Sources & standards
We prioritize primary metrology references and clearly defined unit relationships.
BIPM — International System of Units
The BIPM SI Brochure is our primary reference for the SI and its defining constants. The current 9th edition was updated in 2026.
NIST — Guide to the SI
NIST Special Publication 811 and its conversion-factor appendices are used for many SI/non-SI relationships, including customary and engineering units.
How we use sources
When a relationship is exact by definition, we preserve the defining value. When a source publishes a rounded factor, or a unit has multiple conventions, we avoid describing the displayed decimal as mathematically exact.
See Conversion methodology for how these references are applied in the calculators.