Every calculator on Wealth Growth uses standard financial formulas and clearly stated assumptions. This page documents the math behind each tool, where our default values come from, and what the results do — and do not — account for.
Each calculator runs entirely in your browser using JavaScript. The formulas are standard financial mathematics — the same ones used in finance textbooks and by banking regulators. We do not use proprietary or "black box" models. Every input you see is editable, and the result updates instantly based on the numbers you enter.
Important: The default values pre-filled in each calculator (interest rates, contribution limits, tax brackets) are realistic examples, not personalized recommendations. Always replace them with your own numbers before relying on any result.
Below are the core formulas behind our most-used calculators.
Future value is calculated using the standard compound interest formula:
A = P × (1 + r/n)n×t
Where A = final amount, P = principal, r = annual interest rate (decimal), n = compounding periods per year, t = years. For compound interest, we default to monthly compounding (n = 12) and an example 7% annual return, reflecting long-run historical S&P 500 returns after inflation.
M = P × [r(1+r)n] / [(1+r)n − 1]
Where M = monthly payment, P = loan principal, r = monthly interest rate (annual ÷ 12), n = number of monthly payments. The mortgage calculator does not include property taxes, insurance, PMI, or HOA fees unless you add them manually.
The loan calculator and auto loan calculator use the same amortization formula as the mortgage calculator above. Credit card payoff calculations assume a fixed monthly payment applied to a declining balance at the stated APR — it does not model new charges or variable rates.
The retirement calculator and 401(k) calculator project future value using the future value of an annuity formula, assuming a constant monthly contribution and fixed annual return. The Roth IRA calculator uses the same method. These models assume steady returns and do not simulate market volatility, sequence-of-returns risk, or inflation-adjusted withdrawals.
The salary calculator converts between hourly, daily, weekly, monthly, and annual figures using standard work-hour assumptions (2,080 hours per year for a full-time schedule). Tax estimates, when shown, are rough approximations — actual take-home pay depends on filing status, deductions, state taxes, and benefits elections.
Pre-filled values in our calculators are sourced from:
Rates and limits that change over time include a "last verified" date on the relevant page. Market return assumptions are long-run historical averages, not predictions of future performance.
To keep results transparent and easy to understand, our calculators make simplifying assumptions. They generally do not model:
We review default values and formulas at least quarterly, and whenever IRS limits, FDIC averages, or benchmark rates change materially. Each calculator page displays a "last verified" date for any time-sensitive default value. The formulas themselves are standard and do not change.
Disclaimer: Our calculators are educational tools that produce estimates based on the inputs you provide and standard financial formulas. They are not financial advice and do not guarantee any outcome. Always consult a qualified financial professional before making important financial decisions.