Self-Employment Tax Calculator - Methodology

Sources (checked 9 August 2026)

• Schedule SE (Form 1040) and instructions: 92.35% net-earnings factor, Social Security and Medicare portions, optional methods not modelled here. irs.gov/forms-pubs/about-schedule-se-form-1040
• IRS Publication 334: Tax Guide for Small Business context for Schedule C net earnings. irs.gov/publications/p334
• SSA Contribution and Benefit Base: 2026 Social Security wage base $184,500. ssa.gov/oact/cola/cbb.html
• Additional Medicare Tax: 0.9% above filing-status thresholds. IRS Topic No. 560
• Rev. Proc. 2025-32: cited for tax-year 2026 alignment across the site; this SE-only tool does not apply income-tax brackets or standard deductions. IRS 2026 inflation adjustments

Calculation steps (production)

Inputs: net self-employment income, filing status (single, MFJ, MFS, head of household, or qualifying surviving spouse), and optional other Social Security wages already subject to FICA. Steps:

1. Taxable SE earnings = net SE income × 0.9235.
2. Remaining Social Security base = max(0, $184,500 − other SS wages). Social Security tax = min(taxable SE earnings, remaining base) × 12.4%.
3. Medicare tax = taxable SE earnings × 2.9%.
4. Additional Medicare tax = max(0, taxable SE earnings − threshold) × 0.9%, where thresholds are $200,000 (single / HoH / qualifying surviving spouse), $250,000 (MFJ), and $125,000 (MFS). This tool compares taxable SE earnings alone to the threshold; it does not add W-2 wages into the Additional Medicare base.
5. Total SE tax = Social Security + Medicare + Additional Medicare. Deductible half = total ÷ 2. Monthly set-aside = total ÷ 12. Quarterly payment estimate = total ÷ 4.

Reconciled worked example

Spec fixture used in production tests: $80,000 net SE income, filing status single, no other SS wages. Results match seTaxCalculator.test.ts:

Wage-base coordination check (also tested): $200,000 net SE income with $100,000 other SS wages leaves $84,500 of Social Security base, so Social Security tax = $10,478.00.

Limitations

• Federal and state income tax, QBI, credits, and AMT are not calculated. Use the 1099 Tax Calculator for a stacked federal + state + SE estimate.
• Additional Medicare uses taxable SE earnings only; concurrent W-2 wages are not added into that surtax base in this tool (they are considered for the Social Security wage cap when entered).
• Optional methods on Schedule SE, church employee income, and farm optional methods are not modelled.

Results are estimates for planning purposes only. Consult a licensed CPA or Enrolled Agent for your actual tax liability.

Edge cases the engine does handle

• Zero or blank net income returns no result (null), so the UI does not invent a $0 SE tax breakdown.
• Other SS wages at or above $184,500 leave a zero remaining Social Security base; Medicare and Additional Medicare can still apply on taxable SE earnings.
• High earners (for example $300,000 single) correctly produce Additional Medicare tax once taxable SE earnings exceed the filing-status threshold.

Testing, updates, responsibility, corrections

Line items above are pinned by Vitest in seTaxCalculator.test.ts. Before this page ships or a constant changes, the same inputs are re-run in the live calculator UI and compared to the fixture table. Constants refresh each autumn with SSA wage-base and IRS inflation releases, and sooner if Schedule SE guidance changes. Publishing responsibility rests with FreelanceMath as an organization (no personal byline and no invented reviewer credentials). Report errors to freelancemath.mail@gmail.com with the inputs used and the figure you expected.

Version log

• 2026-08-09: Expanded methodology; documented W-2 Social Security wage-base coordination (previously misstated as unsupported); added reconciled $80,000 worked example and version log.
• 2026-05-18: Initial SE methodology page.