Calculator Methodology

Below is the formula or model each calculator class uses, with primary references. We do not auto-generate calculations from large language models — every result is produced by deterministic code against a named formula.

Arithmetic and trigonometry

Addition, subtraction, multiplication, division, exponentiation, square root, absolute value, sine/cosine/tangent and their inverses, degree-radian conversion — implemented against IEEE-754 double-precision floating-point with explicit handling for boundary cases (zero division, domain limits on inverse trig).

Date and time

Birthday and date-difference calculators use the proleptic Gregorian calendar with standard leap-year rules. Years before 1900 and after 2100 are out of range. Time zones come from the operating system's tz database; conversions for business-hour overlap are exact down to the minute.

Pregnancy

Due-date estimation uses Naegele's rule: due date = last menstrual period (LMP) + 280 days (40 weeks). For cycle lengths other than 28 days, we adjust by (cycle length − 28). Trimester boundaries follow ACOG conventions (first trimester through week 12, second through week 26, third from week 27).

Finance

  • Loans: Standard amortization with monthly compounding. Monthly payment = P · r(1+r)n / ((1+r)n−1) where r is the monthly rate and n the total number of payments.
  • Investments: Compound interest formula A = P(1+r/n)nt, plus periodic contributions if specified.
  • Debt payoff: Snowball and avalanche methods on user-supplied debt list.
  • Net worth percentile: Based on Federal Reserve Survey of Consumer Finances data.
  • Emergency fund: N-months × monthly essential expenses.

Drug half-life

We use a one-compartment pharmacokinetic model for elimination: C(t) = C₀ · e−kt where k = ln(2)/t½. When a substance's preset specifies an absorption rate constant, we switch to the Bateman equation for absorption + elimination kinetics. Substance half-life values come from FDA drug labels and the NIH PubChem BioAssay database.

Unit conversions

Conversion factors come from NIST Special Publication 811 (Guide for the Use of the International System of Units). Where the US and imperial systems differ for the same unit name (e.g. US vs imperial gallon, short vs long ton), we default to US units and document the distinction on the relevant converter page.

File size and download time

File sizes use both decimal (kB = 1,000 B) and binary (KiB = 1,024 B) prefixes per IEC 80000-13. Download time accounts for a configurable network overhead factor; the default is 10%.