Does a solo 401(k) contribution reduce your QBI deduction? Yes, and here is by how much

Retirement contributions cut AGI and qualified business income at the same time. Multiply by your bracket and you will overstate the saving by about a quarter.

ExactSplit · Updated April 8, 2026

The pitch everyone hears

Every article about retirement contributions for the self-employed says a version of the same thing. You are in the 24 percent bracket, so a 24,500 dollar contribution saves you 5,880 dollars in tax. Contribute the max and pocket the difference.

For an employee with a workplace 401(k), that arithmetic is roughly right. For a sole proprietor it is too high, and the reason is the qualified business income deduction under section 199A. The contribution does not just reduce your taxable income. It also reduces the income that the 20 percent QBI deduction is calculated on, and that second effect takes back part of the saving.

What QBI is, briefly

Section 199A gives owners of pass-through businesses, including sole proprietors, a deduction equal to 20 percent of their qualified business income, subject to limits. For a solo business under the income threshold, which is 201,750 dollars for single filers and 403,500 dollars for joint filers in 2026, the deduction is simply 20 percent of QBI, capped at 20 percent of taxable income before the deduction.

The question is what counts as QBI. The Form 8995 instructions are clear that QBI is the net income from the business after all deductions attributable to it. That includes three items that show up above the line on your 1040 rather than on Schedule C: the deductible half of self-employment tax, the self-employed health insurance deduction, and contributions to qualified retirement plans.

So when you contribute 24,500 dollars to a solo 401(k), your QBI goes down by 24,500 dollars, and your QBI deduction goes down by 20 percent of that, which is 4,900 dollars. That 4,900 dollars of lost deduction is taxed at your marginal rate. At 24 percent, the clawback is 1,176 dollars.

Measured rather than estimated

The clean way to see this is to run the full tax calculation twice, once without the contribution and once with it, and take the difference. That is what ExactSplit does, and here is a representative result.

Single filer, 180,000 dollars of net earnings, standard deduction, no other income. A maximum 24,500 dollar deferral saves 4,617 dollars of federal tax. Divide by the contribution and the effective saving is 18.8 percent per dollar. The marginal bracket at that income is 24 percent.

The bracket estimate says 5,880. The real number is 4,617. The estimate is high by 1,263 dollars, or about 27 percent. If you were deciding whether to contribute based on the estimate, you were deciding on a number that was a quarter too good.

Why the gap is not always the same

The size of the clawback depends on where you sit relative to the QBI limits, and it is not constant.

Below the threshold, the QBI deduction is 20 percent of QBI or 20 percent of taxable income before the deduction, whichever is less. If taxable income is the binding constraint, a contribution reduces taxable income and QBI together, and the deduction falls by 20 percent of the contribution either way. The clawback is roughly 20 percent of your marginal rate, so at 24 percent the effective saving is about 19.2 percent, and at 22 percent it is about 17.6 percent.

Above the threshold things get more complicated. The W-2 wage and property limitations phase in, and for a specified service business the deduction phases out entirely. A sole proprietor with no employees has no W-2 wages, so above the threshold the wage limitation can cut the deduction to almost nothing regardless of contributions. In that case a contribution costs you very little QBI deduction, because there was very little left to lose. The clawback shrinks and the effective saving climbs back toward the bracket rate.

The point is that the effective rate is not a fixed discount off your bracket. It depends on income, filing status, business type, and which limit binds. The only reliable way to know it is to compute it.

Health insurance does the same thing

Retirement contributions are the largest item, but they are not the only above the line deduction that reduces QBI. The self-employed health insurance deduction under section 162(l) does too, and so does the deductible half of SE tax.

That means a sole proprietor paying 12,000 dollars a year in premiums is already seeing a smaller QBI deduction because of it, before any retirement contribution. The ordering matters as well. Retirement contributions come out first, and the health insurance deduction is then limited to whatever earned income from the business remains. If you contribute enough to retirement, you can cap your own health insurance deduction.

It is still worth doing

None of this is an argument against contributing. An 18.8 percent return on the contribution is real, the money is still yours, and it grows tax deferred. The deferral is one of the best deals available to a self-employed person.

The argument is about making the decision on the right number. If you are choosing between a retirement contribution and paying down debt, or between contributing this year and next, or simply deciding how much you can afford to put in, the difference between 24 percent and 18.8 percent changes the answer. A number that is a quarter too optimistic is not a small error.

How ExactSplit handles it

ExactSplit's contribution optimizer does not multiply your room by a rate. It takes your current inputs, adds the contribution, re-runs the entire tax calculation including SE tax, the QBI deduction with its limits, and the federal brackets, and reports the difference in total tax. The effective rate per dollar is shown alongside your marginal rate so the gap is visible rather than hidden.

It does the same for each account separately and for the combined maximum, and it respects the shared employer ceiling between a SEP and a solo 401(k) so the combined figure is never the naive sum. The result is a number you can actually plan around.