Units of Time

Sometimes units of time measurement are abbreviated, and sometimes they are not. As you may remember from Tip #57, “Units of Measure,” time units are abbreviated differently in the English and Metric systems:

Units of Time English Metric
Second sec s
Minute min min
Hour hr h
Day D d
Year / Annum yr a

When a unit of measure contains an element or factor of time along with another unit of measure, abbreviate the time unit of measure.


ft/hr,  m/h;  ft/sec, m/s;  ft3/D, m3/d;  tons/yr, tonnes/a

However, when a unit of time measure stands all by itself after a numeral, spell it out.


3 seconds, 4 minutes, 5 hours, 6 days, 7 years

If you are using a word rather than a numeral, spell out the unit of measure, even if it has more elements than just time in it


several feet per second, a few meters per day

Exception: when the unit of measure is a really long phrase like “several standard cubic feet per day.” For that, you may use “several scf/D,” according to the SPE Style Guide.


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