Tools to convert units of measurement and conversion data is abundant, but what databases/datasets exist which aim to consolidate unit conversion data?

For example:

1 pound equals 0.45359237 kilogram per "Refinement Of Values For The Yard And The Pound", US Nat Bureau of Standards, 1959

Ideally it should:

  • indicate which coefficients are prescribed by standards (rather than derived from the application of multiple other coefficients)
  • cite the standard for each conversion coefficient
  • carefully indicated precision of conversion coefficients
  • contain standard unit abbreviations/symbology
  • document units clearly to avoid confusion (ex: aliases and formal names)
  • be expressed/published in a portable machine-readable format (i.e. JSON)

Fantastically it would:

  • also contain outdated standard historical values
  • contain symbology for hybrid units
  • describe context in which particular units are used (professions, regions of the world, etc.)
  • cite standards for the unit symbology (ex: "lb")

This would make implementing a unit conversion library much easier since the step of aggregating conversion coefficients would be unnecessary.

  • Are you sure you want to implement your own unit conversion library, when udunits and similar libraries already exist?
    – gerrit
    Feb 7, 2017 at 19:06
  • I don't want to; that's why I'm asking the question :-) I'm more interested in the quality of the unit conversion dataset rather than tools themselves. One of the issues with similar libraries is incomplete data and lack of verification of / references for the contained data.
    – ebpa
    Feb 7, 2017 at 19:29
  • Sorry, in that case I'm not quite sure what you're asking for. A meta-database that describes the quality of other databases?
    – gerrit
    Feb 7, 2017 at 19:34
  • By references I mean that sources for the data contained in the database are explicitly cited. The fact that UCAR maintains that tool offers evidence to its validity; it suggests that it is carefully maintained / endorsed. Many tools are maintained by an individual and only contain coefficients (which may have been copy-pasted and may or may not be expressed to appropriate precision). The presence of references indicates an academic diligence and offers a mechanism for verification. The best database that exists may likely be embedded in a tool like udunits; it needn't be stand-alone.
    – ebpa
    Feb 7, 2017 at 19:53

1 Answer 1


You can use the database that comes with the udunits library, on which many other unit conversion libraries rely. For example, see SI derived units and non-SI units. udunits is developed by the University Corporation for Atmospheric Research (UCAR). Its license appears quite open to me.

See also the udunits github page.

P.S. I don't know why you would want to implement your own units conversion library when udunits already exists.

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