How TaxQuarters works — every figure and its source

This calculator is deliberately transparent. It runs a small, unit-tested engine using only published U.S. government figures for the 2026 tax year. Nothing is invented, and the same code you see here runs in your browser.

The calculation, in order

1. Net self-employment profit × 92.35% = the base for self-employment tax.
2. Social Security 12.4% on that base up to the 2026 wage base of $184,500; Medicare 2.9% on all of it; +0.9% Additional Medicare above the high-income thresholds.
3. Deduct one-half of the 15.3% SE tax from income.
4. Subtract the 2026 standard deduction for your filing status (and the optional 20% QBI deduction) to get taxable income.
5. Apply the 2026 marginal brackets to get income tax; add SE tax for the total; divide by four for the quarterly amount.

Sources (all public domain)

• Social Security wage base — $184,500 (2026)
  U.S. Social Security Administration (SSA) — 2026 cost-of-living adjustment fact sheet.
  https://www.ssa.gov/cola/ ↗
• Standard deduction & federal income-tax brackets (2026)
  Internal Revenue Service (IRS) — Rev. Proc. 2025-32 — Annual inflation adjustments for tax year 2026.
  https://www.irs.gov/newsroom/irs-releases-tax-inflation-adjustments-for-tax-year-2026-including-amendments-from-the-one-big-beautiful-bill ↗
• Self-employment tax (15.3%), 92.35% factor, one-half-SE-tax deduction
  IRS — Schedule SE & Publication 334 — 12.4% Social Security + 2.9% Medicare on 92.35% of net self-employment earnings.
  https://www.irs.gov/businesses/small-businesses-self-employed/self-employment-tax-social-security-and-medicare-taxes ↗
• Additional Medicare Tax (0.9%)
  IRS — Applies above $200,000 (single/HoH), $250,000 (MFJ), $125,000 (MFS). Not indexed to inflation.
  https://www.irs.gov/businesses/small-businesses-self-employed/questions-and-answers-for-the-additional-medicare-tax ↗
• Qualified Business Income (QBI) deduction — 20% (Sec. 199A)
  IRS — Up to 20% of qualified business income, subject to income thresholds; simplified here.
  https://www.irs.gov/newsroom/qualified-business-income-deduction ↗
• Estimated tax & payment due dates — Form 1040-ES
  IRS — Quarterly estimated-tax vouchers and safe-harbor rules.
  https://www.irs.gov/forms-pubs/about-form-1040-es ↗

What it does not do

• No state or local income tax (varies by state — add yours separately).
• No tax credits (child tax credit, EITC, education, energy, etc.).
• QBI is simplified (20% below the Sec. 199A income thresholds); complex cases need a professional.
• Additional Medicare is applied to self-employment + reported wages only, without spousal-wage interplay.

Bottom line: a fast, honest ballpark for federal self-employment planning — not a filed return. Confirm with a tax pro before you pay.