Measurements

Use numerals for all measurements

Use numerals for all measurements, even if the number is under 10. This is true regardless of whether the measurement unit is spelled out or abbreviated. Measurements include distance, temperature, volume, size, mass, pressure, energy, power, bytes, picas, and such, but generally not units of time.

Correct

5 centimetres

0.5 cm

8 kPa

three years

3 gigabytes

7 m

Hyphenating number and unit

Hyphenate a number and its unit of measure when it is preceded by another number. In such cases, spell out one of the numbers. Also hyphenate a number and unit when the combined phrase is used as an adjective. Do not hyphenate if it contains an SI symbol—spell out the unit if necessary. Spelled-out compound adjectives containing non-SI measurements are, otherwise, usually hyphenated.

Correct

24 one-kilogram weights, a 1-kilogram weight, a 1 kg weight

twenty-five 150-tonne trucks

two-year outlook, 100-year forecast

five-page report, 50-page report

one-quarter-of-an-inch tolerance, a tolerance of 0.25 inches

12-point type

10-kilometre radius or 10 km radius

50-metre grid or 50 m grid

256 kB RAM

Symbols

The shortened form of an SI unit of measure is known as a symbol and obeys different punctuation rules to abbreviations. Non-SI units use abbreviations but, for consistency, should be treated as symbols. There are many exceptions and should be studied carefully. For more information, see Symbols.

Correct

1 metre, 1 m

5 tonnes, 5 t

0.5 centimetre, 0.5 cm

0 inches, 0 in.

38 percent, 38%

15° 45′ 10″

38.6 degrees Celsius, 38.6 °C

Spelling out units of measure

If the symbol is well known (°C, %, m, and kg, for example), it can be used without definition. Always use the spelled-out form when discussing units of measure—“The results are shown in megawatt hours (MW·h)”. In general, it’s okay to use symbols when stating a measurement (“The pipeline has a capacity of 35 TJ/d”) or if you have space constraints, in a table for instance. But, unless it is in common use, always define your symbols at first use and in the list of abbreviations in the front matter.

Ranges

In general, use a spaced en dash – or spell out to show ranges of measurements. In general, show the unit of measure with each value. Only use an (unspaced) en dash when the unit of measure is the same for each value and the unit is spelled out.

Correct

50–75 cm

250 kPa – 25 MPa

between 250 kPa and 25 MPa

10% to 12%

10–12 percent

from 10 through 12 percent

85–120 t

85 to 120 tonnes

Dimensions

In general, spell out “by”. The multiplication symbol (ANSI code 0180) is acceptable in informal communications and technical documents. In general, show the unit of measure with each dimension, with the exception of some widely used phrases, such as four-by-two.

Correct

42 cm × 23 cm

4.5 m wide by 3.7 m high

4.5 m × 3.7 m (height by width)

8.5-by-11-inch paper

100-metre-by-200-metre grid

an area of 8.5 kilometres by 3 kilometres