Methodology

Last updated 21 October 2026.

Trust in a calculator comes from being able to check it. This page describes how results are produced, how they are tested and what each default means.

One engine, one answer

Financial tools share a single amortization and interest engine. Payment math uses the standard annuity formula M = P × i / (1 − (1 + i)^−n), where P is the principal, i the monthly rate (annual rate ÷ 12) and n the number of monthly payments. Because every tool calls the same code, a mortgage page and a loan page can never disagree about the same loan.

Rounding

Balances are carried at full precision through the schedule and rounded only for display.
Displayed money values are rounded to the cent; yearly table cells and summary bars may round to the dollar.
A final payment is never allowed to overshoot: the last row is capped at the remaining balance, so a schedule can never end with a negative balance or a phantom payment.
Percentages are shown to three decimals where a rate is entered and one or two decimals elsewhere.

Worked examples and tests

The “worked examples” on every page are generated by the calculator itself when the site is built, using the inputs shown next to them. If a formula changes, the printed example changes with it. On top of that, the shared engine is covered by unit tests that check payment amounts, schedule totals, the effect of extra payments, APR solving and debt payoff ordering against independently calculated values.

Default assumptions

Defaults are mainstream published averages for the United States, chosen so the page shows something realistic before you type: for example a 30-year fixed term, a 20% down payment, a 1.2% annual property tax rate, home insurance of $1,800 a year and borrower-paid mortgage insurance at 0.5% of the loan per year when the down payment is below 20%. Each tool lists its own sources in the “Assumptions and sources” box, and every assumption is an input you can change.

Privacy by construction

Calculations execute in your browser. There is no calculation API and no database of user inputs, which is why the site can be used on real salary, debt and home-price numbers without sharing them. See the privacy policy.