How the calculators work

Each calculator shows its inputs, a breakdown of the result, its assumptions and what it excludes. Some models step through time monthly; others use annual cash flows or a single formula. This page explains which is which.

How each model handles time and interest

Monthly savings models. The compound interest, savings goal, savings time, down payment, investment fee, retirement savings, and FIRE tools divide the entered annual rate by 12 and compound once a month. The compound interest tool lets you choose whether deposits arrive at the beginning or end of the month; the others add deposits at the end of the month, after that month's interest. The retirement withdrawal tool also applies the annual return divided by 12 each month, then takes the planned withdrawal. These tools accept a nominal rate, not an annual percentage yield (APY), which already includes compounding.

Selected compounding frequency. The APY and CD tools divide the nominal annual rate by the compounding frequency you choose (annually, semiannually, quarterly, monthly, or daily) and compound at that frequency. They use equal periods, not the actual-day conventions an institution may use in its disclosures.

Annual and single-period models. Simple interest applies the rate to the original principal only, with no compounding. The inflation tool compounds its rate once per year. CAGR solves for one constant annual rate between a beginning and an ending value. NPV and IRR treat each entered cash flow as an end-of-year amount. Rent vs. buy steps through the horizon one year at a time; its mortgage payments are monthly, but rent, ownership costs, and investment of the yearly cost difference are handled annually. The emergency fund tool adds no interest.

Loans. Loan tools estimate equal monthly payments on a fixed-rate, fully amortizing loan, using the annual rate divided by 12. Each month, interest is calculated on the outstanding balance, with the rest of the payment reducing principal. Additional payments reduce principal after that month's interest. The final payment is reduced to the amount still owed. A balloon loan uses the same payment but stops at the balloon month, leaving the remaining balance due.

Debt strategies, side by side

The avalanche method directs extra money toward the highest interest rate. The snowball method targets the smallest balance. Both keep your starting monthly repayment budget constant and roll payments from cleared debts into the remaining debts. Minimum payments stay fixed in this model; actual card issuers may calculate them differently.

Other models in the library

The retirement and FIRE tools project from the returns, contributions, expenses, inflation, and withdrawal assumptions you enter. The home tools use entered purchase, financing, ownership-cost, and rent assumptions. Business tools work from entered prices, costs, and cash flows. These models simplify real decisions: they do not predict market returns, guarantee a withdrawal strategy, or determine a lender's offer.

What the estimate leaves out

Rates are assumptions you supply, not current offers or forecasts. Unless a field explicitly includes them, calculations exclude taxes, inflation, fees, insurance, penalties, and changes in rates or payments. Loan comparison includes the upfront fees you enter, paid separately rather than financed. The original loan-payment calculator covers principal and interest only; the mortgage-payment calculator separately includes any taxes, insurance, HOA, and PMI amounts you enter.

Credit-card and some loan interest is assessed daily. This site's monthly model will differ from those statements. A lender may also apply extra payments to future installments rather than principal; check its instructions and any prepayment charges.

Precision and limits

Calculations retain full precision internally and round displayed currency to cents. Downloaded CSV files round currency to cents and other numbers to at most four decimal places, and they begin with a UTF-8 byte order mark so spreadsheet apps read the encoding correctly. Adding displayed or exported rows can therefore differ slightly from an unrounded total. Each calculator's field limits and result notes describe its modeled horizon. The debt-payoff tool stops at 600 months and explains when a scenario cannot be repaid within that horizon.

Where the calculation happens

Financial inputs are processed in your browser. They are not submitted to us, included in page URLs, or stored across visits. Scenario comparisons last only while the page is open. Downloaded schedules stay wherever you save them on your device.

Primary references

These sources explain the underlying concepts. They do not endorse this site. Estimates are educational planning tools, not personalized financial advice or lender quotes.