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|
|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.