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.

80 of the 110 calculators state the formula they implement and what it assumes, and 80 of those cite a published source — 93 citations in total. Finance and health pages carry them because the results are consequential; physics and statistics because they rest on defined constants and published algorithms; and tools that implement a standard name the standard, whether that is IEC 80000-13 for data units, RFC 4632 for subnetting or the IANA time zone database.

Citations are written so the source can be found without the link, because links rot. Where a stable official link exists it is included as well.

The remaining calculators are deliberately uncited. A source for the area of a circle, for solving a quadratic, or for converting a number to hexadecimal would be padding rather than provenance, and inventing authorities for arithmetic would devalue the citations that are real. Where a page does state a convention rather than a definition — that a tip is customarily figured before tax, that significant figures round half away from zero — it says whose convention it is, because that claim is not arithmetic.

Where the conversion factors come from

The 234 unit converters rest on a different kind of source. A conversion factor is not a formula someone published and others validated; it is a definition adopted by a named body on a known date, and it is exact. The inch has been exactly 25.4 millimetres since the 1959 international yard and pound agreement, the nautical mile exactly 1,852 metres since the 1929 hydrographic conference, and the standard atmosphere exactly 101,325 pascals since the tenth General Conference on Weights and Measures in 1954.

Every conversion page names the instrument that defines its two units — 8 distinct sources across the set — and cites nothing else, so a page carries the SI Brochure alone where that is the whole answer and adds the 1959 agreement only where an imperial unit is actually involved. Each citation was checked against the factor in the code before it was written, and the constants those sources name are pinned by tests, because a citation that does not match the number it claims to support is worse than no citation at all.

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.

The worked examples and reference tables are audited the same way. Every number written into them is checked against the engine that computes it, rather than against the arithmetic that produced it — a habit adopted after that check found figures in published pages that were wrong in the second decimal place. Unit conversion factors are separately verified against their defining values: the inch as exactly 2.54 cm, the pound as 0.45359237 kg, the knot as 1852/3600 m/s. The 234 conversion pages are generated from those factors, so a single wrong constant would be wrong on every page that uses it — which is why they are checked against their definitions rather than against each other.

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 relate

DailyCalc began as an Android app, and the finance, health, date and everyday engines here were ported directly from it along with their tests. For those calculators a figure computed here matches the one computed in the app, and both are verified against the same expected values.

The site has since grown well beyond the app. Physics, statistics, trigonometry and the newer converters exist only on the web and have no app counterpart, so cross-checking against the app is not available for them. They are tested against published worked examples and reference values instead — the normal and t distributions against standard critical-value tables, XIRR against the documented spreadsheet example, and the physical constants against BIPM and CODATA.

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 contact@thedailycalc.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.