Transparency is our core standard. On this page, we document the primary statutory authorities, mathematical formulas, and annual update schedules governing all PlainFig calculators.
Primary Statutory Authorities
PlainFig uses exclusively primary government and regulatory data sources:
- Federal Income Tax Brackets: IRS Revenue Procedure 2025-11 (covering the 2026 tax year).
- FICA Taxes: Social Security Administration (SSA) 2026 Fact Sheet (Social Security wage base of $176,100 at 6.2%; Medicare rate of 1.45% plus 0.9% Additional Medicare Tax per Internal Revenue Code Section 3101(b)(2)).
- Mortgage Benchmarks: Freddie Mac Primary Mortgage Market Survey (PMMS) weekly index.
- Auto Loan Rates: Federal Reserve G.19 Consumer Credit Statistical Release (commercial bank loan terms).
- State Income Taxes: State Revenue Departments and Tax Foundation 50-State Individual Income Tax Indices.
Standard Amortization Equation
Fixed-rate mortgages and auto loans are computed using standard fixed-principal amortization:
M = P * [r(1 + r)^n] / [(1 + r)^n - 1]Where M is monthly payment, P is financed principal, r is periodic monthly interest rate, and n is total scheduled payments.