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CalcDock Calculation Methodology

How every CalcDock calculator is built, sourced, verified, and kept up to date. This page exists so a reader, an editor, or a reviewer can understand the formula behind a result — and decide whether to trust it for their situation.

Last reviewed: 2026-05-16

Our editorial principles

CalcDock is built around three commitments that show up in every tool we publish:

  • Formulas are stated, not hidden. Each calculator shows the exact equation it uses, named variables, and any rounding or sign conventions. Readers should never have to guess what the result means.
  • Limits are stated up front. If a tool cannot account for taxes, fees, leap-year edge cases, regional rules, or jurisdictional differences, we say so before the result, not in fine print.
  • External claims have a source.Where a number depends on a standard (ISO 8601 dates, BMI ranges, common unit definitions) or a public reference, the source is linked from the calculator's "Sources" block.

How we build a calculator

Every calculator on CalcDock goes through the same five-step process before it ships:

  1. Define the question.We start with the literal question a user is trying to answer ("how many days until my exam?", "what is the monthly payment on this loan?"), not with a generic formula.
  2. Pick a reference formula.We choose a single, well-known formula — typically the form taught in finance, statistics, or standards documents — and document it in the calculator's "Formula" section.
  3. Write worked examples. Before any code, we hand-compute at least two examples covering a normal case and a tricky edge case (negative result, rounding boundary, leap year, zero divisor).
  4. Cross-check against a known source. The implementation is compared to a textbook, a published table, or a widely-used reference tool. Differences are tracked and resolved before publish.
  5. Add interpretation notes.A result without interpretation is half a tool. Each calculator ships with "how to read the result", "common pitfalls", and "what this cannot tell you" sections.

Formula families we use

CalcDock's calculators fall into a small number of formula families. Knowing the family helps you judge what the tool can — and cannot — do for you.

Date and time tools

Date tools (D-Day, date difference, age, time duration) compute calendar differences using the proleptic Gregorian calendar as defined in ISO 8601. The unit of difference is whole days unless explicitly stated otherwise. We do not adjust for daylight-saving transitions inside the day count; if you need that nuance, the calculator's "common pitfalls" section describes when it matters.

Finance tools

Finance tools (loan, mortgage, compound interest) use the standard textbook formulas:

  • Equal-payment loan: M = P · r / (1 − (1 + r)−n), where r is the periodic rate and n the number of periods.
  • Compound interest: A = P · (1 + r/m)m·t, where m is compounding periods per year and t the number of years.
  • Effective annual rate: EAR = (1 + r/m)m − 1.

Finance calculators are educational. They do not include taxes, insurance, origination fees, prepayment penalties, or jurisdiction-specific APR disclosure rules. For an actual loan or mortgage decision, talk to a licensed adviser.

Percentage, ratio, and average tools

Percentage tools (percentage, percentage change, average, ratio) use direct algebraic definitions. Percentage change is computed as (new − old) / |old|, which keeps the sign meaningful when the baseline is negative. Averages default to the arithmetic mean; if a tool offers a different average (median, weighted) the option is labeled explicitly.

Conversion tools

Unit conversions (unit, file size, binary) use exact ratios where they exist (1 inch = 25.4 mm exactly) and IEC binary prefixes for file size (1 KiB = 1024 B), with SI prefixes shown alongside where readers commonly mix the two.

Health tools

Health tools like BMI use the World Health Organization adult classification by default and label the unit system (metric or US customary) in the input. BMI is a screening tool, not a diagnosis; the calculator's "what this cannot tell you" section spells out where the metric breaks down (athletes, elderly, children, pregnancy).

Sources we rely on

The references below are the primary sources behind CalcDock's formulas and ranges. Where a calculator depends on a specific source, it is linked again on that page.

  • ISO 8601 — date and time interchange format used for all date math.
  • BIPM SI Brochure — SI base units and prefixes used by the unit converter.
  • IEC 80000-13 — binary prefixes (KiB, MiB, GiB) used by the file-size converter.
  • World Health Organization — BMI — adult BMI classification used by the BMI calculator.
  • Standard finance textbooks (Brealey, Myers & Allen; Ross, Westerfield & Jordan) for the amortized loan, compound interest, and effective rate formulas.

How we verify results

Before a calculator ships, and on every meaningful change after, we run three checks:

  1. Hand-computed examples.The worked examples shown in each tool's "Examples" section are computed by hand and then run through the live calculator. If the numbers don't match, we fix the implementation, not the example.
  2. Cross-tool consistency. Tools that touch the same math (e.g., the percentage and discount calculators, or the loan and mortgage calculators) are tested with shared inputs to confirm they agree on the cases where they should.
  3. External reference comparison. Results are compared against a published table, a textbook answer, or a widely-used reference tool. Discrepancies are investigated; usually they come down to a different convention (inclusive vs. exclusive day counting, nominal vs. effective rate) and we document that choice on the page.

When and why we update

Each calculator page carries an "updated" date that reflects the most recent meaningful change to its formula, copy, examples, or warnings. We update a calculator when:

  • A reader reports a real bug or an unclear explanation, and we can reproduce it.
  • A referenced standard changes (for example, an updated WHO classification or a revised SI definition).
  • We notice that a common edge case (negative numbers, very large inputs, leap-year boundaries) was not handled clearly.
  • The interpretation notes need to be sharper based on the questions readers actually ask.

We do not bump the "updated" date for cosmetic changes that don't affect the result or its explanation. The date is a signal of substance, not activity.

What CalcDock is not

CalcDock is a teaching and quick-reference site. It is not:

  • A licensed financial, tax, legal, or medical advisory service.
  • A tool of record for any decision with binding consequences (loan applications, tax filings, clinical diagnosis, contract deadlines).
  • A replacement for jurisdiction-specific disclosures, such as APR disclosure under regional consumer credit law, or food-label conversions under regional regulation.

For high-stakes decisions, treat a CalcDock result as a sanity check and confirm with the relevant professional or official source.

Errors, corrections, and feedback

If a result, formula, or interpretation note looks wrong, please tell us. Use the contact page with the calculator name, the inputs you used, the result you got, and what you expected. We respond to verified corrections by fixing the underlying tool, updating the "updated" date, and — for non-trivial fixes — adding a short note to the calculator's warnings section so future readers see the edge case clearly.

For the editorial side of this work — who writes, who reviews, and what we will not publish — see the Editorial Policy. For how advertising fits into the site, see the Advertising Disclosure.