Conversion methodology

Our goal is simple: conversions you can verify.

Each converter uses a documented base unit and converts through that base. Where an SI definition or a legally defined relationship is exact, we preserve the defining value in the data rather than shortening it for convenience.

Exact, derived, and approximate values

Exact definitions include relationships such as 1 inch = 0.0254 metre and the SI defining constants. These values are treated as exact in the conversion data.

Derived values are calculated from exact relationships. For example, a square foot follows from the exact foot definition, and a US liquid gallon follows from its defined cubic-inch relationship.

Approximate or conventional values are labelled or described when a unit depends on a convention, a historical definition, a context, or an irrational number represented numerically.

Display precision

The calculator keeps more precision internally than is normally useful to display. Results may be rounded for readability. A displayed decimal should not be interpreted as more precise than the underlying unit definition or input measurement.

Ambiguous units

We identify variants when the same everyday name can mean different quantities. Examples include US liquid versus Imperial gallons, US customary cups, avoirdupois mass, and 30-day months versus calendar months.

Temperature

Temperature conversions use affine formulas rather than simple multiplication because Celsius, Fahrenheit, and kelvin have different zero points.

Sources

Primary references include the Bureau International des Poids et Mesures (BIPM) SI Brochure and NIST conversion guidance. See our Sources & standards page for details.