Trust

Calculation Methodology

Last updated: July 29, 2026

This page documents how every calculator on xyz calculatorz is designed, sourced, tested and maintained. It applies to all tools on the site unless a page states otherwise.

How calculations are created

Scope the question. We define exactly what the tool answers, in what units, and for whom.
Select a primary formula. Preference goes to a regulator, standards body, textbook or peer-reviewed publication over a secondary blog summary.
Model inputs explicitly. Every variable gets a label, a unit, sensible bounds and validation so impossible inputs cannot silently produce a number.
Implement and test. The implementation is checked against reference values published with the source, plus manual spot checks at boundary conditions.
Write the explanation alongside the code. Definition, formula, variables, example and FAQ are written from the same source used to implement the maths.
Review and publish. A tool ships only when the printed formula matches the executed formula.

Formula sources

We prioritise sources in this order:

Government and regulatory publications — tax authorities, central banks, health departments, building-code authorities.
Standards bodies and industry conventions — day-count conventions, unit definitions, engineering allowances.
Academic and clinical literature — for example the Mifflin–St Jeor equation for basal metabolic rate.
Established reference texts for classical mathematics, geometry and statistics.

Where sources disagree, we use the most widely adopted convention and note the alternative on the calculator page.

Assumptions

Every model simplifies reality. The assumptions we make most often are:

Rates are nominal annual rates unless a page says otherwise, compounded at the stated frequency.
Payments occur at the end of each period unless the tool exposes an option.
Taxes, insurance, fees and local surcharges are excluded unless they are explicit inputs.
Health formulas describe population averages, not individuals.
Material estimates exclude waste unless a waste factor is offered as an input.
Intermediate values are held at full precision; only the displayed result is rounded.

Limitations

Results are estimates and cannot account for your full personal, legal or medical circumstances.
Jurisdiction-specific rules — tax brackets, lending criteria, building codes — vary and change.
Formulas validated on one population (for example adults of average body composition) may be inaccurate for others.
Currency, inflation and market assumptions are user-supplied; we do not forecast them.

See the full disclaimer for what this means legally.

Example calculation

A $300,000 loan at 6% nominal annual interest over 30 years uses the standard amortising payment formula:

M = P · i / (1 − (1 + i)⁻ⁿ)

P = 300,000 (principal)
i = 0.06 / 12 = 0.005 (monthly rate)
n = 30 × 12 = 360 (number of payments)

This gives M ≈ $1,798.65 per month, covering principal and interest only — property tax, insurance and HOA dues are excluded, which is precisely the kind of assumption we state rather than bury.

Accuracy standards

Implementations must reproduce published reference values to the precision the source provides.
Displayed results are rounded once, at the end, using the convention normal for that domain (currency to cents, materials up to whole units).
Any deviation from a source formula must be documented on the calculator page.
Reported errors are reproduced before a fix is shipped, and material corrections are dated.

Editorial policy — content standards, review process and corrections.
FAQ — common questions about accuracy and usage.
Contact — report a formula issue.