Methodology

Correctness is the point of this site, so here is exactly how the calculators work. The same approach applies to all of them; each calculator also has its own methodology, formula, and worked example on its page.

One shared engine

Every calculator imports its formulas from a single package. The math is defined once and reused, so a mortgage payment is computed the same way everywhere, with no copy-pasted formulas to drift out of sync.

Decimal-precise money math

All money math uses decimal.js, not binary floating point. That avoids the rounding errors that creep in with ordinary numbers, so cents add up exactly. Values are formatted for display only at the end, using the Intl standard.

Stated assumptions

Each calculator lists what it assumes (compounding, what costs are included, rounding) right on the page. If an assumption matters to your result, you can see it and change it.

Golden-value tests

Each calculator ships tests that check it against known-correct results to the cent, plus property tests (for example, that amortization principal sums exactly to the loan). A formula that fails a test never ships.

Each calculator's method

Open any calculator to see its formula, a worked numeric example, its stated assumptions, and its sources.

For the plain-language process behind all of this (testing, build gates, and how we keep year-stamped figures current), see how we build and verify. For education, not financial advice. See our disclaimer.