EDIT

There is

• QUDT (http://qudt.org/) (https://terminologies.gfbio.org/terminology/?ontology=QUDT) or

• https://bioportal.bioontology.org/ontologies/UO ( https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3468815/ )

• Ontology of units of Measure and related concepts (OM) http://www.semantic-web-journal.net/sites/default/files/swj177_7.pdf

• obo ontology https://code.google.com/p/unit-ontology/

• MUO http://idi.fundacionctic.org/muo/muo-vocab.html

• w3c

Some links to at: http://answers.semanticweb.com/questions/360/ontology-for-units-of-measure

Also consider the javax-measure api (https://www.baeldung.com/javax-measure) which is already part of the classpath