Tax Sources
Key sources include the IRS for US federal tax and payroll rules, Canada Revenue Agency for Canadian tax/CPP/EI information, and GOV.UK/HMRC for UK Income Tax and National Insurance.
Important
Tax rules change. Always verify current rules with the relevant government authority before relying on a result.
Official references
- IRS Topic 306: Penalty for underpayment of estimated tax
- IRS Form W-2, Wage and Tax Statement
- IRS General Instructions for Forms W-2 and W-3
- US Department of Labor: State Payday Requirements (pay frequency is set by state law)
- IRS Publication 926 (2026): Social Security and Medicare rates, wage base and Additional Medicare withholding
- KFF: 2025 Employer Health Benefits Survey (premiums and worker contributions)
- US Department of Labor: Earnings thresholds for the EAP exemptions ($684 per week)
- US Office of Personnel Management: How to compute rates of pay (2,087-hour divisor)
- IRS Publication 15-T (2026): Federal Income Tax Withholding Methods
- IRS: 401(k) limit increases to $24,500 for 2026, IRA limit increases to $7,500
- IRS Rev. Proc. 2025-19: 2026 HSA limits
- IRS 2026 tax inflation adjustments
- IRS Publication 15 — supplemental wages
- IRS Form 941 instructions — 2026 Social Security and Medicare
- Canada tax information
- CRA 2026 CPP/EI payroll figures
- GOV.UK 2026/27 Income Tax rates
- GOV.UK 2026/27 National Insurance rates