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.
Practical Application
Use “h” as the technical unit symbol and do not pluralize it: write “2 h.” In ordinary prose, follow the governing house style for “hr/hrs,” or spell out “hour/hours” when clarity is more important than abbreviation.
Reader Check
Is this technical unit notation or ordinary prose, and does your chosen form follow the relevant style consistently?
Technical Rule
NIST treats unit symbols as fixed mathematical symbols and says they are not pluralized.
Worked Example
Write “The test ran for 2 h” in technical SI-style notation; in general prose, “The meeting lasted two hours” avoids an unnecessary house-style abbreviation decision.
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