The 15.3% that comes first
Self-employment tax is the contractor's version of payroll tax, and a sole proprietor pays both halves of it. On 92.35% of net profit the page charges 12.4% for Social Security and 2.9% for Medicare, then adds the 0.9% Additional Medicare slice on the part of those earnings and your wages that sits above the threshold for your filing status.
The 7.65% left out of the base stands in for the employer share, and half of the tax that results comes back later as an above-the-line deduction, so the base and the deduction are two halves of the same correction rather than two separate breaks.
Two limits shape the answer. The Social Security wage base is shared with any W-2 job, so employer wages use up the $184,500 cap first and a contract on top of a large salary eventually stops paying the 12.4%. Net earnings below $400 carry no self-employment tax at all, and the page prints $0.00 rather than a token amount while still showing the 92.35% base that produced it.
Income tax underneath, and the deduction most people miss
The income tax under that runs on the ordinary brackets, 10% up to 37%, applied to taxable income after the standard deduction - $16,100 single, $32,200 joint, $24,150 head of household for 2026. The deduction comes off first, so a modest contract year can produce no income tax at all while still producing self-employment tax.
The qualified business income deduction is the piece most contractors miss: 20% of qualified profit, capped at 20% of taxable income before the deduction itself. The page takes the smaller of the two, which is why a small profit can be limited by the taxable-income cap rather than by the 20% rate, and the table prints the deduction as its own line so the cap is visible.
A retirement contribution entered in the last field reduces income tax and the QBI base in the same pass. It does not reduce self-employment tax, which is charged on net profit before any retirement contribution is made.
What "set aside X%" really means
The set-aside share answers one question: if this contract had not happened, how much federal tax would have been avoided? The numerator is the full federal bill minus the income tax the W-2 job would owe on its own, and the denominator is the gross 1099 income you entered, not the profit after expenses. That is deliberate, because gross contract dollars are what arrive in the account and what has to be split between tax and living.
A contractor with no W-2 job reserves a share of everything. One with a strong W-2 already has part of the bill paid through the paycheck, so the incremental cost of the contract falls and the share shrinks with it.
Read the results table with that in mind: its total federal tax line covers the wages and the contract together, so it is larger than the contract's own increment. The hero statistic and the set-aside percentage are the incremental figures, and they are the ones to move into a separate account.
The quarterly bills underneath
Nothing is withheld from 1099 money, so the uncovered balance is expected as four estimated payments on April 15, June 15, September 15 and January 15 of the following year. The page takes what withholding leaves unpaid and divides it by four, printing the same amount at each date: the simplest plan that keeps the April bill funded.
The safe-harbor route - paying the smaller of 90% of this year's tax or 100% of last year's, 110% once income passes the higher threshold - can make each payment smaller than a strict four-way split. That version lives on the quarterly estimated tax calculator, and this page does not price a late-payment penalty on your behalf.
It helps to remember that an estimate is a prepayment of the same tax, not an extra one, so anything overpaid returns when the return is filed. The failure mode is not the rate but the missing withholding, which is why the reserve target and the four dates are printed next to the tax rather than left to memory.
Danfio