceiling than the IRS maximum, and highly compensated structures can add nondiscrimination testing, so the calculator's deferral-limit field stays editable — it defaults to $24,500 but accepts any lower plan-imposed cap. Elective deferrals can be designated pre-tax (traditional) or Roth depending on what your plan document allows, a choice covered in detail below.
Bucket 2: catch-up contributions at age 50+
If you turn 50 or older by December 31, your plan may permit an additional $8,000 of elective deferrals for 2026 (estimate), raising your personal employee ceiling to $32,500 and the overall account cap to $80,000. Under SECURE 2.0, participants ages 60 through 63 may qualify for a higher "super catch-up" of $11,250 instead of $8,000 for 2026 (2026 estimate — confirm on irs.gov and check that your plan allows it). Catch-up contributions must be made through elective deferrals before the end of the plan year — unlike employer profit-sharing, they cannot be topped up the following April. One SECURE 2.0 wrinkle arriving with the 2026 rules: if your prior-year wages from the plan sponsor exceeded $150,000 and your plan has a Roth feature, catch-up contributions must generally go in as Roth rather than pre-tax, so high-earning older freelancers should confirm the Roth plumbing in their plan document before year-end.
Bucket 3: the employer profit-sharing share (20% rule)
The employer bucket is what separates the Solo 401(k) from every IRA-based alternative. A sole proprietor may contribute up to 20% of net self-employment income; an S corporation may contribute up to 25% of the W-2 wages it pays the owner-employee. The 20%-vs-25% gap confuses people, but it is the same economics: the sole proprietor's base is profit after the self-employment tax deduction, while the S corporation's base is salary before that adjustment, and the two formulas converge by design. Formula: employer share = 20% × (net profit × 0.9235), where the 0.9235 factor backs out the employer-equivalent half of self-employment tax — the same 92.35% adjustment that appears in our tax set-aside percentage guide. This calculator simplifies to net × 0.20 and flags the difference; at $100,000 of net income the simplified figure is $20,000 while the precise figure is about $18,470, a gap worth knowing but not worth agonizing over in January when you are still estimating. Two further caps apply: only the first $360,000 of compensation for 2026 counts toward contribution math (irrelevant for most freelancers but binding for seven-figure practices), and the employer share can never push the account total past the $72,000 overall cap.
Worked example: $100,000 of net income
Take a 38-year-old freelance designer, sole proprietor, $100,000 of Schedule C net profit, no day-job 401(k), plan established in January. Step one, adjust for the SE-tax deduction: $100,000 × 0.9235 = $92,350 of adjusted net earnings. Step two, the employer bucket: 20% × $92,350 = $18,470 (the calculator's simplified net × 0.20 shows $20,000 — the $1,530 difference is the simplification, and either figure is fine for planning). Step three, the employee bucket: the full $24,500 deferral, since $100,000 of compensation far exceeds the cap and no other plan used any of it. Step four, stack and cap: $24,500 + $18,470 = $42,970 total, comfortably under the $72,000 overall limit. That is a 43% savings rate on net profit — compare a SEP-IRA on the same income, which allows only the $18,470 employer piece and nothing more. If the designer were 55, add the $8,000 catch-up for $50,970; if she also maxed a $15,000 day-job 401(k) deferral, the employee bucket would shrink to $9,500 and the total to $27,970, with the employer $18,470 untouched.
| Bucket at $100,000 net income | 2026 amount |
|---|---|
| Employee deferrals | $24,500 |
| Employer profit-sharing (precise 20% method) | $18,470 |
| Total contribution | $42,970 — about 43% of net profit |
| Overall account cap | $72,000 |
Roth vs traditional: which bucket gets which tax treatment
Practitioner note: The mistake I see most often is calendar-shaped: freelancers discover the Solo 401(k) in March, when the $24,500 employee bucket for the prior year is already gone because no plan existed on December 31. Open the plan in December even if you fund it later — the employer share waits until April, but the employee share cannot be created retroactively.
Employee deferrals can generally be split between pre-tax (traditional) and Roth however you like, up to the combined $24,500 ceiling — Roth does not raise the limit, it changes the timing of the tax. Pre-tax deferrals reduce this year's taxable income dollar-for-dollar, which pairs well with high-earning years and with the quarterly estimate math in our quarterly estimated taxes guide; Roth deferrals cost you the deduction now but grow and withdraw tax-free, which favors early-career freelancers in low brackets and anyone expecting higher rates in retirement. The employer profit-sharing bucket, by contrast, is always pre-tax — there is no Roth employer contribution under current law, so the $18,470 in the worked example always reduces current-year taxable income. Remember the SECURE 2.0 catch-up rule noted above: older high earners may be forced into Roth for the catch-up slice specifically. A common freelancer pattern is pre-tax employer contributions plus Roth employee deferrals in lean years, flipping the employee slice to pre-tax in peak years — revisit the split every December rather than setting it once.
If lean, choose Roth; if peak, choose pre-tax: in a low-bracket early-career year, direct employee deferrals to Roth — you pay little tax now and the growth withdraws tax-free, while the $18,470 employer share still cuts this year's income. In a peak-earning year, direct them pre-tax instead — the full $24,500 comes off taxable income today, when each deducted dollar is worth the most. Revisit the split every December; the right answer follows the bracket, not your age.
The mega backdoor Roth: stuffing the $72,000 cap
Once the $24,500 employee deferral and the employer profit-sharing share are maxed, any remaining room under the $72,000 overall cap can be filled with after-tax contributions — a third category that is neither pre-tax deductible nor Roth. Those after-tax dollars are then converted to Roth through an in-plan Roth rollover or rolled out to a Roth IRA, where future growth compounds tax-free. That contribute-then-convert sequence is the "mega backdoor" — it does not raise the $72,000 ceiling, it fills it with Roth-bound dollars.
On the page's $100,000 example the arithmetic is straightforward: $24,500 + ~$18,470 = ~$42,970 used, so $72,000 − ~$42,970 leaves roughly $29,000 of after-tax room (approximate — the employer figure itself is the precise-20% estimate). At higher incomes the same subtraction applies: whatever the pre-tax buckets do not absorb, after-tax contributions may fill, up to the cap.
The catch is plan paperwork. Most mainstream prototype Solo 401(k) plans — the off-the-shelf brokerage versions — do not permit after-tax contributions or in-service Roth conversions, and without both provisions in the plan document the strategy is unavailable. It requires a custom plan document that expressly allows both. Name no providers here; before counting on the extra room, ask your plan administrator directly: "do you support after-tax contributions and in-service Roth conversions?"
Frankly, this only matters once the pre-tax buckets are already full — at modest incomes there is no leftover cap to fill — and note that each conversion starts its own 5-year clock for penalty-free access to the converted amount.
Deadlines: the two dates that disqualify people
Miss the first deadline and the year's biggest bucket vanishes. The plan document must exist by December 31 of the contribution year — open a Solo 401(k) on January 15 and you cannot make employee deferrals for the prior year, full stop. Most major brokerages offer Solo 401(k)s with no setup fee, and you will need an IRS Employer Identification Number (free, minutes online) even as a sole proprietor, so there is no paperwork excuse for waiting until December. The second deadline is kinder: employer profit-sharing contributions may be made up until your tax-filing deadline, including extensions — typically April 15, or October 15 on extension — which means you can finalize the employer $18,470 (or precise adjusted figure) after your return is essentially complete. Self-employed employee deferrals are generally also due by the filing deadline, but only within a plan that already existed at year-end. Catch-up deferrals follow the stricter rule: before December 31, no extensions. Put two calendar entries in now — early December to confirm the plan exists and set deferral elections, and March to compute the final employer share from finished books.
Solo 401(k) vs SEP-IRA at a glance
Both plans share the same $72,000 overall 2026 cap, but they reach it very differently. The SEP-IRA has only the employer bucket (20% of adjusted net), so at $100,000 of profit it caps near $18,470 while the Solo 401(k) reaches $42,970 — the $24,500 employee deferral is pure extra capacity. The SEP-IRA counters with simpler paperwork and a later setup deadline (you can open and fund one by the filing deadline, including extensions, with no December 31 requirement). Our freelancer retirement plans guide compares all three options — SEP-IRA, Solo 401(k), and traditional/Roth IRA — with 2026 limits and the S-corp salary interaction; the short version is that owner-only businesses wanting maximum capacity almost always prefer the Solo 401(k), while businesses with employees or a need for last-minute setup lean SEP-IRA.
Common mistakes freelancers make
- Double-counting the employee bucket across two 401(k)s. The $24,500 limit follows the person. Total your day-job deferrals before setting the Solo 401(k) election.
- Opening the plan in January for the prior year. The December 31 establishment deadline is absolute for employee deferrals — the April extension only helps the employer share of an existing plan.
- Using 25% instead of 20% as a sole proprietor. The 25% rate applies to S-corp W-2 wages; sole proprietors use 20% of adjusted net. Mixing them up overstates capacity by a quarter.
- Forgetting the 0.9235 SE-tax adjustment. Flat 20% of gross profit overstates the employer share by about 8% — fine for January planning, wrong on the filed return.
- Ignoring the overall $72,000 cap at high incomes. Past roughly $300,000 of profit the buckets collide with the ceiling and every extra dollar must be redirected to taxable accounts.
- Funding Roth catch-ups as pre-tax when the plan requires Roth. Check the SECURE 2.0 high-earner rule with your provider before your first age-50+ contribution.
Run the numbers
The Solo 401(k) contribution calculator takes four inputs — compensation base, net self-employment income, age, and the deferral limit — and returns the employee share, the simplified 20% employer share, and the capped total with a note explaining which 2026 limit zone you are in. Re-run it in December with near-final profit, again in March with finished books to set the employer share, and whenever a day-job deferral changes your remaining employee bucket. Pair it with the tax set-aside calculator and the quarterly estimated taxes guide so each client payment covers both April's vouchers and December's retirement contributions. Estimates only: confirm all 2026 limits on irs.gov before contributing.