Units of Measurement

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Defining Units of Measurement

The International System of Units (SI) defines the majority of unit names and symbols in use (although there are some common non-SI units). SI also makes recommendations regarding the typical quantity symbols in use.

• Unit names are the names given to an SI measurement. For example, the kelvin, kilometer, or joule.
• Unit symbols are the ways in which values are usually expressed. For example, 200 K, 10 km, 5 J.
• Quantity symbols are the ways in which the seven base quantities(opens in a new tab) (time, length, mass, electric current, thermodynamic temperature, amount of substance, luminous intensity) are typically described.

Unit symbols are standardized across languages (for example, the Italian “chilometro” still uses the symbol “km”).

As such, it is not necessary to define any SI unit the first time it is used. In other words, it would be “The length of the field was 100 m,” not “The length of the field was 100 meters (m).”

When expressing the value of something in a scientific or technical text, the unit symbol is used rather than the name (so, 15 kg not 15 kilogram). In other texts, you should follow any style guide or the most appropriate use and aim for consistency.

Formatting and Punctuating Units

The International Bureau of Weights and Measures (BIPM) sets the formatting and punctuation rules for SI unit names and symbols. It gives the following stipulations for SI units.

Other Guidelines on Usage

Here are some other useful guidelines for using SI units.

• Plurals: SI unit symbols never go in the plural (so 500 Ω, not 500 Ωs). Unit names can go into the plural, but should be in the singular if used as part of a compound adjective (10-watt bulb).
• 1,000s separator: You should use the 1,000s separator where appropriate. Some countries use a space instead of a comma, but English text almost always uses a comma (1,000,000 not 1 000 000).
• Unit names and symbols: You should not mix unit names and unit symbols in the same context; use one or the other (for example, kg/m³ or kilogram per cubic meter, not kg/cubic meter).
• Hyphens: Hyphen rules vary. Unless a style guide says otherwise, use an adjectival hyphen with the unit name but not with the unit symbol (e.g., 10 W bulb but 10-watt bulb).
• Variables: Variables (symbols that represent mathematical objects) are given in italics, e.g., V = 2.5 s. These are not units of measurement.
• Ranges: Most scientific style guides prefer that you repeat the unit symbol (e.g., 15 cm x 40 cm, not 15 x 40 cm) and use “to” rather than an en dash (e.g. 20 °C to 30 °C, not 20 °C–30 °C. APA is an exception.

The Percentage Symbol

Although it’s not an SI unit, the percentage symbol (%) deserves an honourable mention here, as the rules are similar in some ways but not in others. Here’s some things to note.

• Whether to use % or percent is a matter for your style guide, but a general rule is (using example figures) five percent or 20 percent (humanities), 5% or 20% (science), and 5.2% or 20.76% (decimal). You should avoid mixing % and percent in the same document.
• Don’t use an adjectival hyphen (e.g., 10 percent raise, not 10-percent raise).
• There is no space between the number and the percentage symbol (e.g., it is 10%, not 10 %). However…
• Where a % symbol is preceded by a space (e.g. D = 0.2 %) it simply means 0.01. You should leave equations alone anyway, but it’s something to be aware of.

Proofreading Responsibilities

When proofreading text that contains units of measurement, you should:

• Note any instances where measurement units appear to be missing.
• Ensure that the spacing, capitalization, and punctuation of units are correct.
• Ensure that unit names and unit symbols are used appropriately and consistently. Unit symbols should not be written as plural.
• Ensure that ranges are written clearly, with a unit attached to each figure.
• Ensure that adjectival hyphens are used with unit names (but not symbols) where appropriate.
• Ensure that the 1,000s separator is used where appropriate.
• Follow any style guide requirements, including those related to percentages.

You do not (generally speaking) need to:

• Correct which unit is used (unless there is an issue with consistency, which you should comment on).
• Apply formatting such as italics (unless there is an issue with consistency, which you should comment on, or unless formatting has been selected as a service).

Summary

SI and non-SI units of measurement are found often in scientific and technical documents. As a proofreader, your responsibility is to ensure that spacing, capitalization, and punctuation are correct. You should also comment on any inconsistency or instances where units seem to be missing.