numbers: quantities and measures

Use numerals for quantities or measures in the following cases:

• when they consist of two or more elements
  • 5 feet 11 inches tall (no comma between elements)
  • 8½ by 11 inch paper (or 8½ × 11 inch paper)
  • 50 cm × 75 cm × 2 m (unit repeated to avoid ambiguity)
• when they contain a decimal
  • 5.6 km
  • 1.6 m tall (not 1 m 60 cm tall)
• when they are used in technical contexts
  • 20/20 vision
  • a magnification of 50 (or a 50 × magnification)

Otherwise, follow the rule of writing the number out if it is less than 10:

• three miles
• two metres tall
• six feet tall

International System of Units

The International System of Units (SI) is now the norm in technical writing. SI usage requires either that both the number and the unit be written in full or that a numeral be used with the symbol:

• two metres or 2 m

not

• 2 metres or two m

Prefixed units should not normally appear as denominators in expressions of the form g/cm³, which should be re-expressed in terms of cubic metres. An exception to this rule is the symbol kg, since the kilogram is considered the base unit of mass.

Conversion from one unit to another

When one type of unit is converted to another in non-technical work, the converted value should normally be rounded to within five percent of the initial numeral and should be preceded by the word about or some other indication that the value is an approximation:

• 5 lb. or about 2.3 kg

Degree symbol

Note the following conventions for using the degree symbol.

Temperature

• 10 °C
• 10.5 °C
• 30 °C–50 °C (symbol repeated) but 30±2 °C
• 10-15 °C
• -10 to -15 °C (not -10-15 °C)

but

300 K (not 300 °K)

Latitude/longitude

• 10.5° or 10.°5 or 10°30’ or 10°30’00"
• 36°N lat. (36th parallel)

Other

• 10° (of arc)
• 40° proof (40 proof) alcohol

Note that the symbol ° should not be used alone in a denominator. To express millimetres per degree, write mm/degree (not mm/°).