Mortgage and auto loan calculators
The mortgage and auto loan tools use a standard fixed-rate amortization model. In plain language, the monthly payment is calculated from three main inputs: the financed amount, the monthly interest rate, and the number of payments.
payment = P * [r * (1 + r)^n] / [(1 + r)^n - 1]
P is the loan principal, r is the monthly rate, and n is the total number of monthly payments.
This formula is useful because it lets readers compare how rate, term, and down payment change the result. It does not automatically include taxes, insurance, HOA dues, registration, dealer add-ons, or lender-specific fees.
Savings and investment calculators
The savings and investment tools model monthly compounding with recurring monthly contributions. Each month, the balance grows by the assumed monthly rate, then the new contribution is added.
new balance = old balance * (1 + monthly rate) + monthly contribution
This is a clean way to compare contribution levels, time horizons, and assumed returns. It is not a prediction engine and it does not attempt to model fluctuating rates, taxes, account fees, or contribution timing differences.
What each calculator excludes on purpose
| Calculator | Main exclusions |
|---|---|
| Mortgage | Property tax, insurance, HOA dues, PMI, maintenance, utilities, closing costs. |
| Auto loan | Sales tax, title, registration, warranties, insurance, fuel, maintenance, parking. |
| Savings | Taxes on interest, changing APYs, irregular deposits, fees, withdrawals. |
| Investment | Taxes, account fees, market volatility, asset allocation changes, employer match rules, withdrawals. |
Why simplified models are still worth publishing
Simplified models can still be very useful if the site is honest about what they do. Readers often need a fast way to compare scenarios before talking to a lender, opening an account, or changing a contribution plan.
The purpose of the methodology page is not to claim precision where precision is impossible. It is to show the assumptions clearly enough that a reader can decide whether the estimate is good enough for the question they are asking.
How the site checks calculator pages
- The formula explanation on the page should match the logic used by the script.
- The calculator should point readers to the guide that helps them interpret the result.
- The page should explicitly say when the output is only part of the full cost or planning picture.
Sources used alongside the formulas
Formulas provide structure, but consumer finance decisions also depend on terms, fees, and assumptions. The trusted sources page lists outside references used to keep those explanations grounded.