Where our numbers come from, how we check them, and what we do when we get one wrong.
A financial calculator is only worth using if the math behind it is right and the sources are real. This page sets out how we build the tools on LoanCalcTools, the published formulas and federal sources we check our figures against, how often we review them, and the policies that keep advertising separate from the results you see.
None of the formulas here are proprietary. Loan, mortgage, auto, personal loan and refinance payments use the standard amortization formula, M = A·i / (1 - (1 + i)^-n), where A is the amount borrowed, i is the monthly interest rate and n is the number of payments. The APR tool follows the cost-of-credit method behind the federal Truth in Lending Act: it solves for the rate that sets the present value of the payments equal to the money you actually receive after fees, which is why an APR with fees is higher than the note rate. The debt payoff tool steps through the balance month by month, adding interest and subtracting the payment, the way a statement does. The compound interest tool uses FV = P(1 + i)^N for the lump sum plus the future-value-of-a-series formula for contributions. The inflation tool uses the ratio of the Consumer Price Index between two years. We show the worked example on each page so you can follow it by hand.
Where a figure can vary or be argued about, we check it against a primary source and cite it on the page. The ones we lean on most:
When a figure is a genuine range or changes over time, like a typical interest rate or an annual CPI value, we give the figure with its year rather than presenting it as fixed and permanent.
Our tools are for planning. They are not financial advice, a credit offer, or a substitute for the exact terms in your loan documents. Real costs can differ from a clean formula: origination fees reduce what you receive, escrow payments change as taxes and insurance change, adjustable rates move after their initial period, and investment returns vary and can be negative. That is why each page tells you to confirm the final numbers with your lender, and for major decisions, a qualified professional, before you commit.
The five sources above are not a decorative list pasted onto every page; each calculator draws from a specific one. The mortgage calculator and the mortgage rows on average loan rates use Freddie Mac's Primary Mortgage Market Survey, published weekly. The auto loan, personal loan, and refinance calculators check their default scenarios against the Federal Reserve's quarterly G.19 consumer credit release. The APR calculator follows Regulation Z's cost-of-credit method directly, since APR is a defined legal calculation, not a survey figure. The inflation calculator uses the BLS Consumer Price Index series. Investor.gov backs the plain-language definitions on the compound interest guide. If a page cites a source that does not match this list, that is an error, and we want to hear about it.
Every calculator formula is checked when it is built and rechecked whenever the underlying published standard changes, for example if the CFPB updates its APR disclosure guidance. Rate-dependent content is on a shorter clock: the average loan rates page is reviewed monthly against new Freddie Mac and Federal Reserve releases, with the review date shown at the top of that page and in its dataset markup. Guides that reference a specific year's CPI value or rate say so explicitly rather than presenting a moving figure as fixed.
If a calculator returns a result that does not match hand-checked math, or a guide states a figure you can show is wrong, use the contact page. We check it against the primary source within the week and fix the page, and for a rate or dataset error we update the "as of" date so the correction is visible, not silent.
LoanCalcTools is free to use and pays for itself through display advertising and a small number of affiliate links. If you click an affiliate link and complete a purchase, we may earn a commission at no additional cost to you. Advertising and affiliate revenue never influence the formulas, the figures or the recommendations in our tools and guides. Ads are labeled, and the affiliate disclosure appears at the foot of every page. As an Amazon Associate we earn from qualifying purchases.
Want to know who writes and checks this material day to day? See the authors page for the two people behind everything published here and exactly what each of them checks before a page goes live.