How PayCalc calculations are produced and checked

Paycheck calculator

The paycheck tool estimates one pay period. It starts with regular and supplemental gross wages, classifies pre-tax/FICA-exempt deductions, applies federal withholding, Social Security and Medicare, then the selected state's withholding method and modeled employee payroll programs. Supported local calculations are handled separately where required.

For 2026, federal withholding follows the IRS Publication 15-T automated percentage-method framework implemented by PayCalc, including supported Form W-4 Steps 2, 3, and 4. Federal and state rules are stored with tax-year and source metadata. Browse the 2026 source directory.

Mortgage calculator

The mortgage tool models a fixed-rate amortizing loan with monthly periods. It calculates scheduled principal and interest from balance, annual rate, and term. Extra principal is applied through a month-by-month payoff simulation. Property tax, homeowners insurance, and optional PMI are user-entered costs kept conceptually separate from amortization.

Investment calculator

General mode uses discrete compound growth: the initial amount is multiplied by one plus the user-entered return per selected period, raised to completed periods. Rental mode subtracts entered annual expenses from rent and divides the result by purchase value to report simplified net rental yield. Neither model is a forecast or recommendation.

Testing layers

Unit tests exercise functions, integration tests cover rule combinations, and Playwright browser tests operate the actual interface. The live regression workflow is scheduled daily and after successful deployments. The Accuracy & Testing Center publishes representative cases.

Source monitoring

Selected official federal and state sources have scheduled monitor workflows. Automated monitoring is not equally reliable for every government publication, so PayCalc does not claim real-time automatic verification of every state. The source directory records the verification date in each rule record.

Model boundaries

Payroll can differ because of employer configuration, local rules, exemptions, benefit plans, or wage history. Mortgage outcomes can differ because of lender terms, escrow changes, closing costs, or PMI rules. Investment outcomes depend on uncertain returns, fees, taxes, inflation, and cash flows outside the simple model.

Important: This methodology describes software models for educational estimates. It does not convert output into professional tax, legal, lending, accounting, or investment advice.