Methodology

A calculator is only worth as much as the reasoning behind it. This page explains how the formulas here are chosen, how they are tested, and what happens when one turns out to be wrong.

How formulas are selected

Every calculator implements a published, named formula rather than an approximation invented for this site. Where several competing formulas exist — as with basal metabolic rate or ideal body weight — the one with the better validation record is used, and the alternatives are shown alongside it rather than hidden.

Each of the 23 finance and health calculators states its formula, its assumptions and, where the subject is medical, the published research it comes from. Citations are written so the source can be found without the link, because links rot.

How calculations are tested

The calculation engines are separated from the interface entirely: they are pure functions with no knowledge of the page they render on, which makes them directly testable. They are covered by an automated suite that runs on every change and blocks release if anything fails.

The tests deliberately avoid checking the code against itself. Results are verified against published worked examples, against closed-form identities, and against a second method of computing the same thing — an amortization schedule is checked by running the balance forward month by month until it reaches exactly zero, not by re-running the formula that produced it. Every calculator is additionally driven through boundary values — zero, negative, absurdly large, blank and non-numeric — to confirm it produces a sensible message rather than a wrong number.

That process has caught real errors, including one in the original Android implementation, where floating-point drift caused a tile-quantity estimate to bill one tile too many whenever the exact count landed on a whole number.

How rounding works

Calculations are performed at full double precision and rounded only for display, so a rounded figure never feeds into a later step. Currency is shown to two decimal places; ratios and scientific results to more, since rounding them early would be misleading.

Where a quantity must be a whole number — tiles, bricks, cement bags — the result is rounded up, because you cannot buy part of one. Float noise is removed before that rounding so a value that is mathematically exact is not pushed to the next unit by a rounding artefact.

How assumptions are chosen

Every model leaves something out, and the honest thing is to say what. Loan calculations exclude lender fees and insurance because those vary by lender and cannot be guessed. Investment projections assume a constant return, which no market delivers. Health formulas are derived from population averages and can be substantially wrong for an individual.

These limitations are stated on each calculator rather than collected in a disclaimer nobody reads, because an assumption only helps if it is visible next to the number it affects.

How the app and the site stay in agreement

The DailyCalc Android app and this website share the same calculation logic, ported directly with its tests. A figure calculated here matches the one calculated in the app, and both are verified against the same expected values.

How errors are handled

Reported errors are treated as the highest priority. Accuracy is the entire value of a site like this, and a wrong result is worse than no result. When a formula is corrected, a regression test is added so the same error cannot return.

If a figure looks wrong, please report it to futuretekapps@gmail.com with the calculator name, the values you entered, the result you saw and the result you expected.

How often formulas are reviewed

Formulas are reviewed when the underlying standard changes — as when a health body revises a threshold — and when a reader reports a discrepancy. The "last updated" date on each calculator reflects when that page's content genuinely changed, not when the site was last deployed.

Who maintains this

DailyCalc is built and maintained by FutureTek Apps, which also publishes the DailyCalc Android app. It is an independently developed product rather than a content network, and it carries no sponsored calculators or paid placements.