Two separate taxes come out of the same number, and mixing them up is the usual reason a first 1099 year hurts.
Self-employment tax is 15.3% of your net earnings — 12.4% for Social Security and 2.9% for Medicare. It is charged on 92.35% of your net profit, not all of it. The Social Security half stops at the annual wage base ({{SS_WAGE_BASE}} in {{SS_WAGE_BASE_YEAR}}); above that only the 2.9% Medicare portion continues, with no cap.
Income tax is charged on that same profit at your ordinary rate — on top of self-employment tax, not instead of it.
🧮 The calculator: Self-Employment Tax Calculator. A "1099 tax calculator" and a "self-employment tax calculator" are the same estimate under two names — the 1099 is just the form the income was reported on. The tool estimates SE tax and income tax together.
What drives the number down: net profit, not gross income, is what the calculation runs on. Every deductible business expense you actually captured lowers the profit and therefore both taxes. Expenses you forgot are the most common reason a real bill comes in above an estimate.
Half of your self-employment tax is deductible against income tax, which the calculator accounts for.
Quarterly payments: nothing is withheld for you, so the payments are yours to make. The IRS threshold is expecting to owe $1,000 or more when the return is filed. See quarterly estimated taxes for the detail, and when the tax filing deadline falls for the annual dates and how filing extensions work.
What the estimate does not cover: it does not file anything, and it does not apply state or local tax, credits, another job's withholding, or a spouse's income. Treat it as a set-aside figure.
Sources: IRS Topic 554 (self-employment tax), IRS Estimated taxes, SSA contribution and benefit base. Estimates — confirm with your tax professional.