How Our Calculations Work
How formulas are chosen, tested, rounded and kept up to date, and how to report an error.
1. Formulas come first
Each calculator is built from a written formula with defined variables and stated assumptions. Standard mathematical and financial formulas are used as published; rules defined by an authority, such as tax rates, are taken from the official source and shown with their effective date.
2. Every calculator is tested against known answers
Before a calculator can be published, its calculation code must reproduce reference values worked out independently of the code: published standard examples, hand calculations and exact unit definitions. The tests also check edge cases such as zero, negative values, leap years and invalid input.
The worked example on each page is part of those tests. If the example and the calculator ever disagreed, the build would fail and the page could not be published.
A record of the passing test run is tied to the exact calculation code that was tested. If the code changes without the tests being run again, affected calculators are withdrawn automatically until they pass.
3. Rounding
- Money is shown to two decimal places (no decimals for currencies without minor units, such as the yen). Calculations keep full precision internally and round only for display.
- Where an answer must be a whole number, such as break-even units, it is rounded up and the page says so.
- Unit conversions use exact definitions (for example 1 inch = 2.54 cm) and are shown to 12 significant figures.
4. Dates
Date calculators use plain calendar dates, without time zones, so results do not change with where you are. Leap years follow the Gregorian calendar.
5. Keeping rules current
Calculators that depend on government rates carry a review date. After that date the page shows that its rates are due for review until they have been checked again.
6. Reporting an error
If you think a result is wrong, tell us the calculator, the values you entered and the result you expected. Corrections are made in the calculator and its tests together, and the page’s “Last updated” date changes when its content changes.