Section 01
Formula selection
Each calculator starts with a standard mathematical, academic, scientific, or business formula. The formula shown on the page is the same logic used by the interactive tool.
Where a convention varies—such as inclusive date counting, GPA scales, or whether a tip is applied before tax—we state the convention directly instead of hiding it behind the result.
Section 02
Input validation
Inputs are checked for missing values, impossible denominators, and ranges that would make the chosen equation invalid. The calculator provides a readable prompt when it cannot return a meaningful answer.
We avoid silently changing a user's input. Rounding is applied to displayed results while calculations retain greater internal precision.
Section 03
Worked examples and tests
Every calculator includes a worked example that can be checked independently. Core formula paths are also tested against known values, including normal cases and important boundary conditions.
A calculator is published only when its result, displayed formula, and explanatory steps agree.
Section 04
Review and updates
Calculator pages include a review date. Tools that depend on external policies, grading scales, fees, or changing rates are separated from timeless formula tools and require a source-and-version review before updates.
If a formula or assumption changes, the calculator logic and explanatory content are reviewed together.