Expertise & Methodology
How LevyIO tax calculators work — data sources, calculation methods, editorial standards, and update cadence.
Why this page exists
Tax math drives real-life decisions: whether to take a bonus this year or next, whether to convert to a Roth, whether to file jointly. When a calculator influences those decisions, you deserve to know exactly where the rates and brackets come from. This page documents LevyIO's data sources, methodology, editorial process, and update cadence.
Primary data sources
LevyIO is built on IRS, state revenue, and Social Security Administration primary sources.
- IRS Publication 15-T (Federal Withholding)
Federal income tax withholding tables (percentage method + wage bracket method). Source for /paycheck-calculator/ and /withholding-calculator/. Updated annually.
- IRS Publication 15 (Employer Tax Guide)
FICA rates (Social Security 6.2% / Medicare 1.45% / additional Medicare 0.9% above $200K single), supplemental wage rules (22% flat / 37% over $1M).
- IRS Revenue Procedure (Annual Tax Brackets)
Annual federal income tax brackets, standard deduction, capital gains thresholds. Released November-December for following year.
- IRS Topic 409 (Capital Gains)
Long-term vs short-term capital gains rules, 0%/15%/20% brackets. Net Investment Income Tax (NIIT 3.8%) thresholds. Powers /capital-gains-tax-calculator/.
- State Department of Revenue Sources (50 states)
State income tax rates, brackets, standard deductions, exemptions for all 50 states + DC. Updated annually + ad-hoc legislation.
- Social Security Administration (Wage Base)
Annual Social Security taxable wage base limit (e.g., $176,100 for 2026). Required for accurate FICA calculation in /payroll-tax-calculator/.
Calculation methodology
Federal income tax (progressive brackets)
Apply marginal rates to income tranches: 10% / 12% / 22% / 24% / 32% / 35% / 37% (2026 single filer brackets: $0–11,925 / 11,925–48,475 / etc). Subtract standard deduction ($15,000 single, $30,000 MFJ in 2026) before applying brackets.
Bonus/supplemental wage tax
Per IRS, two methods allowed: (1) flat 22% federal withholding (37% above $1M annual supplemental wages) — most common for one-off bonuses. (2) Aggregate method — adds bonus to most recent paycheck and applies normal withholding tables. State rules vary (CA imposes 6.6% / 10.23% on stock options; some states no supplemental rate).
FICA (Social Security + Medicare)
Social Security 6.2% applies up to wage base ($176,100 in 2026 — indexed to AWI). Medicare 1.45% on all wages. Additional Medicare 0.9% on wages above $200K (single) / $250K (MFJ). Self-employed pay both halves (15.3% combined SE tax) on /self-employment-tax-calculator/.
Capital gains taxation
Held >12 months = long-term. LTCG rates 0%/15%/20% based on filing status and total taxable income. NIIT 3.8% adds for AGI above $200K single / $250K MFJ. Short-term = ordinary income rates.
Editorial standards
- All tax calculators run client-side. No income, SSN, or filing details transmitted off-device.
- Federal tax brackets reflect the most recent IRS Revenue Procedure within 30 days of publication.
- State tax brackets reflect each state's most recent published rates within 30 days.
- Bracket year (2025/2026/etc) is shown prominently on every calculator output.
- Articles cite primary sources (IRS Pubs, USC Title 26, state revenue departments) — not aggregator sites.
- When tax law is in flux (e.g., TCJA 2017 sunset 2025), we model both scenarios and disclose assumptions.
Update cadence
| What | When |
|---|---|
| Federal tax brackets | Annual (IRS Rev. Proc. November–December for next year) |
| FICA wage base limit | Annual (Social Security Administration October announcement) |
| State tax brackets | Annual + ad-hoc when state legislation passes mid-year |
| Capital gains brackets | Annual (IRS Rev. Proc.) |
| Glossary terms | Quarterly review + within 14 days of major IRS guidance |
| Article fact-checks | Quarterly + within 14 days of major tax legislation passage |
Disclaimer
LevyIO calculators are informational tools, not tax advice. Always consult a CPA or tax professional for your specific situation. We are not affiliated with the IRS, any state Department of Revenue, or the Social Security Administration.
Corrections and feedback
Spot an error or outdated rate? Email [email protected]. We respond to factual corrections within 14 days.
Who builds LevyIO
See /about/team/ for team backgrounds. Browse our tax glossary for definitions of key terms used across calculators.