Solo 401(k) Calculator

ByDavid K. Sterling, ChFC®, RICP® ✓ Verified 📚 98 Calculators Authored

Retirement Wealth Strategist & Actuarial Consultant · Chartered Financial Consultant (ChFC®), Retirement Income Certified Professional (RICP®)

📅 Published: Sep 28, 2026

The formula

Deferral + 20%-of-comp SE-converted profit share (×0.9235 ÷1.2) ≤ $70,000 total

What the solo capacity tells you

Computes the two-hat plan for owner-only businesses: you are both employee (deferral track) and employer (profit-share track), stacking to the highest dollar-per-dollar retirement deduction available.

Calculation: Deferral + 20%-of-comp SE-converted profit share (×0.9235 ÷1.2) ≤ $70,000 total. Enter your inputs and every substitution is shown below the result.

Reading the result

Profit-share room is what the SEP gives too, but the deferral track is uniquely the 401(k)’s — Roth sub-accounts and mega-backdoor pipes live here.

Rules of thumb: 2026: $23,500 deferral (+$7,500 at 50+), total additions $70,000 (+catch-up)

Limitations and best practice

Once a single non-owner employee (other than a spouse) exists, the plan must cover them and pass testing; late Form 5500-EZ filings carry penalties since SECURE 2.0.

This calculator gives an estimate based on standard formulas; it is not professional financial, medical, legal or engineering advice. Verify anything you plan to act on with a qualified professional and your own records.

Frequently asked questions

Do I need an accountant to run a solo 401(k)?

Setup is a document and an EIN; annual work is the 5500-EZ once assets pass $250k and the contribution math.

Can my spouse participate?

Yes — each working spouse gets an independent deferral track on their own comp.

Source & methodology

IRC §401(a), §415(c); 2026 limit notice.

Solo 401(k): the number, the method and the caveats

You are here because you need a specific number, and you want to see how it was produced before you rely on it. Searchers land on a Solo 401(k) page for three reasons — to plan, to compare options, and to check that somebody else’s arithmetic is honest. This tool serves all three, and everything after the calculator explains the model instead of hiding it.

A calculation like this one is worth roughly as much as a careful sentence: it forces several vague considerations into one comparable quantity. Once Solo 401(k) is on the table, the argument can stop being about impressions.

The useful way to hold Solo 401(k) is as a ratio with a story attached: Deferral + 20%-of-comp SE-converted profit share (×0.9235 ÷1.2) ≤ $70,000 total says how the parts combine, and the $70,000 figure above is simply that sentence written in numbers. Everything that follows unpacks one clause of the sentence at a time.

Nothing below is stock text pasted around a number: the worked example computes annual solo 401(k) capacity = $70,000 from this tool’s own defaults, and the sensitivity section ranks the inputs that move the result — Net self-employment income first, at ≈6.0 % of the headline for a ten-percent error either way in that input — which is the thread the rest of this guide pulls on.

Inside the Solo 401(k) calculation: formula and every input

The engine implements a single stated model: Deferral + 20%-of-comp SE-converted profit share (×0.9235 ÷1.2) ≤ $70,000 total. There is no hidden coefficient and no login-gated precision — the formula printed is the formula executed, which is also what makes the result citable.

Net self-employment income ($) (example value 340,000) is a plain measured quantity — the unit shown in the label is the unit the formula expects. Use the figure exactly as the source document states it, not a rounded memory of it — an omitted line is the classic silent error.

Employee deferral ($) (example value 22,708) is a plain measured quantity — the unit shown in the label is the unit the formula expects. Replace the example with the figure from your own records — the tool recomputes instantly and the printed steps follow your numbers.

Employer profit-share rate (%) (example value 18) is a measured number in the unit named by the label; the formula does not convert for you. Enter it exactly as your document states it — quoted annual, monthly and effective rates are three different numbers.

Owner age (example value 45) is a quantity rather than a category — precision here is cheap and worth having. If the period is partial, decide whether you mean elapsed or planned time and keep that meaning across every field.

Years (example value 17) is a plain measured quantity — the unit shown in the label is the unit the formula expects. If the period is partial, decide whether you mean elapsed or planned time and keep that meaning across every field.

Solo 401(k) on the example inputs: the complete substitution

The tool prints its own working; this is the same arithmetic on the example values, reproduced so you can follow it by hand. Starting figures: Net self-employment income ($), Employee deferral ($), Employer profit-share rate (%), Owner age. The endpoint of that chain is $70,000 of annual solo 401(k) capacity, sitting in the “At $340,000 of net SE income you can bank $70,000/yr across both hats — the deferral track is what a SEP cannot offer” band.

SE base = $340,000 × 0.9235 = $313,990
Deferral $22,708 (incl. $0.00 catch-up) + profit share $47,897 (18% ÷ (1+0.18))
Total $70,000 ≤ cap $70,000; grows to $2,309,932

Employee deferral + catch-up — $22,708.00. Read next to the headline, this is the figure that shows the shape of the trade-off.

Employer profit share — $47,896.78. This is the number experienced users quote when the headline alone would mislead.

Balance in 17 yrs at 7% — $2,309,932. If your inputs change, this is usually the first companion figure to move.

2026 absolute cap — $70,000. Read next to the headline, this is the figure that shows the shape of the trade-off.

Keep this worked example open while you enter your own figures: matching the shape of the substitution is the fastest way to spot a unit or convention mistake.

Scenario table: Solo 401(k) at ±10 % and ±20 % of the key input

Single-point answers invite false confidence, so the same engine was run across a band around the input that matters most here. Each row is a full recalculation of Solo 401(k), not an interpolation.

Net self-employment income setting	Annual solo 401(k) capacity
80 % of the example value	$61,025
90 % of the example value	$65,815
example inputs	$70,000
110 % of the example value	$70,000
120 % of the example value	$70,000

That spread — $61,025 at the low end, $70,000 at the high — is what a careful read of Solo 401(k) looks like: a band you can state out loud, with the reason for each edge. Reproduce it in the form above by editing Net self-employment income alone; the substitution steps will follow your figures exactly.

Sensitivity: which inputs change Solo 401(k) the most

Generic advice says “it depends on your inputs”; this section says exactly how much. Each field was perturbed by ±10 % in isolation and the annual solo 401(k) capacity recomputed, so the ranking below is measured rather than asserted. It is the fastest honest answer to “what matters most in Solo 401(k)?” — and it is specific to these defaults, because sensitivity moves with the starting point.

Net self-employment income +10 % → Solo 401(k) = $70,000 · −10 % → $65,815 (the annual solo 401(k) capacity shifts ≈6.0 %)
Nudge Employer profit-share rate by ±10 % and Solo 401(k) lands between $66,483 and $70,000 (sensitivity ≈5.0 %)
Employee deferral: ten percent up gives $70,000, ten percent down gives $68,334 — ≈2.4 % of the annual solo 401(k) capacity either way

Net self-employment income is the dominant lever here: ten percent either way moves the result by about 6.0 %, from $65,815 to $70,000. That is where a careless figure does the most damage, so check it against a source, and treat Employer profit-share rate as the second priority (≈5.0 % per ten percent). Everything below that can stay approximate, which is a licence to stop chasing data that would not change the decision.

Is your Solo 401(k) good — reading the result bands

Nobody acts on a bare figure — they act on a figure with a comparison attached. The default case lands in the “At $340,000 of net SE income you can bank $70,000/yr across both hats — the deferral track is what a SEP cannot offer” band, and bands are deliberately wide because real situations overlap. The sensitivity list shows how far a modest input error moves you along that ladder: for Net self-employment income, ±10 % covers $65,815 to $70,000. The practical reading rules: read the band as a starting hypothesis, then test it: change the two dominant inputs by ten percent and see whether the verdict survives. Verdicts that survive stress are the ones worth acting on.

Where Solo 401(k) sits next to similar measures: this tool answers “how much is the Solo 401(k)”, while the related calculators answer “what do I do about it”. Reading the result alone tends to over-weight a single snapshot; pairing it with the companion tools above turns the number into a decision.

Why Solo 401(k) results go wrong — and how to prevent it

Once a single non-owner employee (other than a spouse) exists, the plan must cover them and pass testing; late Form 5500-EZ filings carry penalties since SECURE 2.0. This calculator gives an estimate based on standard formulas; it is not professional financial, medical, legal or engineering advice. Verify anything you plan to act on with a qualified professional and your own records.

Comparing a Solo 401(k) computed here with a figure from another source that uses different conventions. Same name, different denominator — the disagreement is usually in the definitions, not the data.

One mistake is specific to this engine: assuming every input matters equally. The computation above shows Net self-employment income dominating — a careful figure there beats false precision everywhere else.

Another is comparing across contexts: a Solo 401(k) computed with one set of conventions should never be lined up against a figure computed with another. Check the inputs behind both before ranking them.

Six things to confirm before acting on Solo 401(k)

State, in one sentence, which convention each rate, unit or period in the form is using.
Compare the output against one published benchmark or a colleague’s independent estimate.
Re-verify Net self-employment income against a source document — ten percent of error there moves the headline by ≈6.0 %.
Compare the result with one independent method or published benchmark.
Confirm the example values are plausible for your own situation before replacing them.
Repeat the ±10 % test on the inputs that matter and see whether the decision — not just the number — still holds.

How to use the Solo 401(k) tool properly

The tool does the arithmetic; the routine below is what makes the arithmetic trustworthy. Five steps, a minute total — and afterwards you will know not just what the number is, but which assumption produced it.

Step 1 — gather before you type. Solo 401(k) depends on net self-employment income ($), employee deferral ($), employer profit-share rate (%), owner age, years. Collect them from a source document — a statement, a report, an invoice, a log — rather than from memory, because recalled inputs are the single largest cause of wrong results. A result worth acting on survives having its two shakiest inputs halved.

Step 2 — compute the version you would not be embarrassed by. If a stranger audited your inputs, which set would you hand over? Enter that set, read the result, and treat everything else as upside. Numbers are cheap to produce and expensive to retract; the extra minute is the cheapest insurance available.

Step 3 — isolate your variables. Change one field, observe, revert. The live sensitivity list does the ranking for the default inputs; verify it against your own, because dominance shifts when the starting point shifts. When in doubt, prefer the boring input: the interesting one is usually the one you misread.

Step 4 — compute a corridor, not a point. Three runs — worst, likely, best — turn Solo 401(k) from a snapshot into a range. Plan against the pessimistic end and treat the optimistic end as the upside story. A figure nobody can reproduce is a figure nobody will act on twice.

Step 5 — write the one-line memo. “Annual solo 401(k) capacity = $70,000 on this date, from these inputs.” That line is the entire deliverable; everything else on this page is how you earned the right to write it. The point of transparency is that a reader can disagree with a specific line rather than with you.

The real-world conversations Solo 401(k) settles

Most Retirement Accounts disagreements are arithmetic disagreements in disguise. When Annual solo 401(k) capacity of $70,000 sits on the table with its working shown, the conversation stops being about who sounds more confident and starts being about which input is wrong. Numbers are cheap to produce and expensive to retract; the extra minute is the cheapest insurance available.

Professionals in Retirement Accounts use this style of calculation as a screening filter before spending real money or effort: a quick Solo 401(k) check eliminates weak options in minutes, and the survivors earn the deeper, slower analysis. Comparisons are cheap: run the identical calculation for the alternative and look at the gap.

Re-running this page costs nothing and catches the slow drifts that never announce themselves. In Retirement Planning the expensive mistakes are rarely sudden; they are small changes in Net self-employment income ($) that nobody recalculated for a year. If the verdict changes with a ten-percent nudge, report the nudge alongside the number.

Key terms on this page

Employee deferral + catch-up: $22,708.00 for the Solo 401(k) worked case; it sits next to the main result because it carries context the headline leaves out.
Employer profit share: $47,896.78 for the Solo 401(k) worked case; it sits next to the main result because it carries context the headline leaves out.
Balance in 17 yrs at 7%: $2,309,932 for the Solo 401(k) worked case; it sits next to the main result because it carries context the headline leaves out.
Annual solo 401(k) capacity: The primary output of this tool: $70,000 for the default inputs, computed exactly from Deferral + 20%-of-comp SE-converted profit share (×0.9235 ÷1.2) ≤ $70,000 total.
Worked example: Every substitution in order, from the example inputs to $70,000 — the audit trail for the headline figure.
Verdict band: A named zone on the range of plausible results; the example inputs land in “At $340,000 of net SE income you can bank $70,000/yr across both hats — the deferral track is what a SEP cannot offer”, and your inputs may not.
Sensitivity (±10 %): How far the Solo 401(k) moves when a single input is nudged ten percent either way; the ranked list on this page is computed live from this engine.

Method, sources and what this tool cannot tell you

There is no black box on this page — the engine evaluates Deferral + 20%-of-comp SE-converted profit share (×0.9235 ÷1.2) ≤ $70,000 total. Source of record: IRC §401(a), §415(c); 2026 limit notice. Specialist software layers extra coefficients on top of the same algebra; none are hidden here, and that transparency is what makes a disagreement traceable to a specific input instead of a black box.

This page is an estimation aid, not professional advice. The design rule is simple: two people entering identical figures must get identical answers. The sensitivity table above is part of that promise rather than a bonus: it lets you see how much of the result depends on how carefully you measured.

Turning this Solo 401(k) into a decision

Re-run the calculation with your own figures, stress the two most sensitive inputs by ±10 %, and only then act on the result.

The audience here is people who need one trustworthy number before their next decision. Everything you need to defend the result — formula, substitution, sensitivity, source — is printed above, so the only remaining variable is the honesty of your inputs.