How we calculate

Last reviewed: June 2026

Every apitpi calculator uses standard, published financial formulas — no black boxes. This page documents exactly how each tool works, the assumptions we make, and the authoritative sources behind our figures, so you can trust (and verify) the numbers. All calculations run in your browser; we never see your inputs.

Loans & mortgages

Monthly principal & interest uses the standard amortization formula M = P·r·(1+r)ⁿ / ((1+r)ⁿ − 1), where P is the loan amount, r the monthly rate (annual ÷ 12), and n the number of payments. We build a full month-by-month amortization schedule, add property tax, insurance, HOA and PMI (estimated at ~0.5%/yr when the down payment is under 20%) for the total payment, and model extra payments against principal.

Paycheck & taxes

Take-home pay applies 2026 federal income tax brackets and the standard deduction (per the IRS), Social Security (6.2% up to the annual wage base) and Medicare (1.45%, plus 0.9% above high-earner thresholds) per the SSA, and an estimated effective state income tax rate. State figures are approximations and exclude local taxes, credits, and itemized deductions — treat paycheck results as a close guide, not an exact filing.

Savings, investing & retirement

Growth tools compound monthly: each month the balance earns interest at the annual rate ÷ 12, then contributions are added. The savings-goal tool solves the annuity formula for the required monthly contribution. Retirement projections add employer match (capped at your contribution rate) and estimate income using your chosen safe withdrawal rate (the "4% rule" by default). Investment returns are assumptions you control, not guarantees.

Debt payoff

The debt calculator simulates month-by-month payoff for each strategy: snowball targets the smallest balance first, avalanche the highest APR first. Minimum payments are made on every debt, and freed-up payments roll into the priority debt as each is cleared.

Sources

A note on accuracy

Our calculators are reviewed and tested against published examples from established financial calculators. They provide estimates for educational purposes only and are not financial, tax, or legal advice. Tax brackets, rates, and limits change yearly — always confirm current figures and consult a qualified professional before making decisions. Found an error? Tell us.