When referencing a distance, weight, volume or temperature, attempt to use metric as the primary system of measurement, with U.S. imperial units in parentheses. Typically, a conversion is only necessary the first time a number is mentioned: The earthquake was 27 km (16.7 miles) deep.
Use figures. Spell out imperial (U.S.) units. For metric units, spell out base units: meter, gram, liter and abbreviate others on second mention. Note space between number and unit.
At least 80 mm (3.14 inches) of rain is expected.
The room is 5 meters (16.4 feet) long.
Wrong: Sixty kilometers, 25.3ft., Two-hundred and twenty-four centimeters
Spell out units of measurement when they are not accompanied by a figure: He did not know how many kilometers he walked that day.
For approximations, use approximate conversions. Round up if the decimal is higher than 0.5:
The room is about 5 meters (16 feet) long. Not 16.4 feet.
It was around 550 km (342 miles) away. Not 341.8 miles.
Common metric abbreviations:
- mm – millimeter
- cm – centimeter = 10mm
- m – meter = 100 cm = 1000 mm
- km – kilometer = 1000 m
- m² – square meters (not meters square). Spell out.
- km² – square kilometers (not kilometers square). Spell out.
- g – gram
- kg – kilogram = 1000 g
- t – tonne or metric ton = 1000 kg. Usually, the word or phrase is easier to understand: The elephant weighed 4 tonnes (3.9 tons). The van weighed 10 metric tons (9.8 tons).
- ml – milliliter
- l – liter = 1000 ml. Note the word should be used if there is a possibility of confusion with the number 1: 1 liter, 21 liters
- m³ – cubic meters (not meters cubed). Spell out.
- C – celsius, not centigrade
Common Imperial (U.S.) units:
- inch, plural inches: 20 cm (7.9 inches)
- foot = 12 inches, plural feet: 20 m (65.6 feet)
- yard = 3 feet: 200 m (219 yards)
- square feet (not foot/feet square)
- square yards (not yards square)
- square miles (not miles square)
- ounce: 200 g (7.1 ounces)
- pound = 16 ounces: 200 kg (441 pounds)
Volume (note there is a difference between U.S. and British Imperial volume measurement)
- gallon = 8 pints
- F – Fahrenheit