How we check the math

One shared, tested math library

All 500 calculators draw from a single math library. No calculator carries its own private copy of a formula, so there is one place where each calculation lives, one place it gets reviewed, and one place a fix applies everywhere at once. When we improve the amortization routine, every mortgage, auto loan, and debt payoff tool that uses it improves in the same build.

The library holds the standard, well-established formulas of consumer finance: future value of a lump sum and of regular contributions, loan amortization, minimum-payment simulation, snowball and avalanche payoff, effective annual rate, APR and APY conversion, straight-line and declining-balance depreciation, and the rest. These are not proprietary or invented. They are the same equations taught in finance courses and published by regulators, which is why we can print them on the page for you to check.

Checked against independent reference math

Before any calculator ships, an automated test harness runs its calculation across a range of inputs and compares the output against an independent reference implementation of the same formula, written separately from the code the site actually runs. If the two disagree by more than a rounding tolerance, the build fails and the calculator does not go live.

This catches the errors that matter: an off-by-one in a payment schedule, a compounding period applied incorrectly, an interest charge that drifts a few dollars over 30 years. Every calculator on the site has passed this check on the day it was last built.

You can check our work

Trust should not require taking our word for it, so every calculator is built to be audited by the person using it:

• The formula is on the page. Each calculator shows the exact equation it uses, with every variable defined, in the "How it works" section directly below the tool.
• A worked example traces the default inputs. We walk the starting numbers through to the result in plain language, so you can follow the arithmetic yourself.
• Simplifications are stated, not hidden. Where a tool rounds, assumes monthly compounding, or leaves out taxes and fees, the explanation and FAQ say so plainly.
• Authoritative sources are linked. Most pages point to the relevant government or regulator guidance, so you can confirm the rules behind the math.

What the results are, and are not

The numbers here are careful estimates, not quotes or guarantees. A real lender, bank, insurer, or employer works from details a general calculator cannot know: your exact rate, your contract terms, your local rules, your credit, and the day-count and rounding conventions of their own systems. Use these tools to understand the shape of a decision and to compare options on equal footing, then confirm the specifics with the institution involved. Nothing on the site is financial, tax, or legal advice.

Corrections

If a result ever looks wrong, please tell us. Every report is re-checked against the reference math, and a genuine error is fixed quickly across every calculator that shares the formula. Reader reports are one of the ways the site stays accurate. The fastest way to reach us is the contact page, and more about who builds this is on the about page.