Methodology

Scope and tax year

Unless a tool page states otherwise, calculator constants reflect US federal tax year 2026. Self-employment tax follows Schedule SE. Federal income tax uses the inflation-adjusted brackets and standard deductions published for 2026. Qualified business income (QBI) follows the Form 8995 lesser-of limitation where a tool models QBI. State income tax, where shown, uses a documented flat-rate approximation rather than full progressive state schedules.

All calculator math runs in the browser. Inputs are not sent to FreelanceMath servers as part of the calculation itself. Tools are educational estimators; they are not a substitute for Form 1040 preparation software or advice from a licensed professional.

Primary data sources

Every rate, bracket, threshold, and limit used in production calculator code is traced to a government primary source. Secondary blogs are not used as sole citations for live figures. Source URLs below were checked on 28 September 2026.

• IRS Rev. Proc. 2025-32 (tax year 2026 inflation adjustments, including OBBBA-related amendments): federal income tax brackets and standard deductions ($16,100 single / $32,200 MFJ / $24,150 head of household). IRS newsroom release.
• SSA Contribution and Benefit Base: 2026 Social Security taxable maximum of $184,500. ssa.gov/oact/cola/cbb.html.
• Schedule SE (Form 1040) instructions: 92.35% net-earnings adjustment, 12.4% Social Security and 2.9% Medicare portions, deductible half of SE tax, and coordination of the Social Security wage base with W-2 wages where a tool accepts those wages. About Schedule SE.
• IRS Publication 334 and Publication 505: small-business and estimated-tax context for Schedule C / SE filers and quarterly planning. Pub 334; Pub 505.
• Form 8995 / QBI instructions: the QBI deduction is the lesser of 20% of qualified business income or 20% of taxable income computed before the QBI deduction. It is not an above-the-line AGI reduction. Instructions for Form 8995.
• Additional Medicare Tax (Topic 560 / Form 8959): 0.9% above filing-status thresholds ($200,000 single / HoH, $250,000 MFJ, $125,000 MFS). Topic No. 560.
• State revenue department rate tables: used only for the flat top-marginal approximation in tools that estimate state tax. Progressive state brackets are not fully modelled in the current production engines.

Core calculation framework

Self-employment tax tools share the Schedule SE stack. Income-tax tools add above-the-line SE tax deduction, standard deduction, optional QBI, federal brackets, and (where enabled) a state estimate. The production sequence is:

1. Net SE income = gross self-employment / 1099 income minus the business expense inputs that tool accepts (business expenses, and on the 1099 calculator also mileage). Self-employed health insurance and retirement contributions are not subtracted here; where a tool accepts them, they reduce AGI only.
2. Taxable SE earnings = net SE income × 92.35%.
3. Social Security tax = Social Security taxable base × 12.4%, where the taxable base is the lesser of taxable SE earnings and the remaining 2026 wage base ($184,500). The Self-Employment Tax Calculator reduces the remaining base by W-2 Social Security wages when entered. The 1099 Tax Calculator reduces it by the W-2 wages entered in its single W-2 field.
4. Medicare tax = taxable SE earnings × 2.9% (no wage-base cap).
5. Additional Medicare tax = 0.9% of the amount above the filing-status threshold. The Self-Employment Tax Calculator compares taxable SE earnings alone to the threshold. The 1099 Tax Calculator adds W-2 wages to taxable SE earnings before comparing to the threshold and caps the taxed amount at taxable SE earnings.
6. Deductible half of SE tax = 50% of the Schedule SE tax (above-the-line adjustment used by income-tax tools). In the 1099 Tax Calculator that is Social Security plus Medicare; the 0.9% Additional Medicare Tax is not part of the deductible half.
7. AGI (income-tax tools) = net SE income − half of SE tax + any W-2 income entered. The 1099 Tax Calculator also subtracts any self-employed health insurance and SEP or Solo 401(k) contributions entered. QBI is not subtracted in AGI.
8. Taxable income before QBI = max(0, AGI − standard deduction for the selected filing status).
9. QBI deduction (1099 Tax Calculator) = lesser of 20% of qualified business income or 20% of taxable income before QBI. Qualified business income is net SE income minus the deductible half of SE tax, self-employed health insurance, and SEP or Solo 401(k) contributions. SSTB and high-income wage/capital limitations on Form 8995-A are not modelled.
10. Federal taxable income = taxable income before QBI − QBI deduction.
11. Federal income tax = progressive 2026 brackets on federal taxable income. In the 1099 engine, single, married filing jointly, and head of household each use their own 2026 tables. Married filing separately uses its own 2026 brackets, which match the Single brackets up to the 35% rate; the 35% bracket ends at $384,350 (Rev. Proc. 2025-32). Standard deductions remain status-specific.
12. State income tax (1099 tool) = federal taxable income × the state's stored flat top-marginal rate (zero in no-income-tax states).

Per-tool calculation basis

Approximation and limitation register

The following simplifications are intentional in the current production engines. They are not bugs, but they can make estimates diverge from a full return:

• State income tax uses a flat top-marginal rate applied to federal taxable income, not full progressive state brackets, credits, or locality taxes. Moderate incomes in progressive high-tax states are often overestimated.
• Married filing separately uses its own 2026 brackets, which match the Single brackets up to the 35% rate; the 35% bracket ends at $384,350 (Rev. Proc. 2025-32). Head of household uses its own 2026 brackets. The 1099 vs W-2 comparison documents its own bracket handling on its page. Head of household in that comparison still uses the Single table.
• QBI models the Form 8995 taxable-income limitation. It does not model SSTB phase-outs, W-2 wage limitations, or UBIA of qualified property under Form 8995-A.
• The 1099 Tax Calculator uses one W-2 field for AGI, the Social Security wage base, and Additional Medicare. It does not separate Social Security wages (W-2 box 3) from box 1 wages, and it does not count a spouse's wages toward the joint Additional Medicare threshold. Health insurance and retirement inputs are used as entered, without the Form 7206 or SEP limits.
• Alternative Minimum Tax, most tax credits (CTC, EITC, education credits), itemized deductions, multi-state allocation, and entity types other than sole proprietor / Schedule C-style net earnings are out of scope.

Testing and review method

Calculator engines are pure functions covered by Vitest fixtures. Acceptance scenarios pin line items to the dollar where published (for example, the Self-Employment Tax Calculator $80,000 single case and the 1099 Tax Calculator Denver and Chicago worked examples). Before a methodology page ships or a constant changes, FreelanceMath reconciles prose against those tests and against the live calculator UI with the same inputs. A mismatch between methodology copy and production code is treated as a blocking defect on a YMYL surface.

Editorial review checks primary-source URLs, tax-year labels, filing-status coverage, and whether each listed limitation still matches the code. Independent licensed review of the mathematics (for example by an Enrolled Agent) may be commissioned separately; until such a review is completed and disclosed with accurate credentials, pages do not invent reviewer names or licenses.

Update cadence

• October–November each year: refresh brackets, standard deductions, and related inflation-adjusted figures when the IRS publishes the next-year revenue procedure; refresh the Social Security wage base when SSA publishes the contribution and benefit base.
• Within one week of any IRS or SSA guidance change that materially affects a shipped calculator.
• Immediately on discovery of an error: correct code and prose together, revise the page last-updated date, and record the change in the version log below when the change affects published methodology.

Publishing responsibility

FreelanceMath publishes as an independent editorial project under the organizational name FreelanceMath. There are no personal bylines and no invented professional credentials on this page. Publishing responsibility for calculator constants, methodology statements, and corrections rests with the FreelanceMath project editors. Broader publishing and source-verification standards are described in the Editorial Policy.

Correction route

To report a calculator bug, a stale constant, or a contradiction between this methodology and a live result, email freelancemath.mail@gmail.com. Include the tool URL, inputs used, the figure you expected, and the primary source you believe controls. Confirmed errors are corrected in production as soon as practicable; the affected page's last-updated date is revised when the fix ships. The Corrections Policy explains the full request and review process. See also the disclaimer.

Version log