In scientific applications, physical quantities and units of measurement are used regularly. If the inherent incompatibility between these units is not handled properly it can lead to major, and sometimes catastrophic, problems. Although the risk of a miscalculation is high and the cost equally so, almost none of the major programming languages has support for physical quantities. Instead, scientific code developers often make their own tools or rely on external libraries to help them spot or prevent these mistakes.

We employed a systematic approach to examine and analyse all available physical quantity open-source libraries. The search results were then condensed into 82 libraries, chosen from approximately 3700 search results across seven repository hosting sites, as being the most comprehensive and well-developed. In this group, 30 different programming languages are represented. Out of these 82 libraries, 38 have been updated within the last two years. These 38 are summarised in this paper as they are deemed the most relevant.

The conclusion we draw from these results is that there is clearly too much diversity, duplicated efforts, a lack of code sharing and harmonisation to encourage use.

Unlike Landin~\cite{landin700} we are not proposing an ISWIM for unit descriptions but emphasising the need to create a standard application interface for unit libraries to encourage adoption, especially within the scientific programming community.