Units of Measurement

General rules

You can write values in one of two ways: journal style or scientific style.  Do not mix styles.

  • Journalistic style: use the prefix and unit names spelled out. Example: eight kilohertz
  • Scientific style (preferred for technical writing): use the digit, the prefix, and the unit symbols. Example: 8 kHz

For scientific, technical, or engineering writing, the use of the International System of Units (SI) is preferred.  These are not considered abbreviations; they are considered symbols; as such, they follow the rules for mathematical symbols instead of traditional grammar rules for writing.

Do not abbreviate units; instead, use their symbols or unit names. Here are a few quick examples. It can be confusing because a lowercase unit, like hertz, uses a capitalized symbol or abbreviation/symbol.

The table below is for Symbol use.  For more about abbreviations, go here.

Correct SI symbol Incorrect usage
ampere A amps
curie Ci Cu, ci, cu
dalton Da da, DA
hertz Hz hz or HZ
inch in  or   ” in.
kelvin (degree absolute) K k or Kelvin
meter/metre m met
micrometer/micrometer (not micron) µm µ
millimeter/millimeter mm mlmm
minute/minutes min min. , mins, mins.
nanometer nm
second (unit of time) s sec

 

Do not mix a prefix with a unit symbol or abbreviation.

Correct: kHz

Incorrect: kiloHz, kHertz


Sentences and measurements

NEVER begin any sentence with digits or symbols or units.

Correct:  The result was 56 feet.

Incorrect:  56 feet was the result.

 

Correct: The most famous equation by any scientist is likely E=mc2.

Incorrect: E=mc2 is likely the most famous equation by any scientist.

 

Correct: The atomic mass of Na (sodium) is 22.989 769 28(2), which was revised in the year 2005.

Incorrect:  Na (sodium) has an atomic mass of 22.989 769 28(2), which was revised in the year 2005.

Incorrect:  2005 was the year that the atomic mass of Na was revised to 22.989 769 28 (2).


Space between digit and unit

When writing a measured unit, the numerical value comes first, followed by a space, followed by the unit.

Correct:  5 V, 1.17 V, 8 ohm, 8 W, 17 cm, 2700 mm, 44 °C

Incorrect:  5V, 1.17Volts, 8ohm, 17cm, 2700mm

Good example sentences

  • We measured the output to be 18 V.
  • Under normal operation, the LED housing temperature can increase by as much as 39 °C above ambient conditions.
  • The results fell into the −20 to +100 ppm/°C range.
  • Using NaCl as the press-transmitting medium, a single crystal, measuring ∼9 μm, was positioned in a diamond anvil cell.
  • The calculation showed the semiconducting properties with an indirect band gap of ∼25 eV.

Exceptions to the guidelines

The exceptions to this rule are for the degree sign (°), minute for plane angle (‘), second for plane angle (”), inch (”), foot (‘), and percent (%). For these, no space is needed, but a space can be used for consistency inside a document or article.


Units as adjectives

When units are used as adjectives, a hyphen is appropriate. Thus, the phrase 60-watt bulb is correct, because 60-watt is modifying the word “bulb.”

Good example sentences

  • A 32-watt 4-foot long fluorescent tube emits 2,600 lumens.
  • A 12-W LED light can produce up to 800 lumens.
  • A horsepower-hour can be measured as 2.6845 MJ.

Specifics

foot or feet

Inside of a sentence, there are several ways to represent foot/feet.  Choose a method and be consistent throughout your document.

  • Journalistic style: use the full word of foot or feet.
    • Example: The trebuchet measured ten feet high.
  • Scientific style: use the symbol “ft” for either foot or feet. Do not use a period after “ft” for the scientific style symbol. (Note: use numerical digits for measurements, which are different than count.)
    • Example: The trebuchet measured 10 ft high.
  • Scientific style: use the symbol of (‘) for foot or feet, with no space. In Microsoft Word, use insert/advanced symbol (not an apostrophe) for this symbol.  It is also called the prime symbol (‘).
    • Example: The trebuchet measured 10′.

gallon

If you are using a gallon, be sure to specify US gallon or a UK (imperial) gallon (which is about 1.2 US gallons).  The abbreviation is “gal” with no period in scientific practice. There is no symbol for gallon.


inches

Inside of a sentence, there are several ways to represent inches.

  • Journalistic style: use the full word inches.
    • Example: The box was four inches wide.
  • Scientific style: use the symbol “in” for the word inches. Do not use a period after “in” for the scientific style symbol. (Note: use numerical digits for measurements, which are different than count.)
    • Example: The box was 4 in wide.
  • Scientific style: use the symbol of (”) for inches, with no space. In Microsoft Word, use insert/advanced symbol (not a single set of quote marks) for this symbol.  It is also called the double prime
    • Example: The box was 4”.

kelvin

As a unit of temperature, use the degree symbol + K symbol (24 °K). The word kelvin is not capitalized unless you are referring to the man. There is a space between the numerical digit and the °K. For exceptions to the spacing rule, see Spaces and units herein.

Correct:  19 °K

Incorrect:  19°K, 19°Kelvin, 19°k

 

Good example sentence

The boiling point of water is 373 °K. It’s also important to know that 0 °K is commonly known as “absolute 0.”

Historical note

William Thomson, known also as Lord Kelvin, was a British inventor who coined the term “kelvin.”  In older publications, you will likely see “degree Kelvin” in use. However, in 1967, that practice changed so that when you write out the unit of measurement, uppercase K is used for the symbol.


kiloohm, kilohm, kilo-ohm, kilo ohm

To designate kiloohm (or any version) in any technical writing, it is best to use kΩ.

There is no designated preferred format for the word kiloohm for journalistic style writing. Variations can include kiloohm, kilo ohm, kilohm, and kilo-ohm. Use the preferred format in your field, staying consistent within the document.



laws and theories and phenomena, capitalization of

Overall, there are a few ways that technical authors, editors, and copy checkers approach the issue of capitalization with laws, theories, phenomena, and related categories that are name for specific people.  Our best advice is to consistently one set of the guidelines below.  Of course, if your target publication has preference, follow that style.

laws and theories

  • Chicago Manual of Style says….

If a law or theory is named after a person, capitalize that person’s name but not the general word “law” or “theory.”

Correct:  Bott index, Einstein’s theory of relativity, Kirchoff’s laws, Floquet’s theorem, Fourier’s law, Ohm’s law, Pauli matrices, Planck’s constant, Reynolds number

Incorrect: Bott Index, Einstein’s Theory of Relativity, Kirchoff’s Laws, Floquet’s Theorem, Fourier’s Law, Ohm’s Law, Pauli Matrices, Planck’s Constant, Reynolds Number (or Reynold’s Number)

Note

You will often see the words “law” and “theory” and such capitalized, which is incorrect.  Think of it this way—do we write a “Chevrolet or Ford Car”?  No, we would write “Chevrolet or Ford car.” It’s the same rule in play.

  • Science and Technical Writing by Philip Rubens says…

If a law or theory is named after a person, capitalize that law or theory and  person’s name using an apostrophe to show ownership.

Correct:  Bott Index, Einstein’s Theory of Relativity, Kirchoff’s Laws, Floquet’s Theorem, Fourier’s Law, Ohm’s Law, Pauli Matrices, Reynolds Number

phenomena

Because phenomena are general, their discoverers’ names remain capitalized but the phenomena themselves are in lowercase.

Example:  Hall effect, van der Waal’s force, Planck’s constant



ohm

As a unit of measurement, use ohm for technical writing, which can be plural. An alternative is the omega symbol (Ω).  There should be a space between the number and the unit. Be consistent internally within the document.

Correct: 8 ohm, 8 ohms, 8 Ω  (see your target publication for preferred usage and be consistent within your own document)

Incorrect: 8Ohm, 8Ohms, 8Ωs

Contextual note: If you are referring to the actual person of Georg Simon Ohm, who defined the term “ohm” for technical purposes, capitalize his name.  But when “ohm” is used as a unit of measurement, no caps should be used.

Ohm’s law is a principle named after the scientist. To use it in a sentence, capitalize “Ohm” but not “law.” This is similar in principle to Einstein’s’ theory of relativity, where we don’t capitalize the theory but we do capitalize his name.


seconds

For measuring time in seconds, the symbol is “s” even when plural. The same is true for inches, feet, hours, and minutes.

Correct:  9 s

Incorrect:  9secs, 9 secs, 9S


volt

For scientific style, the symbol for volt is V. There should be a space between the number and the V.

Correct:  8 V

Incorrect: 8V, 8v, 8volts, 8Vs, eightV


watt

For scientific style, the symbol for watt is W. There should be a space between the number and the W.

Correct:  60 W

Incorrect:  60W, 60Ws, 60watts, 60w

Good example sentence

For this project, we only used bulbs with a 100 W rating.


Common abbreviations

Common abbreviations (not symbols)

 

  Preferred for clarity Other* Not acceptable
ampere hour A·h A h Ah, amp hr or variants
bit b
byte B
cubic feet per minute ft3/min CFM, cfm
gallon gal define between US gallon and UK gallon gl, GL, gal.
gallon gas equivalent

 

GGE gge, g-ge, gg eq
litre, liter l or L lowercase is fine; uppercase can be used to distinguish the letter “l” from the number “1” for clarity li, lit
per / (slash or solidus) per per
pounds per square inch lb/in2 psi
square symbol2 symbol sq (example: sq mm is incorrect)
tesla T t
ton/tonne t T
*These uses should be clearly defined in the text with a nomenclature on in-line definition.

 

Sources

 More information

 

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