Hour Abbreviations: h, hr, or hrs?
For scientific and technical measurement, the recognized unit symbol for hour is “h,” and unit symbols are not pluralized. Forms such as “hr” or “hrs” may appear in general prose or house styles, but they should not be presented as the SI-style unit symbol.
Use “h” as the technical unit symbol and keep it unchanged in the plural (for example, 2 h). In ordinary prose, follow your house style for “hr” or “hrs,” or spell out “hours.”
Key details
Important caveats
Boundary
A scientific unit symbol and a prose abbreviation are different style systems; the page keeps them separate.
Further guidance
General Writing
Outside technical measurement, “hour” or “hours” is often clearer than mixing unit-symbol rules with informal abbreviations.
Technical Rule
NIST treats unit symbols as fixed mathematical symbols and says they are not pluralized.
Sources and evidence
Sources are shown with the role they play in this guide. Historical or style-sensitive claims are kept within the evidence boundary described above.
- Writing with SI (Metric System) Units (opens in a new tab)National Institute of Standards and Technology · NIST unit-symbol rules, including no plural s on symbols